BLACK-HEADED GROSBEAK (Pheucticus melanocephalus)
Prepared by: Michael Lynes (email@example.com)
of California - Hastings College of Law
San Francisco, CA 94102
Lynes, M. 1998. Black-headed Grosbeak (Pheucticus melanocephalus). 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
shortcut to references
Two subspecies, both breed in California:
The Pacific-coast Black-headed Grosbeak, P. m. maculatus, occupies California west of the eastern deserts and the Sierran Ridge (Grinnell and Miller 1944, Hill 1995).
Rocky Mountain Black-headed Grosbeak, P. m. melanocephalus, is found east of the Sierras, and may be transient in more western areas during migration. P. m. melanocephalus may hybridize with the Rose-breasted Grosbeak (P. ludovicianus) where their ranges overlap (AOU Checklist 1983).
An intermediate of the two sub-species occurs in the Great Basin area (Grinnell and Miller 1944).
MANAGEMENT STATUS: No special status. Protected under the Migratory Bird Act.
HISTORICAL BREEDING DISTRIBUTION
CURRENT BREEDING DISTRIBUTION
P. m. melanocephalus, breeds in the central eastern part of state, including Inyo and Mono counties. Range extends east to northwestern North Dakota (AOU Checklist 1983).
P. m. maculatus, breeds west of the eastern deserts of California and the Sierran Ridge (Grinnell and Miller 1944, Small 1994).
The information below derives from literature on the Black-headed Grosbeak (BHGR) and on the riparian habitat of California and from data collected by biologists from the Point Reyes Bird Observatory (PRBO). Primary sources include Geoffrey E. Hill's addition to the Birds of North America (1995), Weston's survey of BHGR breeding ecology in Berkeley (1947), and Ritchisson's similar study in northern Utah (1983).
PRBO data sets are the source for all unreferenced descriptive statistics given below. PRBO's studies include work in the Bay/Delta, Klamath, Sacramento Valley, and San Joaquin Valley bioregions of California. Because of the similarity of the study sites in the San Joaquin and Sacramento regions, I combined data from these locations. The category "All sites" includes data from all the above mentioned bioregions. While this increased sample size for some analyses, it is down-plays some significant differences between BHGR breeding sites in the different bioregions. Finally, as is apparent for the bias toward northern central and coastal California, more data are needed from other bioregions around the state.
AVERAGE TERRITORY SIZE
No data available for California. Hill (1998, 1995) reported mean territory size in New Mexico as 0.79 ha (n=28, range=0.43-1.63ha). Ritchisson (1983) reported mean territory size in northern Utah to be about 2.7 ha (n=12, range=1.9-30ha).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS:
Data from 20 years of banding at PRBO's Palomarin Field Station (in the Bay/Delta bioregion) show that mature adult males, in at least their third year, and females (ages unspecified in data) typically begin arriving in the first week of April. The first capture of a female with a brood patch is April 16. Males in their second year of life (first breeding season, with dull plumage) typically arrive about a week later. Each age class shows an increase in capture rates about three weeks later (toward the end of April, beginning of May). This higher level is maintained until about June for yearling males and adult females, and extends into July for adult males (PRBO data).
Weston(1947) reported records from 1911 to 1946 that show the arrival of BHGR in Berkeley (Bay/Delta bioregion) as early as April 4 and as late as April 23. In 1945, he first observed a male in his study area on April 14. Only males were observed for the following six days. On April 20, the first female was seen (Weston 1947). Hill (1989) and Weston (1947) observed males arriving about a week before females. Hill elaborated, reporting that mature adult males arrived 1-3 days before younger males, and that yearling males arrived in his study area about two weeks after the first elder male (Hill 1995). Ritchisson (1983) observed the first arrivals in his study area in Utah during the first two weeks of May. The area was not completely settled for another few weeks. Hill (1995) reported that BHGR arrival on breeding grounds varies greatly with geographic region and altitude, but that the arrival pattern of age classes and sexes remains fairly consistent in every region studied.
In the Bay/Delta bioregion (Berkeley area), adult males generally depart in late July. Females continue to tend to the young, and both start fall migration in mid-August (Weston 1947). Hill (1995) reports that adult males begin leaving their breeding grounds in late July to early August and that females and young of the year leave later, in less regular intervals. PRBO banding data support this: In 25 years of banding at the Palomarin Field Station (Bay/Delta region) there are no capture records of a mature male after July 22, while captures of female and young regularly extend into October (n=1339 total captures).
