Prepared by: Christopher D. Otahal, USDA Forest Service, 22830 Foresthill Road, Foresthill, CA 95631(530) 367-2224.
SPECIES: MacGillivray’s Warbler (Oporornis tolmiei)
The American Ornithologists’ Union checklist (Am. Ornithol. Union 1957) currently recognizes only two subspecies of MacGillivray’s Warbler: O. t. tolmiei and O. t. monticola. However Phillips (1947) originally described 4 subspecies breeding in North America: O. t. tolmiei, O. t. monticola, O. t. austinsmithi, and O. t. intermedia. Subspecies status needs further investigation (Pitocchelli 1995). According to the AOU checklist, the ranges of the two recognized subspecies are as follows:
O. t. tolmiei – breeds from southern and central California east to western South Dakota and north to southeast Alaska, British Columbia, and southern Yukon.
O. t. monticola – breeds from parts of southern Oregon, Idaho, and Wyoming to Arizona, New Mexico, and northeast Mexico.
This is a fairly common species in appropriate habitat. This species is not on federal or state threatened, endangered, or species of concern lists (USFS 1994, NatureServe 2000).
RANGE IN CALIFORNIA
1. Historical References
This species appears to have increased and expanded in the northern parts of its range where timber harvest practices have created extensive brushy second-growth stands, however, it may be of local concern in riparian habitats where willow communities and shrubby understories are degraded or eliminated, particularly in the more arid areas of its range (NatureServe 2000). MacGillivray’s Warbler populations have expanded since the late 1880’s in northwestern California (Raphael et al. 1988). Populations have increased in the mountains along the coast of California, apparently in response to regeneration after logging (Shuford 1993). This species is expanding its range in parts of Southern California – San Gabriel and San Bernardino Mountains (Pitocchelli 1995).
2. Current Breeding Distribution
MacGillivray's warblers breed throughout the mountains of western North America from southern Alaska and southwestern Yukon east to southwestern Saskatchewan and the Rocky Mountains south to southern California and central New Mexico (Peterson 1961, Farrand 1983). About one sixth of the total breeding distribution of MacGillivray's warblers is in California (USFS 1994) but given the great geographic variation in densities reported in Zeiner et al. (1990) it is not known what proportion of the population is found in California. In California, MacGillivray's Warblers breed in interior mountains up to about 9000 feet in the south (Garret and Dunn 1981) and to 8000 feet in the north (Gaines 1977). They are fairly common summer residents and confirmed breeders on both the east and west slopes of the Sierra Nevada (Siegel and DeSante 1999). They also breed in the coast range from Monterey County north and locally in the mountains of southern California south of the Sierra Nevada (Zeiner 1990). Breeding populations may also be present in the eastern Mohave Desert region (Garrett and Dunn 1981, Small 1994). Much of the suitable habitat for MacGillivray's warblers is found on National Forest System lands (USFS 1994). The California Wildlife Habitat Relationships database lists them as spring and summer residents of the following National Forests: Eldorado, Inyo, Klamath, Lassen, Los Padres (northern), Mendocino, Modoc, San Bernardino, Sierra, Sequoia, Six Rivers, Shasta-Trinity and Tahoe National Forests (Timossi 1990).
1. Average Territory Size
There has been a wide variety of territory sizes and breeding population densities reported and this aspect of MacGillivray's Warbler biology deserves further study (Pitocchelli 1995). Much of the variation reported seems to be related to habitat characteristics – especially extent of ground cover and low shrubs. Average territory size varies from 2.00 acres in Utah (Blakesely and Reese 1988) to 4.25 acres in Oregon (Morrison 1981, Morrison and Meslow 1983). In Idaho Johnston (1949) reported 2.5 pairs per 100 acres in a Douglas-fir clearcut with no groundcover, and 10 pairs per 100 acres in a selectively logged Douglas-fir area with dense ground cover. Density in Wyoming was 10 per 100 acres in a willow-sedge swamp, 30 per 100 acres in a flatland aspen stand, and 85 per 100 acres in a scrub-meadow (Salt 1957). Territory sizes and population densities were not reported for California.
