Black-throated Green Warbler

Distribution and Abundance

• BBS Map
• Breeding range northeastern United States and extending westward into southern Canada; also extends southward along Appalachian Mountains at relatively high altitudes. 
• Breeding range expanded westward in Canada since mid 1950s (comparison of 5th AOU Checklist 1957 and 6th AOU Checklist 1983).
• Breeding densities range from 0.3-2 pairs/ha; numbers of breeding individuals differ markedly with habitat.
• Overall numbers remained nearly constant over first 25 years of BBS; slight decrease (0.8%) during 1982-1991.
• Winters primarily in Mexico and Central America; recent winter range expansion in southern Florida and southern Texas. 

Habitat

• Characteristic inhabitant of boreal coniferous forest and transitional area between coniferous and deciduous forests, associated with conifers, but also extending into mixed coniferous-deciduous forests or completely deciduous forests at southern edge of range. 
• In Wisconsin, of 550 observations from 1995-2000 where habitat was reported, 82.7% were in upland mixed forest (with equal preference for mixed forests with hemlock, pine or spruce) or upland hardwood forest, particularly maple (WSO 2002). In Michigan, this species breeds most commonly in mature mesic mixed forest; secondarily in mature mesic deciduous and pure coniferous forest (Brewer et al. 1991).
• Found in red and white spruce forests in Maine; white pine forests, pitch pine coastal forest, and red cedars invading old fields in Massachusetts; jack pine barrens in Michigan; middle-aged and mature forests as well as conifers in West Virginia and New York (Brooks 1940, Kendeigh 1946, Bent 1953, Morse 1976). In eastern Canada, most common in balsam fir, fir-spruce, and fir-deciduous areas (Erskine 1980, 1984).
• Size of territory varies with habitat: smaller where favored conifers are dense than in mixed coniferous-deciduous forest. In favored red spruce habitats, territories may be as small as 0.25 ha, in white spruce may be half again as large (Morse 1976). In Maine, do not breed on islands with forests smaller than 0.35 ha (Morse 1977). Five territories in Ontario ranged from 0.3 - 0.9 ha (Kendeigh 1945).
• Nest site usually in conifers, especially spruce, in coastal Maine (Morse 1968); white pines immediately inland; hemlock in west Pennsylvania and New York (Bent 1953); in central Appalachians, both deciduous and coniferous forests (Brooks 1940, Hall 1983); inland in northeastern United States and eastern Canada, often in predominately deciduous forests.
• During migration, found in all woody habitats, including forest edge (Keast 1980, Graber et al. 1983), both deciduous and coniferous.
• In Central America, usually montane regions (Ridgely 1989) where they occupy primarily canopies of tall forests. Will occupy lumbered-over areas if some emergent vegetation remains (Lynch 1989); will also occupy native canopy trees of coffee plantations (Terborgh 1989).

Behavior

• Diet during breeding season mostly insectivorous with caterpillars often comprising the largest proportion of volume (Morse 1976).
• Adults generally intolerant of approach of conspecifics of same sex during breeding season. Males exhibit elaborate patterns of chasing and fleeing behavior in flight.  Females also seen to attack each other in apparent territorial encounters. Males guard females, often remaining close to them during nest-building and immediately before egg-laying.
• No clear evidence for interspecific territoriality; differences in niche exploitation from co-occurring congeners minimize its likelihood (Morse 1968).
• Mating system monogamous, but occasional presence of quiet second males on territories (Morse 1970) consistent with outside copulations.
• During migration, may join mixed-species flocks (Morse 1970).
• In some areas, individuals solitary on wintering grounds (Bent 1953); in others, appear to be social with others (Tramer and Kemp 1980).

Parasitism and Predation

• Brood parasitism by Brown-headed Cowbird although reported to be an infrequent victim (Friedmann 1963, 1977); probably mostly confined to individuals nesting near edges.  Adults mob female cowbirds on breeding grounds, but accept cowbird eggs and nestlings.
• In Wisconsin, only 2 of 477 confirmed Brown-headed Cowbird observations from 1995-2000 listed Black-throated Green Warbler as host species (WSO 2002). 
• Population densities of host have not declined in extensive forested area in Maine over period when cowbird parasitism has been advanced as important cause of neotropical migrant decline (Morse 1976).
• Sharp-shinned and Cooper's hawks have traditionally been the most important predators of adults; red squirrels and Blue Jays most important predators of eggs and young.