FORAGING STRATEGYIn a study of insect-gleaning birds in the central Sierra Nevada, Airola (1985) reports that BHGR primarily foraged in the higher strata of the canopy, preferring oak-pine stands. While foraging, grosbeaks gleaned (70%), lunged (25%), and hawked (5%). BHGR also forage on the ground, in agricultural fields, and among weeds and exotic thistle patches (Weston 1947, Shuford 1993, Hill 1995).
BHGR seems to be somewhat of a generalist, recorded as having consumed both native and exotic species of insects and plants. BHGR forage primarily on insects during the breeding season and warmer months when they are readily available (Weston 1947, Hill 1995). Over the breeding season, diet will also include spiders, seeds, and fruits. In northern California, BHGR forage on California Blackberry (R. ursinus) and Himilaya Blackberry (R. discolor) when those plants are fruiting (pers. obs.).
NEST SITEVegetation diversity and vertical complexity appeared to be important indicators of high quality BHGR territories in New Mexico (Hill 1988). Older, dominant males held these territories and produced more young, despite an equal distribution of food resources between poor and high quality habitats. The primary differences between "poor" and "high" quality habitats were the heterogeneous vertical structure of the vegetation of the "high" quality areas and a significantly correlated decrease in jay (species unspecified) activity in these more diverse areas. Grinnell and Miller (1944) note the diversity of nest sites: from orchards to oak woodlands, desert washes to coniferous forests. They point out that BHGR favor areas with a cottonwood-willow association. Gaines (1977) confirms these statements in the Sacramento bioregion and goes on to describe the vertical complexity of cottonwood-willow associations: often there exists a primary and secondary canopy, a variety of shrubs of different heights, and patches of forbs, grasses, and sedge.
HEIGHT OF NESTNest height did not significantly affect nest success in the PRBO data set. All sites: mean=3m, range 1.28-7.5m (n=68); Bay/Delta: mean = 3.23m, SD=1.4m, range=1.7-6.0m, n = 16. Weston reviewed 163 nest records from around California and found the average nest height to be 10 feet (3 meters) (Weston 1947); Klamath: mean = 2.3m, SD=.92, range=1.3-5.0m, (n = 18); Sacramento/San Joaquin: mean = 3.7m, SD=1.5, range=1.6-7.5m, (n = 38). Ritchisson (1983) reports: mean= 4.1m (n = 21, range = 2-7m) in northern Utah. Hill found: mean=3.7m (n=38), range = 2.1-7.6, SD = 1.4 in New Mexico (Hill 1995).
HEIGHT OF NEST PLANTPlant height did not significantly affect nest success in the PRBO data set. All sites: mean=4.8m, range=2.0-12.0m (n=64); Bay/Delta: mean=3.2 m, SD=2.6m, range=2.3-12.0m, (n=15). Weston's descriptions (1947) include a thicket of deciduous shrub averaging 6-8 feet, two Coastal Live oaks, one 20 ft. and the other 25 ft., and a 45 ft. California laurel; Klamath: mean=3.5m, SD=1.6m, range=1.0-7.0m, (n=18); Sacramento/San Joaquin: mean=5.6m, SD=1.5, range=1.6-7.5, (n=38).
PLANT SPECIES CONCEALING THE NESTNests are typically built in a live tree or shrub with its foliage providing the basic concealment for the nest. Bay/Delta: Weston (1947) describes nests as being hidden by clumps of leaves of the nest substrate and the varied plants composing a mixed thicket; Klamath: Observers noted that blackberry provided effective cover of nests in Shasta Co (PRBO data); Sacramento/San Joaquin: In the oak riparian forest of these regions, nests were frequently built with Wild Grape (V. californica) providing substantial concealment (PRBO data, pers. obs.).
Authorities most often describe the preference of the BHGR for "open woods" and forest edges (Grinnell and Miller 1944, Weston 1947, Gaines 1977, Hill 1995). Whitmore (1975) found that BHGR in southwestern Utah show affinity for areas of moderate tree density and canopy cover. Densiometer readings were taken from four directions, all within 5 meters of the nest, and averaged: Bay/Delta: mean=59.7%, SD=36.9%, range=0-96% (n=7); Klamath: mean=64.5%, SD=34.6%, range=0-99% (n=13); Sacramento/San Joaquin: mean=38.3%, SD=42.5%, 0-97.8% (n=45).