2. Time of Occurrence and Seasonal Movements
Little is known about the nature of migration because of the species preference for dense undergrowth and its elusive and shy behavior (Pitocchelli 1995). In migration, this species is secretive and difficult to detect except for loud, harsh call notes (Rosenberg et al. 1991). During migration, MacGillivray's Warblers are common in dense shrubs or well-shaded habitats along mountains and deserts of interior California (Garrett and Dunn 1981, Small 1994). They are less common along the coast and on the Channel Islands (Garrett and Dunn 1981, Small 1994).
3. Arrival Date on Breeding Grounds
Breeders arrive throughout the state by mid-April or mid-May at higher elevations (Small 1994). The first birds on the central California coast usually have arrived by 10 April, but arrivals are progressively later to the north with birds usually not on territory in coastal British Columbia until late April or early May (Dunn and Garrett 1997).
4. Initiation of Nesting
Courtship activities in California begin soon after birds arrive on breeding grounds in late May and early Jun (Pitocchelli 1995). Peak nesting season is June (Zeiner 1990). Egg-laying and full clutches are completed by early June (Pitocchelli 1995). Extreme records for California include 28 April and 10 July (Pitocchelli 1995).
5. Departure Date from Breeding Grounds
In California, breeders begin to depart higher elevations
by mid-August continuing through late August to
mid-September at lower elevations (Small 1994). These birds generally leave the breeding grounds by mid-September but small numbers are regularly observed in lowlands into October or early November (Zeiner et al. 1990). Wintering individuals have been reported regularly in San Diego County, but individuals have been seen as far north as Monterey County (McCaskie et al. 1988).
6. Spring Migration Period
Spring migration is quite prolonged with no sharp peak in California (Dunn and Garrett 1997). MacGillivray's Warblers are common migrants in the interior of southern California and the coastal regions of northern California (Zeiner et al. 1990). Relatively few birds are found on the immediate coast of southern California (Dunn and Garrett 1997). They are uncommon to fairly common migrants on the Farallon and Channel Islands (DeSante and Ainley 1980, Garrett and Dunn 1981). Concentrations of spring migrants have been noted in the deserts of southern California (Dunn and Garrett 1997). Earliest spring migrants appear in extreme southern California by the end of March, pass through the lowland areas of the state primarily April to late May with stragglers until early June in the north (Small 1994). In migration MacGillivray's Warblers are found in open woodland undergrowth, scrubby areas, and thickets (AOU 1998). Their preference for riparian and moist brushy habitats suggest they may use riparian corridors for travel (NatureServe 2000).
7. Fall Migration Period
There is a slight post breeding upslope migration through willows and meadows as high as timberline (Gaines 1977). Migration is well underway by late August to mid-September and continuing through mid-September in desert oases, and through early October or occasionally mid-November along the coast (Small 1994). Although fall migrants are numerous in montane areas, large numbers also pass through the lowland deserts (Dunn and Garrett 1997).
8. Nest Type
MacGillivray's Warblers make a cup nest, often loosely constructed. The outer nest is made from coarse grasses, willow bark strips, dry weed stems, leaves, wild oat and straw fibers. Leaves forming the outer nest are often "skeletonized" by birds, leaving only the stem (Pitocchelli 1995). The inner lining is composed of finer grasses, root fibers, and hair (Chapman 1907, Bent 1953). The nest is often fastened to stems of shrubs concealed by twigs and leaves (Salt 1973).
9. Foraging Strategy
MacGillivray's Warblers forage on or low to ground in brushy thickets along streams or in dense second growth (Griscom and Sprunt 1957, Miller et. al. 1972, Pitocchelli 1995). They also forage among branches and leaves of trees and shrubs (Miller et. al. 1972, Hutto 1981). In California, MacGillivray's Warblers feed on different substrates, more commonly on bark (70%) than on foliage (20%) or ground (10%) and tend to forage along inner parts of vegetation (Shuford 1993). Foraging height varies with geography and season. Average foraging height in Oregon was 2.6 feet in deciduous forest, 2.0 feet in coniferous woods; 84-92% of foraging < 3.3 feet above ground in both forest types (Morrison 1981). In Wyoming average foraging height in early summer was 1.7 feet, and increased to 5.6 feet during late summer with the majority of observations being 4.0 feet above ground (Hutto 1981). Also observed foraging 9.9 – 16.5 feet above ground in Wyoming (Cody 1974).
