California Partners in Flight Riparian Bird Conservation Plan
Flycatcher (Empidonax traillii)
Photo by Steve Zack
Prepared by: Diana Craig1 (email@example.com) and Pamela L. Williams2 (firstname.lastname@example.org)
Forest Service, Pacific Southwest Region
Kern National Wildlife Refuge
Craig, D. and P. L. Williams. 1998. Willow Flycatcher (Empidonax traillii). In The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/riparian_v-2.html
SUBSPECIES STATUS: The Willow Flycatcher was named by the American Ornithologist's Union in 1973 (AOU 1973) when the species E. traillii was divided into two species, the Willow (E. traillii) and Alder (E. alnorum) Flycatchers. There are five subspecies of the Willow Flycatcher currently recognized (Federal Register 1995, Browning 1993, Unitt 1987). Three of these subspecies occur in California (Phillips 1948, Unitt 1987).
E.t. brewsteri ("Little Willow Flycatcher") Breeds in California from Tulare County (S. Laymon, pers. comm.) north, along the western side of the Sierra Nevada and Cascades, extending to the coast in northern California.
E.t. adastus: Breeds in California east of the Sierra/Cascade axis, from the Oregon border into Modoc County and possibly into northern Inyo County. (For the purposes of this paper, populations at high elevation just east of the Sierra Nevada crest but south of Modoc County are assumed to be E.t. brewsteri). There has been very little study of E.t. adastus in California and therefore this subspecies is not covered in depth in this report.
E.t. extimus ("Southwestern Willow Flycatcher") Breeds in California from the Mexican border north to Independence in the Owens Valley, the South Fork Kern River, and the Santa Ynez River in Santa Barbara County.
MANAGEMENT STATUS: All three subspecies are listed as State Threatened and U.S. Forest Service Region 5 Sensitive in California. The U.S. Fish and Wildlife Service has designated the Willow Flycatcher a sensitive species in Region 1 (Washington, Oregon, Idaho, California, and Nevada). E.t. extimus is federally listed as endangered (Federal Register 1995).
HISTORICAL BREEDING DISTRIBUTION
Historically, Willow Flycatchers nested throughout California wherever riparian deciduous shrubs, mainly thickets of willows, occurred (Grinnell and Miller 1944). Altitudes of known nestings occurred from within 30 m (100 ft.) of sea level to 2,440 m (8,000 ft.) (Grinnell and Miller 1944). Habitat was most common at lower elevations, rarely occurring at the 1,830 to 2,440 m (6,000-8,000 ft.) range in the Sierra Nevada (Bent 1942). The historic breeding distribution of Willow Flycatchers in California probably included representatives of three subspecies (Phillips 1948, Unitt 1987).
E.t. brewsteri: On the west side of the Sierra Nevada, Willow Flycatchers were considered common summer residents in Yosemite Valley and surrounding areas (Grinnell and Storer 1924, Gaines 1988), and in Sequoia and Kings Canyon National Parks and surrounding areas (Sumner and Dixon 1953).
E.t. adastus: One specimen of E.t. adastus, a Great Basin subspecies, is known from Goose Lake, Modoc Co. (Harris 1997). Phillips (1948) thought that portions of northern California might be a zone of intergradation between E.t. brewsteri and E.t. adastus. The status of populations east of the Sierra/Cascade crest between Goose Lake, Modoc County, and Independence, Inyo County, is unclear, although they have been thought likely to be E.t. adastus (Unitt 1987). On the east side of the Sierra Nevada, Willow Flycatchers were considered common along the lower streams in the vicinity of Mono Lake (Grinnell and Storer 1924).
E.t. extimus: This subspecies occurred in southern California, with its northern limits represented by specimens from Independence (Inyo Co.), the South Fork Kern River near Weldon (Kern Co.), and the San Fernando Valley (Los Angeles Co.) (Unitt 1987). Populations in southeastern California were probably restricted to the Colorado River, but they were common there, e.g. 37 nests collected near Yuma in 1902 (Unitt 1987). They were also common in swampy thickets around Los Angeles (Belding 1890). It is probably this subspecies that Belding (1890) found a common summer resident in the Central Valley, although there are no specimens from the Central Valley in California museums. Other areas represented by museum specimens include the Santa Ana River near Colton, San Bernardino County; San Fernando Valley, Los Angeles County; and areas of Riverside County (Unitt 1987). They were also observed along the Mohave River, San Bernardino County, and at Santa Barbara, Santa Barbara County (Belding 1890). Willett (1912) considered it a common summer resident in willow thickets of lowland southern California occurring up to 1,520 m (5,000 ft) in mountain canyons. Other locations that were probably occupied by the southwestern subspecies include: (1) Goldman's (1908) account of Willow Flycatchers as "rather common in willow thickets and tule marshes" from Summit Lake at the delta of the Kings and San Joaquin Rivers south to Lake Buena Vista in Kern County; (2) Linton's (1908) referring to it as a common breeder in the vicinity of Buena Vista Lake; and (3) Tyler's note of encountering them "occasionally" in the willows along certain sloughs in the Fresno district (Tyler 1916).
CURRENT BREEDING DISTRIBUTION
The Willow Flycatcher breeds in "moist brushy thickets, open second-growth, and riparian woodland, especially with willow and buttonbush."(AOU 1998). Its breeding range extends "from central British Columbia, southern Alberta, southern Saskatchewan, southwestern Manitoba, northern North Dakota, western and southern Minnesota, central Wisconsin, Michigan, southern Ontario, southwestern Quebec, central Maine, New Brunswick, Prince Edward Island, and Nova Scotia (possibly) south to southern California (local, formerly widespread), northern Baja California and northern Sonora (at least formerly), southern Arizona (locally), southern New Mexico, northeastern Oklahoma, Arkansas (rarely), northeastern Louisiana, central Tennessee, northern Georgia, western South Carolina, western North Carolina, and central and eastern Virginia (AOU 1998).
E.t. brewsteri: In California, it is a rare to locally uncommon summer resident in wet meadows and montane riparian habitats from 600 to 2,440 m (2,000-8,000 feet) in elevation and a common spring (mid-May to early June) and fall (mid-August to early September) migrant at lower elevations, primarily in riparian habitats, throughout the state exclusive of the North coast (Zeiner et al. 1990). Most of the remaining breeding populations occur in isolated mountain meadows of the Sierra Nevada and Cascades (Serena 1982, Harris et al. 1988, B. Valentine pers. comm.). The Perazzo Meadow/Little Truckee River/Lacey Valley area, in Sierra County, has had a relatively stable population of breeding Willow Flycatchers known since 1982, with 23 to 36 singing males detected each year; there were 21 breeding territories in the Perazzo and Little Truckee River areas in 1997 (Serena 1982, Harris et al. 1988, Bombay 1998, S. Sanders, pers. comm.). A population of breeding Willow Flycatchers was discovered at Red Lake, in Alpine County, in the early 1990s; there were five territories here in 1997 (Bombay 1998, R. Schlorff, pers. comm.). A possible breeding population occurs along the Klamath River (S. Snaido, pers. comm.).
Two newly discovered populations documented in the Cascades during the past two years almost double the known population of this subspecies in California. One population southeast of McCloud, Siskiyou County, had 72 flycatchers in 1997 (C. Stermer, pers. comm.). Another population in Warner Creek Valley, in northern Plumas County had 42 flycatchers in 1997 (S. Armentrout, pers. comm.).
E.t. adastus: Willow Flycatchers were found breeding on the Owens River between Pleasant Valley and Bishop in the summer of 1993 (Laymon and Williams 1994), and, based on range, it is likely that these were E.t. adastus.
E.t. extimus: Breeds from near sea level on the Santa Margarita River to 805 m (2640 ft.) at the South Fork Kern River (Whitfield et al. 1997) and 914 m (3000 ft,) at upper San Luis Rey River (Griffith Wildlife Biology 1995) in California and to over 2,600 m (8530 ft.) in Arizona and southwestern Colorado (Sogge et al. 1997a). The largest remaining population in California is on the South Fork Kern River, Kern Co. (Unitt 1987).
In southern California, the Southwestern Willow Flycatcher breeds on the San Luis Rey River, San Diego County; on Camp Pendleton, San Diego County on the Santa Margarita River and Pilgrim, De Luz, French, and Las Flores creeks (Griffith Wildlife Biology 1995); and on the Santa Ynez River, Santa Barbara County (Holmgren and Collins 1995; US Fish and Wildlife Service 1997).
In 1996, breeding was confirmed along the Arizona side of the lower Colorado River at Lake Mead Delta (7 nests on 5 territories) and at Topock Marsh (1 nest) (McKernan 1997). Other possible breeding sites on the lower Colorado River included North Lake Havasu, Bill Williams River, Ehrenberg, Cibola Lake, Walker Lake, Ferguson Lake, Mittry Lake, and Gila River (McKernan 1997).
The subspecies E.t. extimus has suffered severe declines throughout its range (Unitt 1987, U.S. Fish and Wildlife Service 1993, 1995).
The winter range (all subspecies) extends from Nayarit and southwestern Oaxaca, Mexico, south to Panama and possibly extreme northwestern Columbia (AOU 1998). Willow Flycatchers prefer pacific slope, arid scrub, and brushland habitats (Fitzpatrick 1980). There is no clear evidence that the subspecies of Willow Flycatchers segregate on the wintering grounds (Unitt 1997).