AVERAGE TOP CANOPY HEIGHT
All sites: mean=12m, with a median= 9, a SD=7.7m, and a range=2-28 m (n=59);Bay/Delta: mean = 11.0 m, SD=4.8m, range=5-15m, (n=6); Klamath: mean = 13.0m, SD=8.1m, range=2-27m (n=18); Sacramento/San Joaquin: mean = 10.8m, SD=4.1m, range=0-28m, (n=41). This wide range probably reflects the fact that BHGR nest in both old growth riparian forest, with large, shady oaks and cottonwoods, as well as in the relatively open areas in early successional riparian zones and along levees (Gaines 1977, PRBO data, pers. obs.).
DOMINANT PLANT SPECIES IN CANOPY
BHGR may be found in as diverse canopy structures as oak riparian, cottonwood groves, conifer forests, oak savannah, and pinyon-juniper (PRBO data, Weston 1947, Shuford 1993, Gaines 1974, Hill 1995). Bay/Delta: Weston (1947) remarks that grosbeaks are found in open riparian forests, oak-laurel woods, and among the live oaks, cottonwoods, and willows bordering steams. Sacramento/San Joaquin: Gaines (1974) detected abundant breeding BHGR in Fremont Cottonwood (P. fremontii)-willow (Salix spp.) forests and none in forest dominated by Valley Oak (Q. lobata). However, PRBO records show BHGR nesting at sites in Sacramento County and Tehema Co. in old growth riparian forests dominated by Valley Oak (Q. lobata) and Fremont Cottonwood (P. Fremontii), with a secondary canopy of Oregon Ash (F. latifolia), Box Elder (A. negundo), and Gooding's Black Willow (S. goodingii), often draped with Wild Grape (V. californica) and occasionally Poison Oak (T. diversilobum) (PRBO data, pers. obs.)
Many levees along California's waterways support thin strips of riparian vegetation. Along the levees passing through the Cosumnes Preserve in Sacramento Co., the canopy is primarily composed of short (about 5m) Sandbar Willow (S. exigua), Arroyo Willow (S. laevigata), Yellow Willow (S. lutea), and occasionally small Valley Oaks (Q. lobata) and Oregon Ash (F. latifolia).
CO-DOMINANT PLANT SPECIES IN CANOPY
PERCENT NEST COVERConcealment of nests positively correlates with nest success (PRBO data). Concealment was estimated by nest finders from six directions: above, below, north, south, east, and west. All sites: mean=39.5%, SD=18%, range=7-80 (n=71); Bay/Delta: mean=41.7%, SD=16.5%, range=8.3-68.3% (n=15); Klamath: mean=52.1%, SD=27.6%, range=10.3-93.7% (n=19); Sacramento/San Joaquin: mean=48.3%, SD=24.0%, range=0-95% (n=37).
AVERAGE SHRUB COVER
Observers recorded the number of stems and percent cover of each species of shrub or small tree (height < 5m, diameter of stem at breast-height < 8cm) within 5m of each nest: All sites: mean=50%, SD=29.6%, range=1.25-100% shrub cover (n=67); Bay/Delta: mean=38%, SD=23% (n=7); Klamath: mean=66.3%, SD=24.5% (n=18); Sacramento/San Joaquin: mean=42.9%, SD=30.3% (n=46).
At study sites along the lower Sacramento River, BHGR nest-site selection was significantly positively correlated with percent shrub cover within 5m of the nest (regression coefficient=0.383, n=18). At the same sites, BHGR nest success positively correlates with percent cover provided by Wild Grape (V. californica) (P < .003) (Geupel et. al. 1997). When all PRBO sites are analyzed together, there is an insignificant positive correlation (P=0.08) between percent shrub cover within 50 m of a point count station and presence of BHGR at that point. Note the variability of average shrub cover for each bioregion. This may be another indicator of the plasticity of BHGR nesting ecology.
DOMINANT SHRUB SPECIES
The table below
summarizes which shrubs were most frequently found within 5m of nests and
the average percent cover provided by those species when they occur.