The main food taken are insects (Pitocchelli 1995). During the breeding season in California, true bugs (Hemiptera), leaf hoppers (Homoptera), beetles (Coleoptera), bees, wasps and ants (Hymenoptera) were taken (Shuford 1993). Elsewhere, prey species include click, dung, and flea beetles; alfalfa weevils (Coleoptera); and caterpillars (Lepidoptera) (Bent 1953, Oberholser 1974). Due to the MacGillivray's Warblers exclusively insectivorous feeding habits, they may play a positive role in control of insects in riparian and early serial forest habitats, but this is unstudied (NatureServe 2000). Young are known to take sap from sapsucker drillings in willows (Zeiner et al. 1990).
Adult calls are given year-round. Singing by territorial males peaks during breeding when they become conspicuous, singing loudly from tree branches or shrubs (Pitocchelli 1995). Highly territorial males sing from perches to advertise territorial ownership, and are often seen fighting with other males during late spring and summer (Pitocchelli 1995). Singing intensity is greatest at dawn and dusk. Singing begins approximately 30 minutes before sunrise, tapers off by midmorning, resumes in early evening, and terminates soon after dusk (Pitocchelli 1995). Males usually sing from within dense vegetation but will work their way up to more exposed perches high in the canopy – dropping back to the understory when disturbed (Dunn and Garret 1997). Males sing from tree perches 16.5 – 23.1 feet above ground (Salt and Salt 1976), with exceptional heights of 45 – 60 feet reached (Dunn and Garret 1997). The male’s song is rarely heard outside of the breeding season and they become shy and elusive and often difficult to detect (Pitocchelli 1995). Males are somewhat less retiring during migration and on wintering grounds (Pitocchelli 1995). Cryptic females and males are often encountered by accident or when scolding intruders near the nest and fledglings (Pitocchelli 1995). Females respond to intruders with distraction displays and feigning injury when near nest or fledglings (Jewett et al. 1953)
11. Mating System
MacGillivray's Warblers are presumed to be seasonally monogamous and polygamy is not known to occur (Pitocchelli 1995).
12. Clutch Size
In California, clutch size is 3-6 with the usual number being 4 (Zeiner 1990, USFS 1994).
13. Incubating Sex (Female/Male)
The nest is built by the female (Baicich and Harrison 1997) and it appears that only the female incubates the eggs, but this needs further study (Harrison 1978, Pitocchelli 1995).
14. Incubation Period
In Alberta, the incubation period was found to be 11-13 days (Zeiner 1990, Semenchuk 1993). Incubation begins with first egg (Bent 1953).
15. Nestling Period
Young fledge 8-9 days after hatching (Bent 1953, Semenchuk 1993).
16. Development at Hatching
Young are altricial and born with eyes closed. Downy feathers appear on top of head, back, and wings 2 days after hatching (Bent 1953).
17. Number of Broods
Normally one brood per year is produced. Some authors report that second broods do not occur (Pitocchelli 1995) while others report second brooding (Baicich and Harrison 1997). This needs further study.
18. Who Tends Young
Incubation is by the female only (Baicich and Harrison 1997). Female feeds nestlings, leaving the nest for 3 to 5 minute intervals to collect food (Pitocchelli 1995). Both parents remain near the nest (Jewett 1953) and tend young until fledging (Harrison 1978). Both sexes tend to fledglings, but the female plays a predominant role (Bent 1953, Johnsgard 1979). Young are dependent on parents immediately after fledging, but there is no information on how long this dependent period lasts (Pitocchelli 1995). Families forage together soon after fledging, but it is not known how long they remain together (Pitocchelli 1995). This aspect of MacGillivray's Warbler biology deserves further study (Pitocchelli 1995).
