Black-throated Blue Warbler Habitat
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Black-throated blue warbler, Dendroica caerulescens
Use of Study Area Resources:
Reproduction. "Breeds from the Great Lakes region ...east through southern Canada to the Maritimes. The range extends south through the New England states, higher elevations of New York and Pennsylvania, then southward through the high Appalachians to northern Georgia" (Cornell Laboratory of Ornithology)". Winters mainly in the Caribbean and parts of Central America (Johns).
Habitat Requirements (Reproduction):
Cover: Considered a forest-interior species (Holmes 1994, Johns). "Breeds mainly in large, mostly continuous tracts of northern hardwood or mixed coniferous forests... A key component of this species' habitat is a dense shrubby understory, especially consisting of hobblebush and other viburnum species, as well as striped maple, rhododendron, or regenerating conifers" (Holmes 1994, Johns).
"The nest is usually located in the dense shrub layer of either deciduous or mixed coniferous/ deciduous forest, frequently in broad-leaved evergreen shrubs (e.g., Mountain laurel, rhododendron, and viburnum), conifer saplings, deciduous shrubs, or saplings" (Holmes 1994).
Foraging: "Black-throated Blue Warblers feed on insects and other small invertebrates. During the breeding season they forage from the ground to high in the forest canopy, with males often foraging higher than females. This bird seems to locate a high percentage of prey from the lower surfaces of leaves. Small fruits are often eaten during the winter. Major foods eaten during the breeding season include larval Lepidoptera and beetles, adult fly species and spiders." (Johns).
Area Requirements: Robbins et al. (1989) observed that forest patches of 1000 ha had 50% of the probability of occurrence of black-throated blue warblers of that at 3000 ha; minimum patch size at which they were seen in middle Atlantic state was 1120 ha.
The Breeding Bird Atlas (BBA) data for Maine, New Hampshire and Massachusetts (courtesy of the University of Vermont COOP Unit) were used to identify the range of the black-throated blue warbler within the study area. BBA blocks and Breeding Bird Survey (BBS) sites in which black-throated blue warblers were known to occur were used to select USDA Forest Service Ecological subunits (Keys et al. 1995); habitat mapping then was restricted to these areas.
Based on the above sources, the preferred vegetation appears to be large tracts of deciduous and mixed forest, with appropriate shrub understory, remote from openings or edges. We obtained BBS data, collected from 1997 through 2000, and subdivided the 372 occurrences in the study area into half for model development, and half for testing the final model. We compared our various land cover classes (see table, below) within 186 occurrence areas to land cover found around all 3723 BBS sites in the study area. Upland deciduous and coniferous forest covers were > 1.3 times more common at black-throated blue warbler occurrence sites than at BBS sites overall ; upland mixed forest was about 1.2 times more abundant than for the BBS sites overall. We also examined the forest composition at 56 study plots in eastern Canada at which black-throated blue warblers were found (Kennedy et al. 1999). Of these, 35 (63%) were hardwood dominated (having > 60% hardwoods), 9 (16%) were mixed (hardwoods 50%<> 40%), and 12 (21%) were coniferous dominated (hardwoods 0 to < 40%). Fewer than half of the sites had information on understory characteristics, and these were primarily deciduous. The corresponding statistics for all 360 plots in this area were 26% hardwoods, 6% mixed, and 23% coniferous, indicating a selection for hardwoods and mixed by this bird.
Using both the published findings and our BBS land cover associations, we rated deciduous, mixed and coniferous forest as appropriate for this species, with decreasing levels of suitability (see table, below). In order to deal with the configuration and size of forest patches, we re-calculated the scores of cells as the sums of these basic cover scores for all cells within a 10 cell by 10 cell window. Interior cells within patches consisting entirely of hardwoods would have a score of '100' (10 times 10 at a score of 1.0); other cells would range down to 0, depending on the proximity and cover types of cells within the window. From this grid we retained as habitat cells with a range of scores encompassing the majority of BBS occurrences reserved for model development. To further select forest-interior areas, we retained only patches over 1 ha and within a contiguous forest area of more than 1000 ha.
|Cover Types||Cover Suitability
(0 - 1 scale)
|Upland deciduous forest||1.0|
|Upland coniferous forest||0.5|
|Upland mixed forest||0.7|
|PEM, L2EM||Lake/pond, emergent vegetation|
|PFOcon||Palustrine forest, conifer|
|PFOdec||Palustrine forest, deciduous|
|PSSdec||Palustrine scrub shrub, deciduous|
|PSScon||Palustrine scrub shrub, conifer|
|PAB, L2AB||Lake/pond, aquatic vegetation|
|L1UB, PUB||Lake/pond, unconsolidated bottom|
|L2US||Lake, unconsolidated shore|
|L2RS||Lake, rocky shore|
|R1UB||Riverine subtidal unconsolidated|
|E1AB||Estuarine subtidal vegetated|
|E1UB||Estuarine subtidal unconsolidated bottom|
|E2AB||Estuarine intertidal algae|
|E2EM||Estuarine intertidal emergent|
|E2RS, R1RS||Estuarine, tidal river rocky shore|
|E2SS||Estuarine intertidal shrub|
|E2US||Estuarine intertidal unconsolidated shore|
|M1AB||Marine subtidal vegetated|
|M1UB||Marine subtidal unconsolidated bottom|
|M2AB||Marine intertidal algae|
|M2RS||Marine intertidal rocky shore|
|M2US||Marine intertidal unconsolidated shore|
Model testing: The 'reserved' black-throated blue warbler occurrences along Breeding Bird Survey routes were used to test the habitat map from the above model. We compared the presence of habitat near these sites to that for a random set of 787 upland points within the species range. Of the 186 sites with birds, 181 had mapped habitat, while 673 sites out of the 787 randomly distributed sites had habitat. Birds occurred in areas mapped as having habitat more frequently than would be expected by chance (Chi-square significant at < .000012).
Cornell Laboratory of Ornithology. Birds in Forested Landscapes. http://birds.cornell.edu/bfl/speciesaccts/btbwar.html; downloaded 11/01
Johns, M. Black-throated blue Warbler (Dendroica caerulescens). North Carolina Partners in Flight, species profiles, http://faculty.ncwc.edu/mbrooks/pif/Bird%20Profiles/black-throated blue.htm; downloaded 10/01.
Keys, J.E., Jr., J.C. Carpenter, S. Hooks, F. Koenig, W.H. McNab, W. Russell and W. Smith. 1995. Ecological units of the eastern United States - first approximation (map and booklet of map unit tables), USDA Forest Service. Atlanta, GA.
Holmes, R.T. 1994. Black-throated blue warbler, Dendroica caerulescens. In A. Poole and F. Gill, (eds.) The Birds of North America, No. 87. The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, D.C.
Kennedy, J.A., P. Dilworth-Christie and A.J. Erskine. 1999. The Canadian Breeding Bird (Mapping) Census Database. Technical Report Series No. 342, Canadian Wildlife Service, Ottawa, Ontario. Report and data available at: http://www.cws-scf.ec.gc.ca/birds/db_cens_e.cfm
Robbins, C.R., D.K. Dawson and B.A. Dowell. 1989. Habitat area requirements of breeding forest birds of the middle Atlantic states. Wildl. Monogr. 103:1-34.