Klamath: Ponderosa Pine (P. ponderosa), Interior Live Oak (Q. wislizeni), Black oak (Q. kelloggii); Sacramento/San Joaquin: Box Elder (A. negundo), Elderberry (Sambuca spp.), and Fremont Cottonwood (P. fremontii); Bay/Delta: Currant (Ribes spp.), Coyote Bush (B. pilularis)
AVERAGE FORB COVER
Vegetation sampling around point counts at PRBO sites show a significant negative correlation between the percent herbaceous cover within 50m of the point and the presence of BHGR exists (P < 0.0001, n=234). However, where BHGR are present, herbaceous cover is not an indicator of abundance. Observers measured total "green" ground cover within 5m of each nest. Total green cover includes forbs, grass and sedge, shrubs, and ferns.
At sites along the lower Sacramento River, grass and sedge cover within 5m of a potential nest-site had significantly negative correlation (coefficient = -0.311) to BHGR nest-site selection. However, a positive correlation exists between grass cover around the nest and nest success (regression coefficient= 0.338, n=18) (Geupel et. al. 1997). All sites: mean=39%, SD=29.6%, range=0-100% (n=69); Bay/Delta: mean = 19.5%, SD=6.9%, range=10.8-28.8% (n=7); Klamath: mean = 65.2%, SD=27.1%, range=4,3-100 (n=18); Sacramento/San Joaquin: mean = 34.5%, SD=27.9%, range=0-90% (n=46). The high standard deviations reveal a wide range of percent cover provided by forbs around BHGR nests. This seems to follow the pattern of varied nest-site selection in BHGR.
Observers recorded the percent ground cover within 5m of each nests and broke its constituents into the following categorize:
All sites: mean=6.6 % SD=11.5%, range=0-56% (n=69); Bay/Delta: mean=0.4%, SD=0.94%, range=0-2.5% (n=7); Klamath: mean=3.6%, SD=8.2%, range=0-35.0% (n=18); Sacramento/San Joaquin: mean=8.6%, SD=12.8%, range=0-56.3% (n=46).
All sites: mean=33.95%, SD=35.82%, range=0-100% (n=69); Bay/Delta: mean=18.9%, SD=26.4%, range=0-67.5% (n=7); Klamath: mean=12.8%, SD=16.1%, range=0-50% (n=18); Sacramento: mean=45.8%, SD=37.8%, range=0-100% (n=46).
Only one nest of the sample had water within 5m of the nest. See Distance to Water below.
4. Leaf Litter:
All sites: Leaf litter: mean=31.5%, SD=28.3%, range=0-100%. (n=69) Litter depth: mean=20.5mm, SD = 15.9mm, range=0-79.8mm; Bay/Delta: Litter cover: mean=47.9%, SD=30.1%, range=1.3-85.0%. (n=7) Litter depth: mean=37.1mm, SD=25.7mm, range=5-80mm; Klamath: Litter cover: mean=18.3%, SD=23.9%, range=0-96.8%. (n=18) Litter depth: mean=10.8mm, SD=12.0mm, range=0-39.8mm; Sacramento/San Joaquin: (n=45) Litter cover: mean = 34.5%, SD=27.9%, range=0-88.8% (n=46). Litter depth: mean = 21.4mm, SD=18.7mm, range=1.2-77.1mm.
No data on presence of rock around nests recorded (some may have been categorized as "bare ground" below).
6. Bare Ground:
All sites: mean=21.7%, range=0-90 (n=69); Bay/Delta: mean=22.5%, SD=18.9%, range=0-43.8% (n=7); Klamath: mean=13.2%, SD=18.4%, range=0-62.5% (n=18); Sacramento/San Joaquin: mean=25.5%, range=25.2%, range=0-90% (n=46).
In riparian areas, BHGR nests are generally placed on flat ground. Some slopes of 1-2 degrees have been recorded (PRBO data).
No data available.
Observers measured the diameter at breast-height (DBH) of all trees within 11.3 meters of each nest, categorizing them as small (8 to 23cm dbh), medium (23 to 38cm dbh), and large (greater than 38cm dbh). ANOVA analysis shows that the number of small trees (8-23 dbh) significantly varies between the bioregions in the PRBO sample (P < 0.05). In the table below, the average number of trees in each size category are listed for each bioregion:
SNAGS AND STUMPS
Number of snags was not significant to nest success or to describe variability between bioregions. All sites: means were 0.7 snags and 0.32 stumps within a 11.3 m radius of the nest site; Bay/Delta: a little more than 1 snag per nest on average; Klamath and Sacramento/San Joaquin samples had less than 0.6 snags per nest.