E.t. extimus: Examination of museum specimens of 578 migrating and wintering Willow Flycatchers located three definite and eight probable specimens of E.t. extimus indicating that Guatemala to Costa Rica constitutes the main winter range (Unitt 1997).
AVERAGE TERRITORY SIZE
Male Willow Flycatchers establish territory boundaries prior to pair formation and maintain them early in the season by singing from elevated perches. Territorial overlap between adjacent males is minimal. Willow Flycatchers conduct most of their foraging and other activities within their territories (KRCD 1985a), however both males and females use adjacent areas (Sanders and Flett 1989). In southern Michigan, 1957-1964, average territory size was 0.7 ha (Walkinshaw 1966).
E.t. brewsteri: Territory sizes for 6 paired males ranged from 0.09-0.38 ha and averaged 0.18 ha in eastern Fresno County, California (KRCD 1985b). On the Little Truckee River in Sierra County, 22 territories ranged from 0.06-0.89 ha and averaged 0.34 ha (Sanders and Flett 1989). On the Little Truckee River, males and females regularly used perches ranging from 4-30 m (13-98 ft) outside their defended territory when feeding the young (Sanders and Flett 1989). Willow Flycatchers may forage as far as 100 m (328 ft) from their territories at this time (Ibid). Fledglings typically range into territories of adjacent pairs, often followed by parents (KRCD 1985a). Little singing or chasing occurs at this time, indicating a general decline of territory defense (Ettinger and King 1980).
E.t. extimus: Estimated territory sizes range from 0.06 to 1.5 ha on the Colorado and Verde Rivers in Arizona (Sogge et al. 1997a). At the South Fork Kern River, territory size for monogamous males averaged 0.6 ha (SD = 0.35, n = 24, range 0.1 to 1.3) and for polygynous males averaged 1.1 ha (SD = 0.68, n = 24, range 0.2 to 2.8) (Whitfield and Placer 1994, Whitfield and Strong 1995, Whitfield and Enos 1996, Whitfield et al. 1997).
TIME OF OCCURENCE AND SEASONAL MOVEMENTS
Little is known about each subspecies' pre-breeding season movements in California. Grinnell and Miller (1944) reported that spring migration occurred in a wide distribution over the lowlands of the State. At the South Fork Kern River from 1992 to 1997 (all subspecies), the average first arrival date was 27 April, the earliest date was 16 April (1992), and the latest date of first spring arrival was 10 May (1997) (S. Laymon, pers. comm.). At desert oases in eastern Kern County, Willow Flycatchers have been found from 7 May to 16 June with the peak migration from mid-May to early June (M. Heindel pers. comm.). In the Sierra Nevada, the earliest documented arrival date is May 15, 1926 (Gaines 1988). At Red Bluff, Tehama County, spring migrants were rare (1976-1979); 13 individuals were recorded, the earliest on 5 April, 1976, and the latest on 13 June, 1979 (Laymon 1981). In a two year banding study on the Middle Rio Grande River in New Mexico, 42 spring migrants were captured from 13 May - 8 June, with the peak of migration in the last week in May (Yong and Finch 1997).
Transients are observed in the state through mid-September (Zeiner et al. 1990), but little is known about each sub-species' post-breeding season movements in California. Grinnell and Miller (1944) reported that post-breeding fall migrations may include invasions of the species into habitat higher in elevation than the highest breeding habitat. At desert oases in eastern Kern County, the earliest fall date is 28 July and the latest fall record is 18 October, with peak of migration from mid-August to early September (M. Heindel, pers. comm.). At Coyote Creek Riparian Station, Alviso, Santa Clara County (data for 1992, 1993, 1995), the earliest fall migration date of 99 Willow Flycatchers captured was 9 August, 1992, and the latest was 9 October 1992, with the median capture date ranging from 29 August (1993) to 31 August (1992) (Bousman 1993, 1994, 1996). In the Sierra Nevada, the latest fall record is 1 October (1926) (Gaines 1988). At Red Bluff, Tehama County, fall migrants occurred from 7 August to 30 September, from 1976-1979 (Laymon 1981). In a two year banding study on the Middle Rio Grande River in New Mexico, 42 fall migrants were captured from August 9 to September 16, with no peak based on weekly captures, and with adults showing a tendency to migrate earlier than hatching-year birds (Yong and Finch 1997).
How to separate migrating from breeding individuals. Willow Flycatchers (all subspecies) sing during migration and E.t. brewsteri is one of the latest spring migrants in North America (Unitt 1987). Thus, while E.t. extimus is already breeding, E.t. brewsteri is still migrating through the region, and in southern California may continue moving north until about 20 June (Garrett and Dunn 1981). Thus, E.t. brewsteri may be present in the range of E.t. extimus during much of the latter's breeding season, and single observations are often useless in identifying breeding sites (Unitt 1987). Also, mated males may stop singing during nesting. For this reason, sites must be surveyed a minimum of three times. For E.t. extimus, Survey 1 should be conducted 15 - 31 May, at which the earliest arriving males may have already established territories; Survey 2 should be conducted 1 - 21 June when later arriving males will be singing strongly and nesting activity may be detected (a male detected in Survey 1, but not Survey 2 may have been a migrant); Survey 3 should be conducted 22 June - 10 July, by which time all breeders will have arrived on their territories and any singing males detected will be residents (Sogge et al. 1997a).
E.t. brewsteri: In the Sierra Nevada, Willow Flycatchers arrive late in the breeding season compared to other passerines nesting in Sierran meadows (Harris 1997, Sanders and Flett 1989). Males arrive in late May to early June, and females arrive about 1 week later (Garrett and Dunn 1981, Stafford and Valentine 1985). Weather conditions on the breeding grounds can affect arrival date and subsequent nesting activities (Stafford and Valentine 1985). In the central Sierra Nevada, the heavy snowfall and long winter of 1983 delayed arrival by nearly a month relative to 1984 (Stafford and Valentine 1985). Nest building in the central Sierra Nevada usually begins within one week of pair formation (Stafford and Valentine 1985, Harris 1997). Nest construction generally takes about eight days and the first egg may be laid before the nest lining is complete.
In the central Sierra Nevada, departure from breeding grounds usually occurs within a week after the young have fledged. If a nest fails too late for the adults to attempt a re-nest, the adults depart the breeding meadow. By mid-August of most years, all Willow Flycatchers have departed (Valentine et al. 1988). Time of departure from the breeding grounds was not synchronous; non-breeders left first followed by breeders. Late departure dates reflect delayed on-set dates (Stafford and Valentine 1985,Valentine et al. 1988).
E.t. extimus: The southwestern subspecies arrives on the breeding grounds between early May and early June (Sogge et al. 1997a). Successful breeders depart from breeding territories as early as mid-August, and fledglings probably leave the breeding area a week or two after adults, but few details are known (Sogge et al. 1997a).
MIGRATION STOPOVER CHARACTERISTICS
While only a single study was found on use of migratory stopover sites, it appears that Willow Flycatchers stay only briefly at stopover sites. On the Middle Rio Grande River in New Mexico, of 84 migrant Willow Flycatchers captured in two years, only seven were recaptured (Yong and Finch 1997). All the recaptures occurred within one day of the initial capture and had added on average 1.6% body mass/day. About 50% of the captures had no fat stores, suggesting that stopovers are brief but frequent (Yong and Finch 1997).
The range of habitats used is much wider than that preferred for breeding and includes narrow, linear riparian strips less than 10 m (33 ft) wide (Sogge et al. 1997a) and shrubs and trees in parks and gardens (pers. obs.). On the Middle Rio Grande River, netting was done in four habitat types: willow, cottonwood-Russian olive, agricultural fields, and cottonwood-other habitats. Willow habitat had the highest capture rate (n=34.1 birds/10,000 net hours), with dense young cottonwood-Russian olive having the next highest rate (n= 16.0 birds/10,000 net hours) (Yong and Finch 1997). Willow Flycatchers were observed flycatching and gleaning from foliage. Vegetation structure probably also played a role, with more captures occurring in habitats with dense shrub vegetation than in open, shrubless habitats (i.e. cottonwood-other and agriculture).
Willow Flycatchers forage by either aerially gleaning (capturing an insect from a substrate while hovering) from trees, shrubs, and herbaceous vegetation or hawking larger insects by waiting on exposed forage perches and capturing insects in flight (Ettinger and King 1980, Sanders and Flett 1989). Hawking appears to be more common than aerial gleaning in mountain meadows and the opposite appears to be the case in lowland riparian areas (Sanders and Flett 1989, J. Harris, pers. comm.).
A local, concentrated source of nutrients in the form of flying insects is required to meet the nutritional needs of territorial establishment and defense, mating, nest building, egg laying, brooding, and nestling rearing. Bent (1942) states that 96% of the diet of this species consists of animal matter, with most of this in the form of flying insects. These insects are either attracted to water for consumptive proposes, or require it for the aquatic phase of their life cycle. After the breeding season, when Willow Flycatcher fledglings are able to forage for themselves and become more mobile, the Willow Flycatchers are not as dependent on a localized food source.