For breeding, MacGillivray's Warblers require dense undergrowth and moderate cover (Morrison and Meslow 1983). On National Forests in northern Idaho and western Montana, MacGillivray's Warblers were most abundant in clearcuts and riparian shrubland; followed by seed tree and shelterwood harvest units; marsh/wetland, cottonwood/aspen and spruce-fir habitats; and group-select harvest units (Hutto 1995). Timossi (1990) characterizes the following habitat types as having high importance to reproduction in California: seedling valley-foothill riparian; seedling and dense canopy sapling mountain riparian; and seedling and dense canopy sapling aspen. Preferred habitat in California also includes coastal Douglas-fir, redwoods, montane riparian vegetation (USFS 1994, Zeiner 1990), and chaparral (AOU 1998). Highest densities occur in riparian habitats, stream bottoms, brushy hillsides and along canyons (Pitocchelli 1995). They are often found in mid-elevation wet meadows with low shrubs in the Sierra Nevada (Verner and Allen 1980). Nesting is positively associated with logged-over clearcuts with second growth (Zeiner et al. 1990), areas recovering from avalanches (Pitocchelli 1988), burned areas (NatureServe 2000), successional brush fields near water (USFS 1994), and other disturbed sites with a dense understory. Although MacGillivray's Warblers use brushy ecotones, they may respond more to vegetation structure than to edge itself (USFS 1994). Dense shrubby habitats are preferred throughout their range, but vegetation parameters are not quantified in most sources (NatureServe 2000). This species deserves attention for its close association with dense brush in moist woodlands, shrubby riparian habitats, and early successional stages of cut-over or burned forests (NatureServe 2000). There is evidence of fidelity to breeding sites in Oregon (Klimkiewicz and Butcher 1989).
1. NEST SITE
a. Nest Substrate2. Vegetation Surrounding the Nest
The nest is a small compact cup of fine blades and stems, dead grasses, weed stems and bark shreds, lined with fine grasses, rootlets and hairs (Harrison 1978, Zeiner at al. 1990).
b. Height of Nest
The nest is usually placed low, 2.0 – 5.0 feet above ground (Terres 1980). The mean nest height was reported by Pitocchelli (1975) as 1.4 feet above ground (n = 110) with extremes of ground level to 8.2 feet above ground.
c. Height of Nest Plant
No information was found.
d. Plant Species Concealing Nest
The nest is often found low in a thick shrub, conifer sapling, weed clump, or fern clump (Harrison 1978, Terres 1980). In Alberta, MacGillivray's Warblers may nest on clumps of grass on or above ground among cedar, alder, hawthorn, willows and flowering shrubs of wild raspberry, stinging nettle, buckthorn, saskatoon, wild rosebush, gooseberry, currant, blackberry vines, huckleberry, wild lettuce, hellobore, salal bush, hazel bush, wild columbine, dry ferns, and wildflowers (Salt 1973). Along the Pacific coast, they favor dense thickets of poison oak or blackberry or other shrubs and tangles (Dunn and Garrett 1997).
e. Percent Nest Cover
The nest is always concealed by shrubs and dense undergrowth (Chapman 1907, Bent 1953, Salt 1973, Salt and Salt 1976). In cottonwood/ponderosa pine riparian breeding habitat in Montana brush cover was 61%, with 6 shrubs per 33 foot radius plot, and 58% ground cover (Mosconi and Hutto 1982). In Wyoming, MacGillivray's Warblers prefer habitats with dense shrub cover (mean distance to nearest shrub less than 6.6 feet) with a high percentage of willow species (more than 70 percent of shrubs) and a high foliage density of small shrubs (0.9- 3.3 foot height interval) (Finch 1989a, 1989b).
a. Canopy Cover
MacGillivray's Warblers require moderate canopy cover (Morrison and Meslow 1983) and the amount of cover is important for assessing breeding habitat (Morrison 1981). In a Oregon conferious forest site, total canopy cover was 74.2%: 3.7% coniferous species + 6.7% deciduous species + 63.8% shrub species (Morrison 1981). In a Oregon deciduous forest, total canopy cover was 60.1%: 7.7% coniferous species, 7.6% deciduous species and 44.8% shrub species (Morrison 1981). In Montana, a cottonwood/ponderosa pine area had a 31% mid-canopy cover (Mosconi and Hutto 1982). In Wyoming, MacGillivray's Warblers breed in shrub willow and thin-leaf alder riparian communities with few trees (less than 400 trees per acre) (Finch 1989a, 1989b). In California, breeding habitat includes seedling valley-foothill riparian, seedling and dense canopy (60-100 percent) sapling montane riparian, and seedling and dense canopy (60-100 percent) sapling aspen (Timossi 1990). In Oregon, the nesting abundance of MacGillivray's Warblers was found to be negatively associated with diameter-cut logging that reduced tree density from 125-138 stems/ac to 73-94 stems/ac (East Slope Cascade Mountains Bird Conservation Plan 2000).
b. Average Top Canopy Height
No information found.