DISTANCE TO WATER
No exact distances available. However, authorities report that BHGR show an affinity for streamsides and riparian groves (Weston 1947, Hill 1988, Zeiner 1990). Weston (1947) reports that BHGR prefer to nest along streamsides. Hill (1995) summarizes by saying that they are not found away from riparian habitat. Most BHGR nests found within the Cosumnes Preserve, in the San Joaquin bioregion, were within 50-300 meters of running water, probably closer on average to standing water (pers. obs.). Since all nest-searching took place in riparian habitat, PRBO data may be biased. Finally, BHGR have been observed drinking (Williams and Koenig 1980) and probably require a regular source of water (USDA Forest Service 1994).
As stated above, BHGR show a strong affinity for willow thickets (Weston 1947, Ritchisson 1983, Hill 1988, PRBO data). Other nest substrates include cottonwoods, varieties of oak, pine, and other deciduous trees (Weston 1947, Zeiner et. al. 1990, Gaines 1974). All sites: 51% were built in a willow species (Salix spp.) total: 18% were in unspecified willow (Salix spp), 12.5% in Sandbar willow (S. exigua), 11% in Arroyo Willow (S. lasiolepis), and 8% in Gooding's Black willow (S. goodingii) (n=72); Bay/Delta: 46.7% unidentified willow (Salix spp.), 20% Red Elderberry (S. callicarpa) and 13.3% Red Alder (A. rubra) (n=15). Weston (1947) reported from records that of 120 nests, 35% were in willows, 12% in Coast Live Oak (Q. wislizeni), and the remainder in deciduous trees and shrubs. It is unclear if his sample was restricted to the Berkeley area. He describes, 6 of 8 nests that were Coast Live Oak (Q. wislizeni); Klamath: 31.1% unidentified willow (Salix spp.), 21.1% Manzanita (Arctostaphylos spp.), and 10.5% Red Willow (S. laevigata)(n=19); Sacramento/San Joaquin: 23.7% Sandbar Willow (S. exigua), 21.1% Arroyo Willow (S. lasiolepis), 15.8% Gooding's Black Willow (S. goodingii), and 10.5% Wild Grape (V. californica) (n=38). Wild grape commonly combines with other plants and provides nest substrate and cover for BHGR nests in the Sacramento and San Joaquin bioregions (pers. obs.).
Open cup. Weston (1947) reports a range in structure from a "saucer-like platform to one resembling a cup". Hill (1995) describes it as "bulky and loosely constructed." Materials include small twigs, rootlets, pine needles, and finer materials for lining (Weston 1947, Hill 1995, Ritchisson 1983, pers. obs.). Bent (1968) reported that some BHGR nests were so thinly constructed that one could see through the bottom. This may be to better thermoregulate the eggs (Dawson 1923). Weston (1947) reported having no such "see-through" nests on his study plot in Berkeley; however, I have observed thin nests with "see-through" bottoms at the Cosumnes Preserve in Sacramento County, CA, where temperatures in the breeding season often exceed 100 degrees Fahrenheit.
INITIATION OF NESTINGWhile there are no published data recording the interval between arrival on breeding grounds and initiation of nesting, first nests may give an indication of arrival dates. Bay/Delta: one nest with first egg laid estimated as 4/27/97 in Marin Co. Klamath: Two nests found while building on 5/5/93 in Shasta Co. Sacramento/San Joaquin: Tehama Co.: An abandoned nest with 6 eggs (no Brown-heade Cowbird (BHCO) eggs reported) was found 5/18/94. Another with 6 eggs (no BHCO eggs reported) was found 5/24/95. Another was found fully built on the same day. At the Cosumnes River Preserve in Sacramento Co., one nest found during building on 5/15/95. DISPLAYSWhen courting a female, BHGR males sing loudly from a perch, occasionally flying upwards with wings and tail spread while singing, and then returning to their original perch (Weston 1947, Hill 1995). Ritchisson (1983) reports similar displays between territorial males in Utah. Females also sing, sometimes while foraging with the male, reacting to other females, or maintaining contact with their young ( Weston 1947, Ritchisson 1983).