In an examination of stomach contents of 135 specimens, 96.05% of the diet was animal food and 3.95% of the diet was vegetable food (elderberry , blackberries, etc.) (Beal 1912 in Bent 1942). Hymenoptera (mostly wasps and bees) made up 41% of the diet, Coleoptera were 18% of the diet, Diptera (such as crane, robber, house, and dung flies) were 14%, Hemiptera were 8%, Lepidoptera (moths and caterpillars) were 8%, and Orthoptera (mostly small grasshoppers) were 4%.
E.t. brewsteri: In the area of Kings Canyon and Sequoia National Parks, the diet was reported to be "wasps, bees, beetles, flies, caterpillars, moths, grasshoppers, and occasionally berries" (Sumner and Dixon 1953).
E.t. extimus: Food analysis using fecal samples from birds in Arizona and Colorado have shown results similar to those reported in Bent (1942) with the addition that Dragonflies (Odonata) composed 6% of the diet (K. Williams, pers. comm., citing draft report by Drost et al.). Fecal samples from California have been collected, but not yet analyzed (M. Sogge, pers. comm.).
There is no documentation that water is required for drinking.
In California, breeding habitat is typically moist meadows with perennial streams; lowland riparian woodlands dominated by willows (Salix spp.), primarily in tree form, and cottonwoods (Populus spp.); or smaller spring-fed or boggy areas with willow or alders (Alnus spp.) (Serena 1982, Harris et al. 1988, Whitfield et al. 1997). Riparian deciduous shrubs or trees, such as willow or alder, are essential elements on Willow Flycatcher territories (Sanders and Flett 1989, Harris et al. 1988). In mountain meadows, willow thickets interspersed with open space are typically utilized, while large, contiguous willow thickets are avoided (Ibid). However, in lowland riverine habitats, contiguous willow thickets are used, possibly because the linear nature of these areas provide sufficient edge and/or the tree-like willows typically found in these areas provide sufficient openings within the willow canopy (Harris 1991, R. Wilson, pers. comm.).
The following quotes with references are from Harris (1997).
"Given the extensive geographic range of the Willow Flycatcher, it is not surprising that there is geographic variation in the characteristics of Willow Flycatcher habitats. Even in the western United States, substantial variation exists between the habitats of Willow Flycatchers in Oregon and Washington as compared to the central and southern Sierra Nevada or the southwestern region."
E.t. brewsteri and E.t. adastus
"In Washington and Oregon, several vegetation types have been mentioned in descriptions of Willow Flycatcher habitat, including the following: deciduous growth around the borders of clearings and brushy lowlands (Jewett et al. 1953); shrubby portions of wooded stream bottoms (Gabrielson and Jewett 1940); willow thickets bordering streamside lakes, woodland edges, young alder forests and tall brush at the margins of fields (Gilligan et al. 1994); riparian hawthorne thickets, shrub stratum of floodplain forest, upland prairie remnants with hawthorne, rose or Prunus, and ninebark thickets at the lower edge of conifer forest (King 1955). Additional environmental features mentioned in these accounts include openness of the shrub stratum (Jewett et al. 1953) and proximity to water (Gilligan et al. 1994), although the immediate proximity of water is not an absolute requirement (King 1955). A wide variety of deciduous plant species form the shrub stratum in these vegetation types, including willow, alder, dogwood, hawthorne, rose, elderberry, and Prunus spp. In Washington and Oregon, the Willow Flycatcher appears to be a lowland species."
"In contrast, habitat descriptions for the central and southern Sierra Nevada emphasize riparian, willow-dominated vegetation (Grinnell and Miller 1944, Gaines 1988, Serena 1982, Harris et al. 1988) Habitat typically includes moist meadows with perennial streams and smaller spring-fed or boggy areas with willow (Salix spp.) or alder (Alnus spp.) (Serena 1982, Harris et al. 1988). Willow Flycatchers have also been found in other riparian environments of various types and sizes ranging from small willow-surrounded lakes or ponds with a fringe of meadow or grassland to various willow-lined streams, grasslands, or boggy areas. Breeding Willow Flycatchers have not been detected above 2,500 m (8,000 feet) elevation in California."
"Non-shrub trees do not appear to be a required habitat component, but Willow Flycatchers will use scattered trees for singing and foraging perches and females will use the foliage of trees as gleaning substrate during the nestling period (KRCD 1985a, Harris et al. 1988, Sanders and Flett 1989). Several authors describe edge, in the form of openings within thickets of riparian deciduous shrubs, as an important component of Willow Flycatcher habitat (Walkinshaw 1966, Serena 1982, Harris et al. 1988, Sanders and Flett 1989)."
"The habitat descriptions presented above suggest that Willow Flycatchers occupy a broader range of habitats in Oregon and Washington than in the central and southern Sierra Nevada. The range of shrub species utilized and the requirement for water are two aspects of habitat that appear to change from north to south. Recent observations in California (J. Villegas, pers. comm.) suggest that northern California Willow Flycatchers might not conform to breeding habitat descriptions based on work in the central and southern Sierra Nevada. In the Modoc National Forest, Willow Flycatchers have been seen occupying breeding habitat with a shrub stratum of Prunus, Ribes and other shrub species. Two nests have been found in Cercocarpus. Surveyors should be aware that Willow Flycatchers may occupy sites in the northern portions of California which more closely resemble habitats described in Oregon and Washington than those described for the central and southern Sierra Nevada. Surveyors should also be aware that migrating Willow Flycatchers may occupy a wide range of environments that differ in shrub species composition, slope and hydrology from those occupied during breeding activities." (End of quotes from Harris 1997).
Valentine (1987) suggested that an important character of Willow Flycatcher habitat in the central Sierra Nevada was the openness of the tree canopy. He suggested that fire may result in early successional stages that may provide usable nesting habitat. This was based on a sighting of a territorial male that behaved as if paired in a thick shrub field that was the result of an old forest fire. The site was mesic and shrub species diversity was high, but the site was not a typical Sierra Nevada Meadow (B. Valentine, pers. comm.).
E.t. extimus: The Southwestern Willow Flycatcher is a riparian obligate species restricted to dense stream-side vegetation. Four general habitat types are used by the Southwestern Willow Flycatcher throughout its range: monotypic high-elevation willow; monotypic exotic (dense stands of saltcedar [Tamarix] or Russian olive [Elaeagnus]; native broadleaf dominated, and mixed native/exotic (Sogge et al. 1997a). Of these, native broadleaf dominated and mixed native/exotic are mainly used in California. The native broadleaf dominated habitat is composed of a single species (e.g., Goodding's or other willow species) or a mixture of broadleaf trees and shrubs, including cottonwood [Populus], willow [Salix], box elder [Acer negundo], ash [Fraxinus], alder [Alnus], and buttonbush [Cephalanthus], from 3 - 15 m (1 - 50 ft) tall, and characterized by trees of different size classes yielding multiple layers of canopy (Sogge et al. 1997a). On the San Luis Rey River, the broadleaf habitat is dominated by coast live oaks (Griffith Wildlife Biology 1995). The mixed native/exotic habitat is composed of dense mixtures of native broadleaf trees and shrubs mixed with introduced species such as salt cedar or Russian olive. The vegetation of occupied sites includes dense patches often interspersed with small openings, open water, or shorter vegetation, creating a mosaic that is not uniformly dense (Sogge et al. 1997a).
In general, willows, alders, and cottonwoods or other riparian deciduous vegetation.
E.t. brewsteri: In mountain meadows, Willow Flycatchers appear to prefer nesting near the edges of vegetation clumps and near streams (Valentine et al. 1988, Sanders and Flett 1989). In meadows along the little Truckee River, nests were built in shrub willows (Salix lemmonii and S. jepsoni) (Sanders and Flett 1989). Nests in these meadows are generally located in riparian deciduous shrubs at least 2 m (6.6 feet) high, with a foliar density of approximately 50-70%, and with about 1 m (3.3 feet) of cover above the site (Harris et al. 1988, Sanders and Flett 1989). Nests are usually placed in a vertical fork of a riparian deciduous shrub and built around supporting twigs (Stein 1963, Flett and Sanders 1987, Valentine et al. 1988, Sanders and Flett 1989, Harris 1991).
E.t. adastus: A vertical crotch or a horizontal or slanting fork with small branches that can be woven into the nest are required (King 1955). Plants commonly used include rose (Rosa), hawthorne (Crataegus), cow parsnip (Heracleum), choke cherry (Prunus), and ninebark (Physocarpus) (King 1955).
E.t. extimus: Nests are typically placed in the fork of a branch with small (1-2 cm in diameter) vertical stems supporting the nest (Sogge et al. 1997a). Nests have been found in willows, box elder, salt cedar, live oak (Quercus), buttonbush, black twinberry (Lonicera involucrata), Fremont cottonwood, alder, blackberry (Rubus ursinus), baccharis or mulefat (Baccharis spp.), and stinging nettle (Urtica spp.) (Sogge et al. 1997a, Sferra et al. 1997). However, we could find no record of flycatchers nesting in tamarisk in California.
On the South Fork Kern River, Willow Flycatchers tend to nest in areas that have more trees greater than 5 m (16 ft) tall, a larger amount of tree canopy cover, and a larger amount of foliage volume from 0 to 4 m (0 to 13 ft) than random areas (Whitfield 1990). Nests there occur typically in areas with multi-layered vegetation and fairly high (60-65%) tree canopy cover (Whitfield and Enos 1996). In a study of 344 nests found during nine years of study on the South Fork Kern River, 244 (73%) of nests were supported by willows, 46 (14%) were supported by a combination of willow and nettle, 30 (9%) were supported entirely by nettle, 7 (2%) were supported by mulefat, with the remaining nests occurring in cottonwood (Populus fremontii), white alder (Alnus rhombifolia), and ash (Fraxinus sp.) trees (Whitfield et al. 1997).