c. Dominant Plant Species in Canopy
Dominant plant species found in the canopy varies with the breeding site. In Canada and northern US, MacGillivray's Warblers breed primarily in coniferous-forest clearcuts with spruce and Douglas-fir or mixed deciduous forests with birch, aspen, and poplar (Oakleaf 1992). In Utah, elevational differences in plant composition were noted with lower elevations dominated by birch, cottonwood, and boxelder and higher elevations by willows, mountain alder, and dogwood (Blakesely and Reese 1988). In the Centennial Mountains, Idaho, breeding sites were associated with xeric willow communities with mixed grasses and forbs, willow-conifer habitats, and willow riparian communities (Douglas et al. 1992). In montane habitats of Colorado, breeding sites were associated with riparian aspen-willow and spruce-aspen communities (Winternitz 1976). In Wyoming, shrub willow and thin-leaf alder dominated the canopy (Finch 1989a, 1989b). In California, willow, alder, and other dense shrubs provided cover at breeding sites (Zeiner 1990).
d. Dominant Shrub Species
Breeding abundance is positively associated with well developed understories (> 268 shrubs, seedlings, saplings/ac) (East Slope Cascade Mountains Bird Conservation Plan 2000). Breeding site undergrowth is composed of shrubs and bushes and the species composition varies throughout the range (Pitocchelli 1995). In Oregon the understory was dominated by salmonberry, thimbleberry, vine maple, salal, Sword fern, tansy ragwort, foxglove, and pearly everlasting (Morrison 1981, Morrison and Meslow 1983). In Alberta, understory plants consisted of buckthorn, wild rose, chokecherry, Saskatoon serviceberry, and currant (Salt 1973, Salt and Salt 1976). In study sites in Wyoming and Utah, the understory vegetation consisted of willows with a dense mix of rushes, horsetails, grasses, and sedges (Hutto 1981, Blakesely and Reese 1988).
Birds breeding along the Pacific Coast are associated with north facing slopes (Dunn and Garrett 1997).
f. Tree DBH
No information found.
g. Distance to Water
In southeastern Wyoming, breeding was associated with moist sites (more than 5.5% water cover) with dense grass cover (Finch 1989a, 1989b). In California, presence of streams, bogs, wetlands, ponds, lakes or other aquatic habitats is preferred (Timossi 1990, Verner and Allen 1980).
3. Landscape Factors
MacGillivray's Warblers breed from sea level to 9,900 feet in Sierra Nevada range and prefer the upper Sonoran and Canadian life zones in California (Pitocchelli 1995). They breed coastally from Monterey County north (McCaskie et al. 1988). In northern California, they breed to 7,900 feet and in southern California they reach 9,200 feet (USFS 1994). They tend to be less common and more local as a breeder in mountains south of Sierra Nevada (Zeiner 1990).
b. Fragmentation and Patch Size
Optimum patch sizes and most aspects of landscape relationships for this species are unknown (NatureServe 2000). Presumably, the patch size necessary for breeding would depend on quality of habitat as evidenced by the great geographic variation in density for these warblers (USFS 1994). There is a positive association with total area of clearcut patches (2 of 3 years old) and a negative association with total perimeter of clearcut patches (2 or 3 years old) and number of clearcut patches (1 or 3 years old) at the landscape level in Oregon (East Slope Cascade Mountains Bird Conservation Plan 2000).
c. Disturbance (Natural or Managed)
MacGillivray's Warblers breeding is positively associated with natural or managed disturbance as long as a dense understory is allowed to develop. Because of their preference for nesting close to the ground in moist shrub areas, MacGllivray 's warblers are likely to be affected by activities that alter these communities (USFS 1994). Nesting is positively associated with logged-over clearcuts with second growth (Zeiner et al. 1990), areas recovering from avalanches (Pitocchelli 1988), burned areas (NatureServe 2000), successional brush fields near water (USFS 1994), power line right-of-ways (Dunn and Garrett 1997) and other disturbed sites with a dense understory. These birds frequent habitats with a high component of natural edge (e.g. riparian areas, brush-meadow or brush-forest ecotones) (USFS 1994). Although MacGillivray's Warblers use brushy ecotones, they may respond more to vegetation structure than to edge itself (USFS 1994). See the section on sensitivity to human-induced disturbance below.
d. Adjacent Land Use
No information found.