Hill (1995) reports BHGR in New Mexico appear to be socially monogamous with no extra-pair copulations observed in 1,500 hours of field observation.
All sites: mean=2.95, with a range of 1-4 (n= 42); Sacramento/San Joaquin: clutch size mean = 3.33, range 3-4 (n = 6); Klamath: mean=3, range=2-4, n= 8; Bay/Delta: mean=2.4, range=1-3, n=5. Weston (1947) estimated an average clutch size of 3.31 (n = 192 clutches). It is unclear if all his sample came from nests in this region. Hill (1995) reports that generally there are 2-5 eggs per clutch (3 is the mode and the median). Ritchisson (1983) found clutches of 2-4 eggs (mean = 2.8, n = 14) in Utah.
Females and males share incubation data during the day (Weston 1947, Ritchisson 1983, Hill 1995, pers. obs.). Females spend somewhat more time on the nest than males during the day and brood the entire night (Ritchisson 1983).
Both sexes are vocal around the nest. Both males and females occasionally sing and call when arriving to, departing from, or even sitting on the nest (Weston 1947, Ritchisson 1983, pers. obs.).
Klamath and Sacramento: range=12-14 days, (n=11); Bay/Delta: 12 days, with all eggs hatching in a 24 hr. period (Weston 1947). Ritchisson (1983) observed incubation periods from 12-14 days in Utah, with all eggs hatching within a 24-48 hour period (usually within a day).
DEVELOPMENT AT HATCHING
All sites: mean=13, range 11-15, n=22; Klamath: mean=13, range =11-15, n=3; Sacramento/San Joaquin: mean=11.5, range=10-13, n=13; Bay/Delta: one record of 11 days. Weston (1947) observed parents incubating and feeding nestlings for 12 days before the young fledged. Ritchisson (1983) reports nestling periods of 9-14 days, with a mean=11.5 days (n=21).
Both parents feed young during nestling and post-fledging periods. Females may use song to communicate with young, in conjunction with calls (Ritchisson 1983). Since males depart earlier from breeding grounds (see above), females may sometimes be left with the duty of feeding the fledged young (Weston 1945, Ritchisson 1983).
NUMBER OF BROODSFrom PRBO data, observed BHGR pairs produce no more than one brood of young each year, but this needs more study. Hill (1995) reports that no second brood attempts have been recorded.
In California, only 2 of 77 nests in the PRBO data set were parasitized by the Brown-headed Cowbird (Molothrus ater). One occurred in the San Joaquin bioregion, and the other in the Bay/Delta. Both nests were depredated while still in the incubation stage (PRBO data). Other sources also report brood parasitism of BHGR as rare (Laymon 1984, Hill 1995).
BHGR show no anti-parasite behavior, despite a historical over lap in range with the BHCO. They seem tolerant of other species within the vicinity of their nest, though they often become agitated and vocal when humans approach (Weston 1947, Hill 1995). Weston (1947) observed both a Wrentit (Chamea fasciate) and a Bushtit (Psaltriparus minimus) gleaning insects from the rim of an active BHGR nest, with no reaction from the BHGR parents. On the Cosumnes Preserve (in the San Joaquin bioregion), I observed a Brown-headed Cowbird sitting on a BHGR nest while the BHGR female silently perched on a nearby branch.
Because both sexes incubate, the nest is rarely left unattended for longer than a few seconds (Weston 1947, Ritchisson 1983), reducing the opportunity for depredation and parasitism. Finally, BHGR may be large enough to raise their own young along with those of the parasite, minimizing the loss in productivity (Laymon 1984).
P. m. maculatus: breed west of the Sierran Ridge from sea level up to about 8000 ft. (Grinnell and Miller 1944, Small 1994)
P. m. melanocephalus: breed in eastern desert regions of California, at elevations between 5000 ft. to 9000 ft. (Grinnell and Miller 1944, Zeiner et. al. 1990, Small 1994).
Grinnell and Miller (1944) describe the BHGR as occurring in areas with "extensive edge" conditions. Weston (1947) remarks that they are seen most often in "open woods." Strong and Bock (1990) report the presence of BHGR in small riparian corridors less than 2 hundred meters in length and often 20-50m in width. While the clearing of some woods, such as those for ski runs (Hill 1995), may be beneficial (at least locally and temporarily), the over-all loss of habitat is not (Small 1994), since they cease to be detected in areas where suitable habitat is destroyed (Klebenow and Oakleaf 1984, Small 1994).