Along the San Luis Rey River in San Diego County in 1994, Willow Flycatchers nested in coast live oaks (Quercus agrifolia; n = 7), blackberry hedges (Rubus ursinus; n = 5), and flowering ash (Fraxinus dipetala; n = 1) (Griffith Wildlife Biology 1994).
Along the lower Colorado River in Arizona, seven nests were found in Goodding's black willow (at Lake Mead Delta) and one in salt cedar (Tamarix sp.; at Topock Marsh; McKernan 1997).
NEST SITE FIDELITY
In Michigan, Walkinshaw (1966) found a return rate of 22.6% of females (n = 31) in the first year after they were banded, 40.9% for males (n = 22) in the first year after they were banded, and 1.4% for young (n=147). Only one female returned for a third year. For males, 22.7% (n = 5) returned for a third year; 13.6% (n = 3) returned the fourth year; and 4.5% (n = 1) returned the fifth year (Walkinshaw 1966).
E.t. brewsteri: In the Sierra Nevada, return rates for adults have been 25% in the Little Truckee River area (Sanders and Flett 1989) and about 31% for adults and nestlings in the Shaver Lake area (5 of 12 banded adults and 4 banded nestlings; Stafford and Valentine 1985). In the Little Truckee River area, of the four banded males from 1986 that returned to breed in 1987, three returned to their 1986 territories and one settled approximately 1 km (0.6 mi) from his 1986 site (Sanders and Flett 1989). On Sierra National Forest, one pair returned to their same territory, while one female on territory in 1983 nested successfully in 1984 at a site approximately 14.5 km (9 mi) from her 1983 territory (Stafford and Valentine 1985).
E.t. extimus: Site fidelity for birds banded as adults on the South Fork Kern River is 35.8%. Of 38 banded adult females, 34% were resighted in subsequent years, as were 41% of 27 banded males (Whitfield and Enos 1996). Fledglings also show a high return rate with 34% of 123 banded young returning the year after fledging, and a few individuals returning for as many as five years (Whitfield and Enos 1996). Young fledging early in the season (before July 20th) are significantly more likely to return in subsequent years than young fledged late in the season (on July 20th or later), 22% vs. 6%, respectively (Whitfield and Strong 1995).
HEIGHT OF NEST
Typical nest placement is such that the nests may be susceptible to damage from wind, cattle, and predators (KRCD 1988, Valentine et al. 1988), although this susceptibility may be more significant in the Sierra Nevada than in lowland riparian habitats (Harris 1991).
E.t. brewsteri: Nests in mountain meadows are generally located at about 1.1 to 2.5 m (3.6 to 8.2 feet) above the ground (Stein 1963, Flett and Sanders 1987, Valentine et al. 1988, Sanders and Flett 1989, Harris 1991). Often, nests are about one meter above the ground and about one meter below the top of the willow foliage (Sanders and Flett 1989, Valentine et al. 1988). In the Tahoe, Toiyabe, and Plumas National Forests in 1997, the mean height of 25 nests was 1.2 m (s = 0.3 m) from the ground (Bombay 1998).
E.t. extimus: Nest height varies considerably, ranging from 0.6 to 18 m (2 to 59 ft) above the ground (Sogge et al. 1997a). In an analysis based on 187 egg sets in museum collections, nest height ranged from 0.6 m to 5.5 m (mean = 2.3 m, SD = 0.92 m; Unitt 1987). At the South Fork Kern River, the average nest height is 2.2 m (SD = 1.36, n = 186) (Whitfield et al. 1997) Most nests (108 of 134) were between 0.6 and 3 m (2 - 10 ft) high, with the highest nest at 10 m (33 ft) (Whitfield et al. 1997). Nests were placed significantly higher and placed farther into willow clumps at the Kern River Preserve (extimus subspecies) than those reported at higher altitude study sites (KRCD 1988, Valentine et al. 1988). This may reflect differences in habitat structure between the short, shrubby willows of montane meadows and the larger willows characteristic of the lowland mature riparian forest (Ibid). The later also provides large openings under the tree willow canopy, allowing nest placement near an edge within the canopy but far from the outer edge of the willow clump.
In 1994 on the Upper San Luis Rey River in San Diego County, nests averaged 2.8 m (9 ft) high, with 7 nests below 2 m (range 0.7 - 8 m; n = 13) (Griffith Wildlife Biology 1995). Nests at Lake Mead Delta and Topock Marsh along the lower Colorado River averaged 2.4 m high (SD = 0.35, n=7) (McKernan 1997).
HEIGHT OF NEST PLANT
E.t. brewsteri: Along the Little Truckee River, nest shrubs averaged 2.1 m tall (n = 20; Sanders and Flett 1989). In the Tahoe, Toiyabe, and Plumas National Forests in 1997, nests were an average of 0.9 m (3 ft) from the top of the shrub (n = 25; Bombay 1998).
E.t. extimus: On the South Fork Kern River, nest tree canopy height averages 5.3 m (SD = 2.95, maximum height 14 m, n=151) (Whitfield and Enos 1996). On the lower Colorado River, nest canopy height averaged 6.4 m (n=7; McKernan 1997). The highest nests were found on the upper San Luis Rey River, where average nest tree height was 7.3 m, range 2 to 20 m (Griffith Wildlife Biology 1995).
PLANT SPECIES CONCEALING THE NEST
Nests are concealed within the riparian deciduous vegetation within which they are placed.
E.t. brewsteri: Riparian deciduous shrubs, such as willow or alder, are essential elements on Willow Flycatcher territories in the Sierra Nevada and Cascades (Sanders and Flett 1989, Harris et al. 1988). Foliage density in the lower 2 m (6.5 ft) of the riparian deciduous vegetation layer should be greater than 25% and 75% and above is optimal (Fowler et al. 1991). In the Little Truckee River area, density of willow foliage around the nest averaged about 70% (n = 11, SD = 25%, range = 10 - 95%; Sanders and Flett 1989). In mountain meadows, willow thickets interspersed with open space are typically utilized, while large, contiguous willow thickets are avoided (Sanders and Flett 1989, Harris et al. 1988). Other plants found on nesting territories include: blackberry (Rubus), azalea (Rhododendron) (Grinnell and Storer 1924), and alders (KRCD 1985b)
E.t. extimus: On the South Fork Kern River, nesting areas are usually dominated by various species of willows.
Willow Flycatchers generally do not occur in areas with dense tree cover although they will use scattered trees on their territories for singing and foraging perches (Bent 1942, King 1955, Walkinshaw 1966).
E.t. brewsteri: Nests in mountain meadows are generally located in riparian deciduous shrubs with only scattered tall trees (KRCD 1985a,b; Sanders and Flett 1989).
E.t. extimus: This subspecies seems to be an exception to the finding that Willow Flycatchers do not occur in areas of dense tree cover. On the South Fork Kern River, mean canopy cover on plots around the nest was 74.4%, range 22% - 100% (Whitfield and Enos 1996). Although canopy cover can be dense, the flycatcher nests are often in large openings under the willow canopy (Harris 1991).
AVERAGE TOP CANOPY HEIGHT
E.t. brewsteri: In the Sierra Nevada and Cascade mountains of California, Willow Flycatchers nest in riparian deciduous shrub assemblages, usually willows, generally between 1 and 3 m (3.3 to 10 ft) in height (Serena 1982). Thus, average height of canopy trees is not an important factor.
E.t. extimus: On the South Fork Kern River, mean plot canopy height is 4.8 m (SD = 1.98, range 0.5 - 10.3 m, n = 153) (Whitfield and Enos 1996).
DOMINANT PLANT SPECIES IN CANOPY
Riparian deciduous vegetation.
E.t. brewsteri: In the Sierra Nevada, the preferred vegetation is willow, alder, and creek dogwood (Fowler et al. 1991). Other phreatophytic shrubs are infrequently used (Bent 1942, KRCD 1985b).
E.t. extimus: On the South Fork Kern River, nesting sites are dominated by either Goodding's black willow (Salix gooddingii) or red willow (S. laevigata). On the upper San Luis Rey River the dominant canopy species is live oak (Quercus agrifolia).
CO-DOMINANT PLANT SPECIES IN CANOPY
E.t. extimus: At the South Fork Kern River, canopy trees in addition to willows include Fremont cottonwood (Populus fremontii), white alder (Alnus rhombifolia), and ash (Fraxinus). On the upper San Luis Rey River, canopy trees in addition to the dominant live oaks are flowering ash (Fraxinus dipetala), cottonwoods, sycamores (Plantanus racemosa), and white alders (Griffith Wildlife Biology 1995). On the Santa Ynez River, canopy trees in addition to red willow (Salix laevigata) are black cottonwood (Populus trichocarpa) and box elder (Holmgren and Collins 1995:60).
PERCENT NEST COVER
E.t. brewsteri: Foliage around nests in Tahoe National Forest averaged 72% (n=20, SD = 25%, range = 10 - 95%; Sanders and Flett 1989).