Conservation and Management

• Numbers thought to have decreased during period of heavy use of DDT. Records of deaths associated with DDT application involved control of Dutch elm disease during spring migration (Nickell and Bernard 1961); also marked decrease associated with application of fenitrothian in spruce budworm control (Pearce et al. 1979).
• Logging curtails populations dependent on coniferous forest for breeding habitat, although this species will occupy middle-aged, second-growth conifers. Disappeared from several small northeastern forests less than 100 ha that became isolated from similar habitat (Askins and Philbrick 1987). Disappeared from mixed forest thinned to 45% of its natural density (Freedman et al. 1981).
• Numbers of spring migrants declined markedly from 1980-1989 in Massachusetts (Hill and Hagan 1991). 
• On the other hand, an analysis from 1969-1986 in unbroken northern mixed forests in New Hampshire revealed no change in numbers of breeding birds (Holmes and Sherry 1988) and 1992 censuses of spruce forests censused in the late 1960s and early 1970s in Maine did not show declines.
• BBS trend results from 1966-2000 (Sauer et al. 2001) in the Northern Spruce-Hardwoods region indicate a significant population increase of the Northern Parula in this region  (2.1, p=0.00 Trend Graph S28); in the Great Lakes Transition region, this species may be experiencing an increase, but more data are needed (1.9, p=0.71 Trend Graph S20). Survey-wide (US and Canada), this species has overall shown a slight but significant increase (0.9, p=0.02 Trend Graph SUR).

References

• Askins, R.A. and M.J. Philbrick. 1987. Effect of changes in regional forest abundance on the decline and recovery of a forest bird community. Wilson Bull. 99:7-21.
• Bent, A.C. 1953. Life histories of North American wood warblers. U.S. Natl. Mus. Bull. 203.
• Brewer, R., G.A. McPeek and R.J. Adams, Jr. 1991. The atlas of breeding birds of Michigan. Michigan State University Press, East Lansing.
• Brooks, M. 1940. The breeding warblers of the central Allegheny Mountain region. Wilson Bull. 52:249-266.
• Erskine, A.J. 1980, 1984. A preliminary catalogue of bird census plot studies in Canada, parts 4 and 5. Can. Wildl. Serv. Prog. Notes Nos. 112, 144.
• Freedman, B., C. Beauchamp, I.A. McLaren and S.I Tingley. 1981. Forestry management practices and populations of breeding birds in Nova Scotia. Can. Field-Nat. 95:307-311.
• Friedmann, H. 1963. Host relations of the parasitic cowbirds. U.S. Natl. Mus. Bull. 233.
• Friedmann, H., L.F. Kiff and S.J. Rothstein. 1977. A further contribution to knowledge of the host relations of parasitic cowbirds. Smithson. Contrib. Zool. 235.
• Graber, J.W., R.R. Graber and E.L. Kirk. 1983. Illinois birds: wood warblers. Biological Notes no. 118. III. Nat. Hist. Surv., Urbana, IL.
• Hall, G.A. 1983. West Birginia Birds. Carnegie Museum of Natural History Special Publication No. 7. Pittsburgh, PA.
• Hill, N.P. and J.M. Hagan. 1991. Population trends of some northeastern North American landbirds: a half-century of data. Wilson Bulletin 103:165-182.
• Holmes, R.T. and T.W. Sherry. 1988. Assessing population trends of New Hampshire forest birds: local vs. regional patterns. Auk 105:756-768.
• Keast, A. 1980. Migratory Parulidae: what can species co-occurrence in the north reveal about ecological plasticity and wintering patterns?, pp. 457-476 in Migrant birds in the neotropics (A. Keast and E.S. Morton, eds.). Smithson. Inst. Press, Washington, D.C.
• Kendeigh, S.C. 1946. Breeding birds of the beech-maple-hemlock community. Ecology 27:226-245.
• Lynch, J.F. 1989. Distribution of overwintering nearctic migrants in the Yucatan Peninsula, I: General patterns of occurrence. Condor 91:515-544.
• Morse, D.H. 1968. A quantitative study of foraging of male and female spruce woods warblers. Ecology 49:779-784.
• Morse, D.H. 1970. Ecological aspects of some mixed-species flocks of birds. Ecol. Monogr. 40:119-168.
• Morse, D.H. 1976a. Variables determining the density and territory site of breeding spruce-wood warblers. Ecology 57:290-301.
• Morse, D.H. 1976b. Hostile encounters among spruce-wood warblers. Anim. Behav. 24:764-771.
• Morse, D.H. 1977. The occupation of small islands by passerine birds. Condor 79:399-412.
• Nickell, W.P. and R.F. Bernard. 1961. Bird mortality in the Dutch elm disease program in Michigan. Bull. Cranbrook Inst. Sci. 41:11-44.
• Pearce, P.A., D.E. Peakall and A.J. Erskine. 1979. Impact on forest birds of the 1976 spruce budworm spray operation in New Brunswick. Can. Wildl. Serv. Prog. Notes No. 97.
• Ridgely, R.S. 1989. Guide to the birds of Panama with Costa Rica, Nicaragua, and Honduras.
• Terborgh, J. 1989. Where have all the birds gone? Princeton University Press, Princeton, NJ.
• Tramer, E.J. and T.R. Kemp. 1980. Foraging ecology of migrant and resident warblers and vireos in the highlands of Costa Rica, pp. 285-296 in Migrant birds in the neotropics (A. Keast and E.S. Morton, eds.). Smithson. Inst. Press, Washington, D.C.
• Wisconsin Society for Ornithology. 2002. Wisconsin Breeding Bird Atlas.