Strong and Bock (1990) found BHGR in both small (200m continuous length x 20-50m width) and large (greater than 1000m continuous habitat) riparian woodlands in southeastern Arizona.
Removal of riparian woodlands for industrial, agricultural, or water management purposes will reduce available habitat. Flood control projects, channelization, and grazing all affect the dynamics of a riparian system (Gaines 1977, Ohmart 1994), preventing the early successional growth (willows, young cottonwoods) that BHGR appear in most frequently. But because BHGR are found in a variety of habitats, the impact may not appear as immediate as for other species with a stronger association for riparian habitat.
See also Brood Parasitism, Predators, and Pesticides sections.
ADJACENT LAND USE
BHGR nests have been found adjacent to and on agricultural areas, ranches, and orchards (Grinnell and Miller 1994, Small 1994, PRBO data). Rural human disturbance may often have a positive impact on the local population of jays and other corvids (Grinnell and Miller 1944). Hill (1988) found that jays were the primary nest predators of BHGR in New Mexico. It may be that any land usage that increases the density of the local jay population may negatively impact on BHGR in the area.
Human disturbance such as grazing and equestrian stables may increase BHCO density in an area (Verner and Ritter 1983), and may eventually have a negative impact on the local BHGR population. While parasitism of BHGR by BHCO is rare (Hill 1988, USDA Forest Service 1994, PRBO data, an increase in cowbird density may eventually result in higher parasitism rates.
Found breeding in California throughout the arid Central Valley and up into the cool elevations of the Sierra range, BHGR seem tolerant of a wide variety of climates. Their large size may lend them resistance to cooler temperatures, and shady areas may provide relief in warmer regions. Weston (1947) observed a female holding her wings out to form an umbrella for her young during rainy days. Others have hypothesized that the thin nature of BHGR nests may be an attempt to cool eggs in hot or humid areas (Dawson 1923). BHGR are seen throughout the state while on migration, seemingly tolerant of the cool coastal islands as well as the heat of the eastern deserts (Small 1994).
No information available. However, as insectivores, BHGR may be affected by pesticide use in adjacent lands. More study is needed.
Adults are probably targets of common avian predators (Hill 1995). Because they may nest on or near ranches and other human settlements, BHGR probably face depredation pressure from domestic and feral cats. Western Scrub-Jay and Steller's Jay are principle predators on nests. Hill (1988) reports that nest success is significantly negatively correlated with the jay activity near the nest. Both sexes defend the nest against known avian predators, as well as humans. Hill (1995) reported that they may use their stout bills to attack smaller predators, such as the Western Scrub-Jay.
DEMOGRAPHY AND POPULATION TRENDS
PRBO banding data (banding years given in parentheses): Bay/Delta: Palomarin Field Station (Marin County): 48% males and 52% females (1971-96); Klamath: Shasta County: 49% males and 51% females (1993-96); Sacramento: Tehema and Glenn Counties: 53% males and 47% females (1994-96); San Joaquin: Sacramento County: 43% males and 57% females (1995-96).
BBS data show no significant trends of Black-headed Grosbeak numbers in California. From 1966-1996, the estimated "trend" is 0.4, with a 95% confidence interval of -1.5.
PRBO banding data taken in the Bay/Delta region from 1978 to 1997 show significant decline. The data were square-root transformed to normalize residuals, and a linear regression showed a significant negative coefficient of -.0066 per year, or -0.66% (P < .004). Over the 22 year period, this describes a 14.5% decline in captures. However, this number may not be biologically significant: the mean and median captures per 10,000 net hours was 3 BHGR (0.03 per one hundred net hours), range = 0-11 per 10,000 net hours, and the SD = 2.6. Variations in captures numbers of a just a few birds every year may bias the data.
In an analysis of 25 years of spring and fall migration data on the Farallon Islands near San Francisco, CA, Pyle (1994) reports no significant changes in the number of BHGR captured. Pyle's findings were generally in agreement with other long-term trend analyses.
MANAGEMENT ISSUES AND OPTIONS
EXOTIC SPECIES INVASION/ENCROACHMENT None reported.