E.t. extimus: In the lowland riverine habitat of the South Fork Kern River, tree canopy cover above the nest averaged 93.4% (n = 150; range = 20 - 100%; Whitfield and Enos 1996). Nests there are typically found in areas with multiple layers of vegetation with fairly high tree canopy cover (Whitfield and Enos 1996).
AVERAGE SHRUB COVER
Several authors describe openings as an important component of Willow Flycatcher habitat (Grinnell and Storer 1924, Meanley 1952, King 1955, Walkinshaw 1966).
E.t. brewsteri: Serena (1982) found that Willow Flycatchers in California had a preference for tall clumps of shrubs separated by open areas. In the Little Truckee River area, "willow flycatcher territories included willow clumps interspersed with clearings. The willow cover on 22 territories ranged from 5% - 80% (mean = 44%, SD = 22%)" (Sanders and Flett 1989). The willows ranged in height from 2 - 3 m. The critical factor was the availability of openings around the willow clumps; no territories were located in areas with a solid contiguous mass of willows (Sanders and Flett 1989).
E.t. adastus: In a study in southeastern Washington, Willow Flycatchers occupied open, park-like areas, avoided dense thickets, and were found on the edges of openings and thickets (King 1955).
E.t. extimus - No data
DOMINANT SHRUB SPECIES
E.t. brewsteri: In mountain meadows, willows are the dominant shrub.
E.t. extimus: At the South Fork Kern River, mulefat (Baccharis salicifolia) is the only shrub besides willows. On the upper San Luis Rey River, the dominant shrub species are California Blackberry (Rubus ursinus), and California Rose (Rosa californica) (Griffith Wildlife Biology 1995).
CO-DOMINANT SHRUB SPECIES
E.t. brewsteri: Azalea, dogwood (Cornus), and alder are present at many sites.
E.t. extimus: On the Santa Ynez River, poison oak (Toxicodendron diversiloba) and mulefat are co-dominant shrubs (Holmgren and Collins 1995:60). On the upper San Luis Rey River poison oak and false indigo (Amorpha fructicosa) are common and other shrubs include mulefat, elderberry (Sambucus mexicana), scrub oak (Quercus dumosa), and currant (Ribes sp.) (Griffith Wildlife Biology 1995).
E.t. brewsteri: The ground cover in Sierra Nevadan meadows occupied by Willow Flycatchers is dominated by grasses, rushes, and sedges (Sanders and Flett 1989). In the Shaver Lake area, vegetation density was relatively dense between 0.0 and 0.5 m (80% of the points had cover) due to grasses, forbs, and willow stems (KRCD 1985b). Duff from the previous season’s growth must be available for nest material.
E.t. extimus: On the South Fork Kern River, the average ground cover is 33.5% (SD = 27.25, n = 153), with a range of 0% - 99% (Whitfield and Enos 1996). Ground cover is often sparse in Willow Flycatcher territories because of the density of the canopy (mean = 93.35%, S.D. = 1.04, range = 20% - 100%, n = 150), as well as the presence of surface water (Whitfield and Enos 1996).
Nesting sites in California are usually near languid streams, standing water, or seeps (Zeiner et al. 1990). However, nests may be placed distant from water (e.g. where river channels or subsurface flows have been modified), as long as the site continues to support riparian vegetation (Sogge et al. 1997a). An example is a nest on the South Fork Kern River that was 250 m (820 ft) from the nearest surface water (Whitfield et al. 1997).
E.t. brewsteri In California, water is always present on Willow Flycatcher territories, in the form of running water, standing water (pools), or saturated soils during the early stages of the breeding season (Harris et al. 1988, Sanders and Flett 1989). Sites with singing Willow Flycatchers were covered by a significantly higher percent of water than sites where no flycatchers were detected (Harris et al. 1988).
E.t. extimus The Southwestern Willow Flycatcher breeds only near surface water or saturated soil, at least during the early stages of pair formation and nest building (Sogge et al. 1997a; Whitfield et al. 1997). Water may dry up and is not necessarily present at the later stages of the breeding cycle.
Little information available.
E.t. extimus: Four of 14 nests studied on the upper San Luis Rey River were high in live oaks with no tall shrubs or herbaceous vegetation under them, only leaf litter (Griffith Wildlife Biology 1995). Leaf litter was not measured at nests on the South Fork Kern, but the prevalence of flooding in the area would likely carry leaves away (see discussion under bare ground).
E.t. extimus: At the South Fork Kern River, Willow Flycatcher territories averaged 66.5% bare ground or water and only 33.5% ground cover (Whitfield and Enos 1996). L. Slope: No information available, but usually in flat or gently sloping areas.
DISTANCE TO WATER
E.t. brewsteri: Water is always present on Sierra Nevadan Willow Flycatcher territories, in the form of running water, standing water (pools), or saturated soils (Harris et al. 1988, Sanders and Flett 1989).
E.t. extimus: Water is almost always present on Southwestern Willow Flycatcher territories, at least at the beginning of the breeding season (Sogge et al. 1997a). At the South Fork Kern River, distance from nest to nearest water averages 21.2 m (SD = 4.4, range = 0-250 m, n = 140 nests; Whitfield et al. 1997). Almost half (46%) of the nests were above water at the time they were built or shortly before they were built (Whitfield et al. 1997).
An open-cup nest is usually placed in a fork of a branch with several small diameter stems supporting the nest. Nests are typically made of coarse and fine plant fibers, fine grasses, and downy material from cattails, cottonwood and willow, and feathers (McCabe 1991). Fibers from dried milkweeds and nettles are commonly used (McCabe 1991, M. Whitfield, pers. comm.). Often there is a loose streamer of material dangling from the bottom (Sumner and Dixon 1953; Harrison 1975, Sanders and Flett 1989).
Adults frequently dismantle a failed nest and move it to a new location (McCabe 1991), suggesting that nesting material supply or the energetics of gathering nest material is a limiting resource.
E.t. brewsteri: Nests are made of grass and sedges. In mountain meadows, duff from the previous growth season must be available when the flycatchers construct their nest (B. Valentine, pers. comm.). Observations of the dismantling of old nests for the construction of new nests were made by Stafford and Valentine (1985).
E.t. extimus: Nests on the South Fork Kern River are often made of stinging nettle fiber (M. Whitfield, pers. comm.), as well as thistle fibers, strands of dried willow bark, grasses, animal hair, and the fluffy "cotton" borne by both willow and cottonwood seeds (Harris 1991). Nests on the Colorado River in the Grand Canyon are made of tamarisk leaves (Sogge et al. 1997b).
E.t. brewsteri: In the Sierra Nevada, sally distances were usually less than 1 m (3.3 ft), but occasionally were as far as 10 m (33 ft), from exposed perches (Sanders and Flett 1989). Males hawked mostly from relatively high perches (greater than 3 m), while females foraged from perches in the lower willow branches, remaining less conspicuous (Sanders and Flett 1989). Individuals were observed to shift hawking perches every few minutes and sometimes use perches outside their territories (Sanders and Flett 1989).
E.t. extimus: At the South Fork Kern River, Southwestern Willow Flycatchers primarily foraged in the middle third of the tree-like willows and tended to glean in canopy spaces within the willows (J. Harris, pers. comm.).
TYPICAL BREEDING DENSITIES
E.t. brewsteri: In the Sierra Nevada, densities at two study sites were 5 pairs per 40 ha (100 acres) and 4 pairs per 40 ha (100 acres) (Sanders and Flett 1989). However, because of the selection of specific microsites within meadows (e.g., wetter areas), breeding density is difficult to calculate for this species. In the Shaver Lake area at Long and Dinkey meadows, the density of Willow Flycatchers in 1984 averaged 0.24 territories per ha or 9.6 pairs per 40 ha. (KRCD 1985b). Single pairs of Willow Flycatchers are known to breed in the absence of other individuals.
E.t. adastus: In 1953, at the Palouse Hills in southeastern Washington, King (1955) observed a density of 14 pairs breeding pairs per 40 ha (100 acres).
E.t. extimus: At the Kern River Preserve, from 1994 to 1997, using all riparian habitat in the study area, average breeding densities were 3 pairs per 40 ha (100 acres). When using only the areas of habitat patches where nests were clustered, the average breeding density was 15 pairs per 40 ha (Whitfield and Placer 1994, Whitfield and Strong 1995, Whitfield and Enos 1996, Whitfield et al. 1997). However, the location of breeding clusters is known to shift among years, and not all sites are used each year (M. Whitfield, pers. Comm.).
INITIATION OF NESTING
E.t. brewsteri: On the Little Truckee River (Perazzo Meadow and Lacey Valley), in 1986 and 1987, the first eggs were laid in the third week in June and the first fledglings appeared in mid-July (Sanders and Flett 1989). The last young fledged on 14 August in 1986 and the last fledgling was seen on 4 August in 1987 (Ibid). In 1997, of 25 nests monitored on the Tahoe, Toiyabe, and Plumas National Forests, the first nests fledged around 21-22 July and the last fledged around 13-14 August (Bombay 1998). The latter nests were the result of renesting after the first nests were lost to depredation (Ibid).
E.t. extimus:Nest building usually begins within a week of pair formation. Egg laying can begin as early as late May, but more often begins in mid-June. Chicks are present in nests from mid-June through early August (Sogge et al. 1997a). On the South Fork Kern River, the breeding season usually begins the last week in May extending into late July, and rarely, into mid-August (Whitfield et al. 1997).