No information found.
4. Special Factors
Parasitized by Brown-headed Cowbird (Bent 1953) but this
seems to be rare in Alberta (Semenchuk 1993).
5. Sensitivity to Human-Induced Disturbance
MacGillivray's Warblers respond differently to different types of human development. Extensive logging in Pacific Northwest should benefit this species (Pitocchelli 1995). The MacGillivray's Warbler has probably benefited from large-scale human development such as logging and mining in boreal U.S. and Canada. Current clearcut logging practices in the northwestern U.S. and Canada will continue to open new breeding habitat for this species, although replanting decisions may affect habitat quality. Many companies have begun replacing Spruce forests with unproductive pine forests. The pine forests prevent dense, lush undergrowth that MacGillivray's Warbler and other boreal species need for breeding. Such unproductive habitat may have commercial value, but massive planting efforts may cause long-term problems for boreal forest species such as MacGillivray's Warbler (Pitocchelli 1995). Activities such as intensive grazing, water developments, recreation or urban development, herbicide treatments, wildfires, prescribed burns, intensive agriculture and ranching that remove or degrade brush and seedling/sapling vegetation in riparian habitats, ecotones, bogs, wet-meadows, and forests or woodlands may be detrimental to local populations. Widespread loss and degradation of western riparian habitats probably affects the species but is unstudied (NatureServe 2000). Cowbird parasitism rates may increase due to encroaching development in the lower evevations of the Sierra Nevada (Siegel and DeSante 1999). Zinkl et al. (1979) studied adverse effects of insecticides on cholinesterase (ChE) activity in boreal forest birds in Montana. MacGillivray's Warblers showed severe depression of ChE activity in the brain when exposed to acephate. The percent depression of ChE ranged from 23% to 51%. Exposure to two other insecticides (carbaryl and trichlorfon), however, resulted in minor effects on ChE activity (Zinkl et al. 1979). The influence of pesticides on populations has not been determined.
MacGillivray's Warblers are preyed on by accipiters, small mammals and snakes (Zeiner et al. 1990). Females give distraction displays and injury feigning near nest or fledglings in response to predators (Jewett 1953). On the Moaollon Rim, Arizona, a nest predation rate of 49 percent was reported in snowmelt drainages of mixed pine-oak woodland (Martin 1993). Nestlings are a known host for parasitic blow flies (genus Protocalliphora) (Revels 1996).
7. Population Trend
North American Breeding Bird Survey (BBS) data for 1966-1996 show a slight, but nonsignificant, survey-wide decline averaging -0.3 percent per year (n = 365 survey routes). Mapped long-term trends, however, show a pattern of declines along the west coast of Canada and U.S., and in Colorado and northern New Mexico; and increases through the northern and central Rocky Mountain corridor, as well as parts of the Sierras and Great Basin. Data show significant long-term declines in Oregon (-3.0 percent per year; n = 65) and Washington (-1.7 percent per year; n = 50), and the Southern Pacific Rainforest physiographic region (-3.6 percent per year; n = 54) for 1966-1996 (NatureServe 2000).
MacGillivray's warblers are easily monitored by Breeding Bird Surveys in California with an average of 1.5 birds detected per route (58 routes in California 1966-1989) (USDI Fish arid Wildlife Service 1990). An analysis of Breeding Bird Survey trends in California showed that these warblers are apparently increasing (by 4.0 % per year, 58 routes, 1966-1989) although there was no difference in the proportion of routes showing increases or decreases (48% and 43% of routes respectively) (USDI Fish and Wildlife Service 1990). Shuford (1993) observed an overall increase in numbers of breeding birds in California, with declines in Monterey County being offset by expansion in northern coastal mountains. In northern California, there is evidence of a historical increase in population numbers in Douglas-fir forests since development in the 19th century, due to an increase in younger seral stages and brushy second-growth, but the species may decline in the future as these forests mature (Raphael et al. 1988). The populations in the southern California mountains also appear to be increasing (Pitocchelli 1995). However, populations at the southern limit of its range in central California and in the mountains of southern California are all small and fragmented and could be vulnerable (Dunn and Garrett 1997).