ASSOCIATED BIRD SPECIES
Other species with strong associations to early successional riparian woodlands and willow stands. The following species appear closely associated with habitats similar to those for which the BHGR shows an affinity:
Pacific-slope Flycatcher (Empidonax difficilis), Western Wood-Pewee (Contopus sordidulus), Spotted Towhee (Pipilo maculatus), Ash-throated Flycatcher (Myiarchus cinerascens)=Cottonwood-willow and oak-cottonwood forests.
Airola, D. A. and R. H. Barrett. 1985. Foraging and habitat relationships of insect-gleaning birds in a Sierra Nevada mixed-conifer. Condor 87:205-216.
American Ornithologists' Union. 1983. Check-list of North American birds. 6th ed. Am. Ornithol. Union, Washington, D.C.
Anderson, B. W., and R. J. Daughtery. 1974. Characteristics and reproductive biology of grosbeaks (Pheucticus) in the hybrid zone in South Dakota. Wilson Bull. 86:1-11.
Bent, A. C. 1968. Pheucticus melanocephalus melanocephalus(Swainson), Rocky Mountain Black-headed Grosbeak. Pp. 55-58 in Life histories of the North American cardinals, grosbeaks, buntings, towhee, finches, sparrows, and allies (O.L. Austin jr., ed.) U.S. Natl. Mus. Bull. 237.
Dawson, W. L. 1923. The birds of California. S. Moulton Co., San Diego, CA.
Grinnell, J. and Miller, A. H. 1944. The distribution of birds of California. Pacific Coast Avifauna Number 27, 608pp.
Gaines, D. A. 1974. A new look at the nesting riparian avifauna of the Sacramento Valley, California. Western Birds 5:61-79.
Gaines, D. A. 1977. The valley riparian forests of California: Their importance to bird populations. in Ann Sands (editor) Riparian Forests in California: Their ecology and conservation. Institute of Ecology Publication 15, Univ. of California, Davis, CA. 57-85.
Hill, G. E. 1988a. Age, plumage brightness, territory quality, and reproductive success in the Black-headed Grosbeak. Condor 90:379-388.
Hill, G. E. 1988b. The function of delayed plumage maturation in male Black-headed Grosbeaks. Auk 105:1-10.
Hill, G. E. 1995. Black-headed Grosbeak (Pheucticus melanocephalus). In Birds of North America, No. 143 (A. Poole and F. Gill eds.). The Academy of Natural Sciences, Philadelphia, and the American Ornithologists' Union, Washington, D.C.
Klebenow, D. A. and R. J. Oakleaf. 1984. California Riparian Systems: Ecology, Conservation, and Productive Management. University of California Press. 1035pp.
Laymon, S. 1984. Riparian Bird Community Structure and Dynamics: Dog Island, Red Bluff, California. California Riparian Systems: Ecology, Conservation, and Productive Management. University of California Press. 1035pp.
Ohmart, R.D. 1994. The effects of human-induced changes on the avifauna of western riparian habitats. Studies in Avian Biol. 15:273-285.
Pyle, P., N. Nur and D. F. DeSante. 1994. Trends in nocturnal migrant landbird populations at southeast Farallon Island, California 1968-1992. Studies in Avian Biol. 15:58-74.
Ritchisson, G. 1983. Breeding biology of the Black-headed Grosbeak in northern Utah. West. Birds 14:159-167.
Shuford, W. D. 1993. The Marin County breeding bird atlas: A distributional and natural history of coastal California birds. Bushtit Books.
Small, A. California Birds: 1994. Their status and distribution. Ibis Publishing Co. 342pp.
Strong, T. R. and C. E. Bock. 1990. Bird species distribution patterns in riparian habitats in southeastern Arizona. Condor 92:866-885.
United States Department of Agriculture Forest Service. 1994. Neotropical Bird Reference Book, Volume 1. United States Forest Service, Pacific Southwest Region. 832 pp.
Weston, H. G. 1947. Breeding behavior of the Black-headed Grosbeak. Condor 49:54-73.
Whitmore, R. C. 1975. Habitat ordination of passerine birds of the Virgin River Valley, southwestern Utah. Wilson Bull. 87:65-73.
Williams, P. L. and W. D. Koenig. 1980. Water dependence of birds in a temperate oak woodland. Auk 97:339-350.
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