Males will use the highest, most exposed perches, such as branches on the tops of willow shrubs or on trees and snags, as singing perches. The distinctive male territorial song is a "fitz-bew." Female Willow Flycatchers are also known to sing, and will do so loudly and repeatedly (Seutin 1987, Sogge et al. 1997b). The songs of females are identical to those of males (Seutin 1987), thus making estimation of population size by monitoring singing birds much less certain, since each singing bird may not represent a separate territory. Also, migrant individuals may also sing, sometimes responding vigorously to tapes (Sogge et al. 1997b).
Generally monogamous, although a few cases of polygyny have been reported (Prescott 1986; Sedgwick and Knopf 1989; Sogge et al. 1997b and see below for references for California).
E.t. brewsteri: A case of polygyny at Dinkey Meadow was reported in 1986 (Valentine et al. 1988) and one case was reported at Perazzo Meadows in 1993 (Powers 1993).
E.t. extimus: Generally thought to be monogamous, but a high proportion of polygyny (10-40%) observed at the South Fork Kern River (M. Whitfield, pers. comm.). In 1996, four males were confirmed and three others were suspected to be polygynous (Whitfield and Enos 1996), while in 1997, 13 of 35 to 37 males were polygynous, and only seven females were mated monogamously (Whitfield et al. 1997).
Full clutches range from two to four eggs, with three to four being most typical (Bent 1942). There is a record of a five egg clutch from southern Michigan (Walkinshaw 1966) and from the South Fork Kern River (M. Whitfield, unpubl. data).
E.t. brewsteri: Clutches of two to four eggs were found in the Little Truckee River area, but of 18 full clutches, only one had two eggs (Sanders and Flett 1989).
E.t. extimus: Full clutches ranged from two to four (n=44) in 1996 and 1997 on the South Fork Kern River. One clutch had two eggs (2.3%), 23 clutches had three eggs (52.3%), and 20 had four eggs (45.5%), for an average clutch size of 3.4 eggs (Whitfield and Enos 1996; Whitfield et al. 1997). Based on dated clutches in museum collections from coastal Southern California, 60% had four eggs and 40% had three eggs (Unitt 1987). Clutches from the Colorado River were smaller; 18% had two eggs, 82% had three eggs, and none had four eggs (n = 28 unparasitized clutches; Unitt 1987).
Mainly females (Ettinger and King 1980), but males have been recorded incubating in Arizona (M. Whitfield, pers. comm.).
Approximately 12 days after the last egg is laid (King 1955). Range of incubation periods for eight nests in Michigan was 13-15 days (Walkinshaw 1966). E.t. extimus: Incubation lasts 12 - 13 days from the date the last egg is laid, and all eggs typically hatch within 24 - 48 hours of each other (Sogge et al. 1997a).
DEVELOPMENT AT HATCHING
Young are altricial. King (1955) gives a detailed description of growth and development and says "the sequence of development does not differ markedly from that outlined by Nice (1943) for open-nesting passerines in general."
Nestling period is 12 to 15 days (Sogge et al. 1997a, King 1955). Chicks can be present in the nest from mid-June through early August, and young fledge typically from late June through mid-August (Sogge et al. 1997a).
Nestlings are brooded by the female only and are fed by both male and female, although the female performs more than half the feedings (Ettinger and King 1980).
POST FLEDGING BIOLOGY OF OFFSPRING
In Michigan, young remained on their parent's territory for 10-11 days after fledging (Walkinshaw 1966). E.t. extimus: Fledglings stay close to the nest and to each other for 3 - 5 days after leaving the nest, and fledglings stay in the area for a minimum of 14 - 15 days (Sogge et al. 1997a).
Little information available, but probably breed as SY (second year) birds. On the South Fork Kern many birds banded as nestlings have returned to breed in their first year as adults (M. Whitfield, unpubl. data).
NUMBER OF BROODS
One brood, with no renesting after a brood is reared, was recorded by Walkinshaw (1966) in Michigan and by McCabe (1991) in Wisconsin.
E.t. brewsteri: No pairs have been recorded as attempting another nest after successfully rearing a brood (Stafford and Valentine 1985, Sanders and Flett 1987, Valentine et al. 1988, Flett and Sanders 1989, Bombay 1998). Renesting attempts after nest failures have been observed in the central Sierra Nevada (Stafford and Valentine 1985). Two males that lost nests attempted renests, one with a new female and the second with the same female. The brevity of the breeding season in the Sierra Nevada probably limits the potential for renesting to only those nests lost early in the breeding season.
E.t. extimus: On the South Fork Kern River in the late 1980s, a probable case of double brooding was documented when the first nest of the pair fledged only one young (Whitfield 1990). In subsequent years of study on the Kern River, from 5 to 10% of successful pairs have been found to double brood (M. Whitfield, unpubl. data). In Arizona and New Mexico there have been suspected cases of double brooding (Sferra et al. 1997).
Willow Flycatchers are frequently parasitized by Brown-headed Cowbirds, especially in the lowland portions of their range (Grinnell and Miller 1944, Friedman 1963). Friedmann (1963) reported 150 instances of Brown-headed Cowbird parasitism of Willow Flycatchers; 41 of these were reports from southern California.
E.t. brewsteri: Nesting failure due to cowbirds appears to be less in Sierra Nevadan montane meadows than in lowland areas (Gaines 1977, Stafford and Valentine 1985, Valentine et al. 1988, Sanders and Flett 1989). At least four instances of cowbird parasitism are documented in the literature for the northern Sierra Nevada: Lake Tahoe area (Gaines 1977), Lacey/Perazzo Meadow area (Sanders and Flett 1989), and in Helen Bombay’s study to the north and south of Lake Tahoe (Bombay 1998). In 1997, two of 25 nests were parasitized, then subsequently depredated (Bombay 1998). Cowbirds in montane Sierra Nevadan meadows nearly complete their breeding season before Willow Flycatchers start nesting, which may explain this apparent low rate of nest parasitism (Verner and Ritter 1983, Stafford and Valentine 1985).
E.t. extimus: Intense parasitism was reported in 1987 on the Kern River Preserve, with 13 to 16 out of 19 nests parasitized by cowbirds (Harris 1991). At the South Fork Kern River, an average of 63.5% of nests were parasitized from 1989 to 1992 with a range from 50% in 1989 to 80% in 1991 (Whitfield et al. 1997).
Cowbird trapping has lowered the parasitism by Brown-headed Cowbirds to an average of 20.8% from 1993 to 1997 (range 38% in 1993 to 11% in 1996), stabilizing breeding populations on the South Fork Kern River (Whitfield et al. 1997). Although cowbird trapping has significantly increased Willow Flycatcher reproductive success at the South Fork Kern, predation has been high enough to prevent an increase in the number of breeding pairs thus far (Whitfield et al. 1997). Populations at Camp Pendleton and the upper San Luis Rey River have increased due to cowbird trapping to protect Least Bell's Vireos (Griffith Wildlife Biology 1995).
Willow Flycatchers are known to breed at elevations from near sea level to 2440 m (8,000 feet) (Grinnell and Miller 1944, Zeiner et al. 1990). Historically the low elevations of the San Joaquin and Sacramento valleys were probably the prime habitat.
Much of the riparian deciduous shrub communities that historically provided habitat for Willow Flycatchers have all but disappeared in California, especially in the Central Valley and coastal southern California. Existing Willow Flycatcher habitat is widely dispersed, mostly at small mountain meadows in the Sierra Nevada. In addition, for E.t. brewsteri much of the remaining habitat in California exists at the geographic and altitudinal extremes of the Willow Flycatcher’s range, where late spring storms, isolation, or other unknown factors reduce the likelihood of successful breeding.
Willow patch dynamics in regard to Willow Flycatcher habitat suitability is still unknown. Various authors describe openings as an important component of Willow Flycatcher nesting habitat (Grinnell and Storer 1924, Meanley 1952, King 1955, Walkinshaw 1966). In Sierra Nevadan montane meadows, willow thickets interspersed with open spaces are typically used for nesting, while large, contiguous willow thickets are avoided (Harris et al., 1988). Bombay (1998) found that 25 nests in the Tahoe, Toiyabe, and Plumas National Forests were an average of 1.3 m (SD = 0.5 m) from the closest shrub canopy opening. Sanders and Flett (1989) felt that openings within willow patches appear to increase habitat suitability. However, in a survey of 125 sites in the Sierra Nevada/Cascade mountain ranges, it was not possible to predict presence or absence of Willow Flycatchers by willow clump sizes (Harris et al. 1988). Nonetheless, some openness in the shrub stratum seems important.
E.t. brewsteri: In California, the smallest area where E.t. brewsteri have been documented nesting is the 0.25 ha Poison Meadow in the south central Sierra Nevada (KRCD 1985a). Fowler et al. (1991) felt that studies on the Little Truckee River and surveys across California suggest this size is an absolute minimum. In two statewide surveys, meadows 8 ha and larger made up the majority of the meadows in which Willow Flycatchers were detected: in 1981, 20 out of 24 meadows with Willow Flycatchers were greater than 8 ha in size (Serena 1982) and, in 1986, 104 out of 111 singing male Willow Flycatchers were detected in meadows greater than 8 ha in size and none in meadows less than 4 ha in size (Harris et al. 1988). In 1997, 25 nests in the Tahoe, Toiyabe, and Plumas National Forests were located in patches that averaged 107 m2 (SD = 91 m2) in size (Bombay 1998).