Breeding bird density varies with elevation, habitat quality, and location. In western Oregon, breeding bird densities varies from 0.55 (Morrison and Meslow 1983) to 1.83 birds/ac (Morrison 1981); in Washington, breeding bird densities were reported as 0.65 males/ac (Miller et al. 1972); and in Wyoming densities ranged from 0.40 - 0.83 territorial pairs per acre (Finch 1989a). These densities are sensitive to the amount of ground cover. For example, in Utah, breeding bird densities range from 2.1 to 3.0 males/ac, increasing with vegetation cover (Blakesely and Reese 1988); and in Idaho, Johnston (1949) found 2.5 pairs per 100 acres in a Douglas-fir clearcut with no ground cover and 10 pairs per l00 acres in a selectively logged Douglas-fir area with dense ground cover. These densities also vary with habitat type. For example, in Wyoming, breeding bird densities were 10 per 100 acres in a willow-sedge swamp, 30 per 100 acres in a flatland aspen stand and 85 per 100 acres in a scrub-meadow (Salt 1957). From 1990-1995, 12 Monitoring Avian Productivity and Survivorship (MAPS) stations in the Sierra Nevada (mostly in forest-meadow interface habitat between 4,300 and 7,900 feet) reported a mean capture rate of 2.1 adults per 100 net hours (one net hour is one standard 40 foot mist net operated for one hour) and young birds comprised 51.0% of these captures (Siegel and DeSante 1999). Using data from the 12 Sierran MAPS stations over a four year period (1993-1996) estimates of annual survival rates of adult birds was found to be 0.5 % (Siegel and DeSante 1999). The longevity record for this species is 5 years 11 months (Klimkiewicz 2000).
9. Management Issues and Habitat/Population Objectives
The following is a list of potential management issues with suggestions on how to minimize the adverse impacts and enhance the positive impacts of these actions on MacGillivray's Warbler populations:
10. Associated Bird Species
a. Reduce the negative effects of human-induced disturbance including:
- Timber Harvest: The MacGillivray’s Warbler has increased and extended its range in northern California Douglas-fir forests that have undergone extensive timber harvesting, creating stands of shrubby second-growth (Morrison and Meslow 1983). This pattern likely holds for most northern conifer forests in its range (Pitocchelli 1995). Populations in forested regions will undoubtedly fluctuate with trends in timber harvest practices and the availability of such brushy seral habitats (NatureServe 2000). Logging practices which open-up dense moist forest or destroy the shrubby understory of such forests provide a serious risk to this species (Siegel and DeSante 1999). Timber cutting should not be permitted within 100 feet of riparian areas (Colorado Partners in Flight Landbird Conservation Plan 2000).
- Intensive Grazing: Intensive grazing that reduces or eliminates willows and other brush and sapling vegetation along streamsides, in bogs, wet meadows, and moist woodlands is detrimental (USFS 1994, Pitocchelli 1995, Siegel and DeSante 1999). In cottonwood and Ponderosa pine riparian woodland of western Montana, MacGillivray's Warblers were absent on a heavily-grazed site but bred on a lightly-grazed site. The percent ground cover, bush cover, mid-canopy cover, bush volume, canopy volume, and numbers of trees and shrubs differed significantly between the sites (Mosconi and Hutto 1982). Grazing had a negative effect on species presence in aspen communities in California and Nevada (Page et al. 1978) and aspen/willow communities in Nevada (Medin and Clary 1991). Nests may also be vulnerable to trampling by livestock (NatureServe 2000).
- Fire: No direct information is available on the effects of wildland or prescribed fire on this species. However, burns that result in the regeneration of brush and deciduous saplings in forest stands and riparian habitats should be beneficial (NatureServe 2000).
- Pesticides: Cholinesterase (ChE) inhibitors (organophosphates and carbamates) used as insecticides can disrupt bird nervous systems, resulting in death. In a Montana and Oregon study, 78 percent of birds sampled (n = 14) showed a 23 percent to 51 percent depression in ChE activity in the brain when exposed to acephate insecticide (Orthene, 0-methyl S-methyl N-acetylphosphoramidothioate) sprayed on forests at 1.13 kilograms per hectare. This insecticide caused a marked response in 13 other passerine species as well. No ChE response was reported from MacGillivray's in plots sprayed with carbaryl or tricholorfon, though it is unclear from the data whether the species was collected on those plots (Zinkl et al. 1977).
- Recreation: In Utah, MacGillivray’s Warblers were less abundant in campgrounds and picnic sites developed in riparian areas where shrub/sapling density was half that of non-campground riparian sites (Blakesley and Reese 1988).