E.t. extimus: Southwestern Willow Flycatchers have nested in patches of riparian habitat as small as 0.6 ha (e.g. Grand Canyon) (Sogge et al. 1995:14) and as large as several hundred hectares (e.g. Roosevelt Lake and Lake Mead, Arizona) (Sogge et al. 1997a). There are no records from narrow, linear riparian habitats less than 10 m (33 ft) wide, although Willow Flycatchers will use such linear habitats during migration (Sogge et al. 1997a). At the South Fork Kern River, the total riparian forest is about 1,130 ha (2,800 acres), but the four major breeding sites range from 7 to 24 ha (17 to 60 acres) in extent (S. Laymon, pers. comm.)
Willow Flycatchers in the Sierra Nevada have been observed nesting near trails created or maintained by livestock. This placement near the edges of willow clumps makes the nests susceptible to being knocked over by cattle (Stafford and Valentine 1985, Flett and Sanders 1987, Valentine 1987, Valentine et al. 1988, Sanders and Flett 1989).
Livestock grazing can also indirectly affect Willow Flycatcher habitat by altering the vegetation and hydrology. Livestock can eat the lower branches of riparian deciduous shrubs and consume or trample young riparian plants (Taylor 1986). A decrease in foliar density within the lower 1.5 m (5 ft.) of riparian deciduous vegetation, where most Willow Flycatcher nests occur, is of particular concern (Fowler et al. 1991). In Utah, Duff (1979) found that livestock exclusion resulted in an increase in the portion of willow plants favored by Willow Flycatchers for nesting. In Oregon, Taylor and Littlefield (1986) found that the increase in a Willow Flycatcher population coincided with a dramatic decrease in the number of cattle using the area and the elimination of willow cutting and spraying. In another area, these authors found a negative statistical correlation between frequency of cattle grazing on an annual basis and the numbers of Willow Flycatchers (Ibid). Most of the studied areas had undergone prolonged (up to 50 years), intensive annual grazing by livestock, as well as cutting and spraying of willows.
Heavy recreational use of Willow Flycatcher habitat, especially uses such as off-road vehicles that can compact soils, can negatively affect the quality of Willow Flycatcher habitat and potentially cause direct disturbance to nesting birds.
ADJACENT LAND USE
Land uses adjacent to Willow Flycatcher habitat that can change the hydrology of the area can have indirect negative effects on Willow Flycatcher habitat. Such uses include timber harvest and associated ground-disturbing activities and ground water extraction. Water impoundments can remove upstream Willow Flycatcher habitat and negatively affect the hydrology of downstream habitat. Brown-headed cowbirds are associated with pack stations and groups of livestock (Verner and Ritter 1983, Stafford and Valentine 1985), and these land uses on lands adjacent to Willow Flycatcher nests can cause an increase in the number of brown-headed cowbirds present to potentially parasitize Willow Flycatcher nests. On the east side of the Sierra Nevada, cowbirds can travel up to 6.7 km from feeding sites to parasitize the nests of hosts (Rothstein et al. 1984).
No specific information found. However, any activity that reduces insect abundance, particularly of Hymenopterans and Dipterans, would negatively affect Willow Flycatchers.
The non-viability of eggs was high in the Dinkey Meadow area (Valentine et al. 1988). Analysis of the combined contents of three of the eggs by the Long Marine Laboratory yielded 0.5 ppm of p,p'-DDE on a wet weight basis (B. Valentine, pers. comm.). Because Willow Flycatchers spend almost 75% of their time outside the United States where pesticide use is regulated differently, productivity of the nests may be affected by out-of-country activities. The incidence of non-viability needs to be followed to determine if the level noted by Valentine et al. (1988) was an exception or a real issue.
There is little information on specific predators in the literature. Cases of documented predators at nests include a milksnake (Lampropeltis triangulum) (McCabe 1991), two common kingsnakes (Lampropeltis getulus) and a Cooper's Hawk (Accipiter cooperii) (Paxton et al. 1997, McCarthey et al. 1997). A long-tailed weasel (Mustela frenata) was seen in the vicinity of a newly depredated nest in the Dinkey Meadow area (Stafford and Valentine 1985). There is evidence that House Wrens may destroy eggs; Stafford and Valentine (1985) found an egg remaining in a nest after the young had fledged that had two punctures in it, suggesting the egg-pecking behavior of house wrens (Belles-Isles and Picman 1986).
E.t. extimus: At the South Fork Kern River from 1989 to 1997, 36.3% of nests were lost to predators, ranging from a low of 14% in 1992 to a high of 57% in 1997 (Whitfield et al. 1997)
DEMOGRAPHY AND POPULATION TRENDS
In Michigan, Walkinshaw (1966) found a return rate of 22.6% of females (n = 31) in the first year after they were banded, 40.9% for males (n = 22) in the first year after they were banded, and 1.4% for young (n=147). Only one female returned for a third year. For males, 22.7% (n = 5) returned for a third year; 13.6% (n = 3) returned the fourth year; and 4.5% (n = 1) returned the fifth year (Walkinshaw 1966).
E.t. brewsteri: In 1983 and 1984 on Dinkey and Long meadows, reproductive success for eight nests is as follows: "Total of at least 24 eggs resulted in at least 18 nestlings (75% egg to nestling success), of 18 nestlings 6 are known to have fledged (33% nestling to fledgling success), another 3 probably fledged (raising to 50% success). These equate to an egg to fledgling value between 25% and 38%. Over this period three of the eight nests are likely to have fledged young (38%)" (KRCD 1985b). In 1997 in the Tahoe, Toiyabe, and Plumas National Forests, 24 nests produced 75 eggs, with 28 confirmed fledglings (Bombay 1998).
There is complete egg to fledging data for 5 of 11 nests at Perazzo Meadows and Lacey Valley in 1986; 14 eggs produced 5 fledglings (36%, Sanders and Flett 1989). For 1987 of 32 eggs (laid by 10 pairs) only six survived to fledging (19%, Ibid.). These rates are significantly lower than the 46% egg to fledging rate for open-cup nesters reported by Nice (1957).
The net reproductive rate (the average number of female young produced by each female during her lifetime) based on data from Sanders and Flett (1989) and KRCD (1985b) is 0.5, indicating a declining population (Sanders and Flett 1989). (A rate of 1.0 indicates a stable population).
E.t. extimus: In 1994, on the upper San Luis Rey River, 7 of 11 nests of known fate succeeded in fledging young (64% nest success), and 4 nests were taken by predators (Griffith Wildlife Biology 1995). In 1996 and 1997, at the South Fork Kern River of 58 pairs observed, 24 pairs (41%) were successful on their first attempt and did not initiate a second brood (calculated from data in Whitfield and Enos 1996, Whitfield et al. 1997). Of the 34 pairs that failed, 27 (79%) renested, and 10 (37%) of these pairs were successful on their second try (Ibid.). Of those 17 pairs not successful on their second attempt, only 2 (11.8%) attempted a third brood, but neither were successful (Ibid.). Therefore, of the 58 pairs studied in 1996 and 1997, 34 pairs successfully fledged young for a 59% pair success rate (Ibid.).
After Brown-headed Cowbird trapping was initiated (years 1994-1997), the egg to fledgling ratio on the South Fork Kern was 40% (168 fledglings from 420 eggs). The average number of young fledged per female per year is 1.77 (low of 1.03 in 1997 and high of 2.54 in 1996); age at first breeding is one year; based on returns of banded birds, 34.2% of females, 40.7% of males, and 34% of banded nestlings return in at least one year to the study area; and the oldest bird detected to date is a six year old male with 4 birds (3 females and 1 male) documented as five years old (Whitfield and Placer 1994, Whitfield and Strong 1995, Whitfield and Enos 1996, Whitfield et al. 1997, Whitfield unpubl. data).
There are insufficient data to determine trends in California of any subspecies of Willow Flycatchers or the species as a whole using Breeding Bird Survey (BBS) data (p = 0.24, n = 14 routes between 1966 and 1996). Throughout North America, Willow Flycatchers have shown a significant 1.2% per year decline from 1966 to 1996 (p = 0.01, n = 1053 routes) (Sauer et al. 1997). During the same time period, the Western BBS Region has experienced a 2.3% per year decline (p<0.01, n = 311 routes). Oregon and Washington, where both E.t. brewsteri and E.t. adastus occur, have both experienced significant declines of 5.8% per year (p = 0.01, n = 61 routes) and 2.5% per year (p = 0.01, n = 57 routes), respectively from 1966 to 1996 (Sauer et al. 1997).
In California, Willow Flycatchers have shown both historic and recent population declines. The primary cause of these declines is probably the loss and degradation of riparian habitats (Gaines 1977, Remsen 1978, Garrett and Dunn 1981, Serena 1982, Stafford and Valentine 1985, Taylor 1986, Taylor and Littlefield 1986, Flett and Sanders 1987, Unitt 1987, Harris et al. 1988, Valentine et al. 1988). Other factors include: Brown-headed Cowbird nest parasitism (especially for E.t. extimus, Harris 1991, Whitfield et al. 1997); grazing disturbance (Valentine et al. 1988); loss of meadow habitat due to reservoir and hydroelectric developments (Serena 1982); loss of willow habitat due to bulldozing, chaining, or intentionally set fires (Serena 1982); lodgepole pine (Pinus contorta) encroachment of meadows (Serena 1982); and habitat loss on the wintering grounds.
E.t. brewsteri: A total of 121 singing males were located in the Sierra Nevada/Cascade mountains of California in 1982 (Serena 1982), and 116 singing males were located statewide in 1986 (Harris et al. 1988). In 1992, the California Department Fish and Game estimated the state population, all subspecies, to be 200 pairs, with local population sizes ranging from about 6 pairs to about 44 pairs (CDFG 1993). A survey of 22 sites in the Sierra Nevada in 1995 detected 44 Willow Flycatchers (26 singing males and 18 non-singing birds) (J. Harris, pers. comm.) The largest population detected in 1995 was at the Little Truckee River (11 singing males, 8 non-singing) (J. Harris, pers. comm.) In 1997, a Sierra Nevadan-wide survey of 145 sites detected 33 singing males at ten sites (M. Flores, pers. comm.). Only three of the sites (Perazzo Meadow in Sierra County, Little Truckee in Sierra County, and Red Lake in Alpine County) had more than one singing male (Bombay 1998).
In the Sierra Nevada, the species has declined (Harris 1987), becoming alarmingly scarce in the Yosemite region (Gaines 1977, 1988). The consensus is that the population in the Sierra Nevada has declined significantly during the past ten years, especially populations on the west slope of the Sierra Nevada (Willow Flycatcher Working Group meeting, January, 1998). However, the discovery of two new populations in 1997 in the Cascades (see current breeding distribution) is encouraging. The densities of willow flycatchers in 1997 at Perazzo Meadow and Lacey Valley seemed about the same as those in 1986 and 1987 (Sanders 1998).
E.t. extimus: In the last four decades, Willow Flycatchers have been eliminated from most lower elevation habitats in the state (Unitt 1987). Recent declining population trends are illustrated by declines from 1989 to 1992 at the South Fork Kern River (Whitfield et al. 1997). However, cowbird control programs are credited with stabilizing or increasing Willow Flycatcher populations at the South Fork Kern River, Kern County, (Whitfield et al. 1997) and Camp Pendleton and the upper San Luis Rey River, San Diego County (Griffith Wildlife Biology 1995).
MANAGEMENT ISSUES AND OPTIONS
(1) Nest disturbance by cattle. OPTIONS: (a) eliminate livestock use during the breeding season. At the South Fork Kern River this would be from 15 May to 20 August (M. Whitfield, pers. comm.); in the Shaver Lake area Valentine et al. (1988) suggest 1 July to 20 August; in the central Sierra Nevada, Harris et al. (1988) suggest 1 June to 15 August in areas where Willow Flycatchers breed. However, these dates will not prevent damage to the understory, the preferred nesting sites, before the Willow Flycatchers chose nest sites. (b) keep livestock from coming in physical contact with nest sites during this period (e.g., by use of electric fences where research is ongoing; Valentine et al. 1988).
(2) Livestock grazing impacts on riparian deciduous vegetation. OPTIONS: (a) manage grazing intensity or location to ensure riparian deciduous shrubs are not high-lined (i.e., the foliar density in the lower portions of the shrubs is maintained) and that recruitment of young riparian deciduous shrubs occurs.
(3) Livestock impacts on hydrology. OPTIONS: (a) protect areas where grazing may be drying meadows by soil compaction and gullying; (b) implement grazing management standards that, if met, will maintain proper hydrologic function.
(4) Upslope activities (e.g., timber harvest, road-building) impacting hydrology. OPTIONS: (a) manage upslope activities so that hydrologic function in nesting areas is maintained.
(5) Recreational activities in meadows impacting hydrology. OPTIONS: (a) implement use standards that, if met, will maintain hydrologic function.
(6) Historic loss or alteration of high quality habitat (riparian deciduous vegetation, loss of hydrologic function). Options: restoration.
(7) Brown-headed cowbird nest parasitism. OPTIONS: (a) manage livestock so that aggregations of livestock do not occur near Willow Flycatcher nest sites; (b) no new construction of facilities such as pack stations, corrals, and salting facilities, which concentrate livestock, within 3-6 miles of areas managed for nesting Willow Flycatcher (Fowler et al. 1991, Verner and Ritter 1983). If these distances are not attainable or if there are landscape features or habitat that concentrate livestock, livestock use of aggregation areas should not occur during the breeding season (USFS 1993); (c) implement a cowbird trapping program in selected areas with both high Willow Flycatcher densities and high cowbird nest parasitism.
ASSOCIATED BIRD SPECIES
Other species nesting in mountain meadows: (1) Shaver Lake area: Orange-crowned, Yellow-rumped, Nashville, Yellow, MacGillivrays's and Wilson's warblers; Song and Lincoln sparrows, Warbling Vireos, and Dark-eyed Juncos (KRCD 1985b).
Other species nesting at low elevations: (1) South Fork Kern River, Kern County - Warbling Vireo, Yellow Warbler, Yellow-breasted Chat, Common Yellowthroat, Lazuli Bunting, Song Sparrow (P. Williams and S. Laymon, pers. obs.); (2) Upper San Luis Rey River, San Diego County - Hutton's Vireo and any other vireo species that may breed there, Yellow Warbler, Common Yellowthroat, Lazuli Bunting, Song Sparrow (Griffith Wildlife Biology 1995); (3) Camp Pendleton, San Diego County - California Gnatcatcher, Swainson's Thrush, Least Bell's, Hutton's and Warbling vireos, Wilson's and Yellow warblers, Yellow-breasted Chat, Blue Grosbeak, and Lazuli Bunting (Griffith and Griffith 1997); (4) Vandenburg Air Force Base, Santa Barbara County - Blue-gray Gnatcatcher, Warbling Vireo, Yellow Warbler, Orange-crowned Warbler, Common Yellowthroat, Wilson's Warbler, Yellow-breasted Chat, Song Sparrow, White-crowned Sparrow (Holmgren and Collins 1995).
MONITORING METHODS AND RESEARCH NEEDS
Although Willow Flycatchers are relatively easy to detect, they are at such low numbers that they can not be adequately monitored solely by multi-species monitoring techniques (e.g., Breeding Bird Survey, Monitoring Avian Productivity and Survival sites). A species-specific monitoring method should be used. An updated survey protocol for E.t. extimus has been recently published (Sogge et al. 1997a, modifying Tibbitts et al. 1994) and a protocol for National Forest Service lands in the Pacific Southwest Region is in preparation. "Please note that federal endangered species permits are required for surveys in all USFWS regions where the southwestern willow flycatcher breeds" (Sogge et al. 1997a). The monitoring strategy should include both wide-scale, periodic species-specific surveys and site-specific monitoring of the demography at the primary population centers.
Research needs include:
(1) Work to clarify subspecies status in northern California and east of the Sierra Nevadan crest. This should include work to identify subspecies genetically. For the purposes of this paper Willow Flycatchers at high elevation on the east side of the Sierra Nevada (Perazzo Meadow, Little Truckee River, and Red Lake) are assumed to be E.t. brewsteri, because of their close proximity to other E.t. brewsteri on the west slope, but this assumption must be tested. These sites support the largest numbers of nesting Willow Flycatchers in the central Sierra Nevada.
(2) A statewide survey for Willow Flycatchers, including all patches of suitable habitat or potentially suitable breeding sites for all subspecies. Most surveys conducted to date include only a portion of their range in the state. A model from statewide surveys conducted in Arizona can be found in McCarthy et al. 1998 and Sferra et al. 1997.
(3) Studies of reproductive success, site fidelity, survivorship, and vegetation structure of selected populations to determine net reproductive rate and contributing factors. These studies will require banding of willow flycatchers. Identification of environmental factors influencing these populations and identification of source/sink areas can be determined from these data.
(4) Comparisons of net reproductive rate and vegetation structure in grazed and ungrazed areas. While curtailment of grazing in riparian zones with nesting Willow Flycatchers from June through mid-August should occur on all sites to prevent physical disturbance to Willow Flycatcher nests, some areas should be protected year round from grazing to maintain high foliage density, eliminate browse lines, and prevent soil compaction and gullying.
(5) Monitor Willow Flycatcher population responses to meadow revegetation and restoration. Monitoring should include other riparian meadow nesting species in addition to Willow Flycatchers.
(6) Study the importance of understory vegetation, particularly grasses and forbs, to Willow Flycatcher nest success. In one year of study on the Tahoe and Toiyabe National Forests, Bombay (1998) found lower nest success at a site with cattle present during the nesting season versus ungrazed sites. This lower success rate was not due to direct nest disturbance. The cause of this lower success rate is unknown, but herbaceous vegetation around some nest sites was reduced to less than two inches and it is known that Willow Flycatchers commonly use fibers from dead standing plants to build their nests.
(7) Study the effects of human and livestock presence around Willow Flycatcher nests on the behavior and nesting success of Willow Flycatchers. In the Tahoe and Toiyabe National Forests, Bombay (1998) observed that "willow flycatchers are easily agitated when humans are in the vicinity (10-20 m) of an active nest. When agitated, they tend to become very vocal, and this in turn could attract nest predators." This could have been a factor in the lower nest success rate observed in an area with cattle present during the nesting season (see (5) above).
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