- Development: Encroaching development in the lower elevations of its breeding range in the Sierra Nevada pose a risk due to the potential for this to increase cowbird parasitism opportunities (Siegel and DeSante 1999).
b. Enhancement opportunities:
- Potential for increasing populations is high where it is possible to restore willow and other shrubby riparian habitats (NatureServe 2000).
- This species has apparently increased where favorable habitat has increased, such as early seral stands in northern California forests (NatureServe 2000).
- Management for MacGillivray's warblers should include retention and restoration of riparian habitats, cessation of brush and willow removal and herbicide use, and reduction in grazing intensity (USFS 1994).
- Where ecologically appropriate, initiate actions in Mixed Conifer forest to maintain or provide the following conditions: dense understory shrub layer (includes shrubs, seedlings, and saplings) dominated by native species with > 40% cover and/or > 270 stems/ac, tree canopy cover < 25%, herbaceous ground cover < 25%. "Ecologically appropriate" refers to the potential vegetation of the site, considering hydrology, soils, topography, and ecosystem processes (East Slope Cascade Mountains Bird Conservation Plan 2000).
c. Priorities for future research:
- A through study of the breeding biology of the MacGillivray's Warbler, similar to Cox’s (1960) work on Mourning Warbler, is noticeably lacking. Mating displays, all aspects of parental care (feeding young, attentiveness at the nest, incubation and brooding, etc.), antipredatory displays, survivorship, and philopatry are some topics that need further attention (Pitocchelli 1995).
- Other aspects of the general biology of this species also deserve further study. Migratory routes are not well defined. Migratory behavior, habitat requirements, and mortality during migration are largely unknown and difficult to obtain because of the secretive nature of this species during migration (Pitocchelli 1995).
- Further information is needed on the relationship to management practices and activities that alter stream vegetation (such as water developments, agriculture, housing developments, and logging) (NatureServe 2000).
- No information is available on effects of timber harvesting in cottonwood riparian or aspen, but practices that reduce shrubs, seedlings, and saplings likely have adverse effects (NatureServe 2000).
- Need an assessment of reproductive success and population viability in early successional habitats created by forest management (East Slope Cascade Mountains Bird Conservation Plan 2000).
- Intensive grazing that reduces or eliminates brush and shrubs, particularly in riparian habitats, is undoubtedly detrimental, but the details on response to different grazing regimes or habitat parameters is unknown. Most available studies examined only species presence/absence and lack treatment replicates, either comparing a single grazed plot with a single ungrazed plot, or comparing several plots within a single riparian zone. Sensitivity to different grazing regimes in various riparian and shrub habitats grazing deserves further study (NatureServe 2000).
- Pesticide effects on population, reproduction, development, and survival is unknown and deserves further study (NatureServe 2000).
- MacGillivray's Warblers are monitored on a broad scale by BBS with relative abundances ranging from an average 0.66 to 9.4 birds per survey route across the species range. However, they may be under represented in some regions because BBS generally does not sample riparian or boreal forest habitats well, and counts in northern areas are typically run in June after peak courtship behavior. There is a need for localized population monitoring (NatureServe 2000).
- Landscape relationships (patch size, area sensitivity, use of corridors) are unknown. More work is needed on vulnerability to predation and parasitism in relation to landscape patterns (NatureServe 2000).
Interspecific aggression between MacGillivray's Warblers and other parulines is rare but does occur on wintering grounds (Hutto 1981). Cody (1974) and Hutto (1981) reported segregation of feeding niches and vertical foraging heights among breeding warblers in the western U.S. thus, this lack of aggressive encounters may be due to exploitation of different ecological niches (Pitocchelli 1995). Competition with other species has not been demonstrated and MacGillivray's Warblers are somewhat restricted in foraging niche. (USFS 1994). Apparently they are rarely hosts for cowbirds (Bent 1953). Other species that may use habitat in a similar way and/or respond similarly to threats, management, and conservation activities include: Spotted Towhee, Green-tailed Towhee, Dusky Flycatcher, Yellow Warbler, Wilson’s Warbler, Fox Sparrow, Lincoln’s Sparrow, and White-crowned Sparrow (Colorado Partners in Flight Landbird Conservation Plan 2000).
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