Bicknell's Thrush Habitat Model
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Draft Date:
March, 2001

Species:
Bicknell's thrush, Catharus bicknelli.  Historically was regarded as a subspecies of the gray-cheeked thrush (Ouellet 1993).

Use of Study Area Resources:
Reproduction. Breeding range is limited to the Appalachian Mountains of New York and New England into portions of southern Quebec and the Maritime Provinces (Ouellet 1993). Within the study area they are found in the Appalachian Mountains of New Hampshire and Maine, but historically occurred on mountains in Massachusetts.  Although there are historical records of scattered occurrences in the 'down east' coast of Maine, Atwood et al. (1996) regards this is not likely to be within their breeding range. Winters in central/south America and in the West Indies (Wallace 1939).

Habitat Requirements:
Cover: Bicknell's thrush is found on mountain tops over 3000 feet (915 m) in the northeastern U.S., but at lower elevations in higher latitudes, nesting in stunted coastal spruce forests in New Brunswick (Wallace 1939, Atwood et al. 1996). Preferred vegetation consists mostly of balsam fir and red spruce, often in thick stands as second growth. In the United States it is a strictly montane bird (Wallace 1939), found in typically steep and rugged terrain (Rimmer et al. 1996). "The zone occupied is typically sub-Canadian, comprising dense coniferous stands of balsam and spruce, mingled with birch. Though tall stands of timber may characterize the 3000' zone, upwardly it merges into the matted, stunted, storm-blasted shrubbery, typically featuring alpine summits.” (Wallace 1939). Atwood and Rimmer (1994) observed about 5 times as many nests in spruce-fir areas than in hardwood conifer vegetation.

Most occurrence data in the study area concurs with the high elevation habitats described (Atwood et al. 1996, Atwood and Rimmer 1994, Ouellet 1993, Wallace 1939), but use of two low-elevation sites has been documented on the extreme eastern coast of Maine (Quoddy Head State Park and Boot Head, Atwood et al. 1996). Atwood and Rimmer (1994) note that these locations are vegetatively similar to the more typical high elevation habitats, and that harsh coastal weather conditions result in a forest structure which resembles mountaintop conditions.

Foraging: Summer foods are predominantly insects, including caterpillars, weevils, ants, wasps, wild bees, grasshoppers, beetles, cicadas, and flies, (Bent 1964, Forbush 1929). Forbush (1929) described the results of stomach content analysis as almost three-fourths animal and just over one-fourth vegetable (including pokeweed, sumac, alder, dogwood, elder, bayberry, black cherries, and wild grapes).

Management issues: Habitat degradation may be caused by acid precipitation, construction of communications facilities, or development of ski resorts (Atwood et al. 1996). Crows and ravens are known to interfere with nesting (Wallace 1939).

Model:
Atwood et al. (1996) developed a habitat model for Bicknell's thrush and found that occurrences were associated with vegetation, elevation and latitude.  Our model applied the same elevation and, as far as possible, cover type parameters.  We also integrated sites of known past occurrences. We selected as 'potential habitat' areas with elevations at or over 3000', based on digital contour maps (Maine) or digital raster graphics (New Hampshire). Lower elevations were regarded as likely to be unsuitable. Bicknell's thrush occurrences in the Northeast are listed by mountain name in Atwood and Rimmer (1994), supplemented with information from Tom Hodgman (Maine Dept. of Inland Fisheries and Wildlife). These were digitized as point locations.  Where 'potential habitat' coincided with observed use, these areas were scored according to their cover type (see table, below). Point occurrences at lower elevations were buffered 100 m, and these areas also were scored according to cover type. Other areas having elevations at or over 3000' and appropriate cover type, but not known to be used, were scored at 0.5 times the nominal values in the following table.

NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest

1.0*

Upland mixed forest

0.2*

Grassland
Upland scrub/shrub

1.0*

Cultivated
Developed
Bare ground
PEM, L2EM Lake/pond, emergent vegetation
PFOcon Palustrine forest, conifer

1.0*

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
Rper Riverine perennial
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
NOTES *at elevations > 3000'

Model testing: Available occurrence information was used in mapping and thus the model was not tested.

Sources:
Atwood, J.L., C.C. Rimmer, K.P. McFarland, S.H. Tsai and L.R. Nagy. 1996. Distribution of Bicknell's Thrush in New England and New York. Wilson Bull. 108(4):650-662.

Atwood, J.L. and C.C. Rimmer. 1994. An Expanded Investigation of the Population Status of Bicknell's Thrush in the Northeastern United States. Manomet Observatory for Conservation Sciences and Vermont Institute of Natural Sciences. 34 pages.

Bent, A.C. 1964. Life Histories of North American Thrushes, Kinglets and their Allies. Dover Pub. NY. pp 199-217.

Forbush, E.H. 1929. Birds of Massachusetts and Other New England States. Commonwealth of Massachusetts, Norwood, MA.

Ouellet, H. 1993. Bicknell's thrush: taxonomic status and distribution.  Wilson Bull. 105(4): 545-754.

Rimmer, C.C., J.L. Atwood, K.P. McFarland and L.R. Nagy. 1996. Population density, vocal behavior, and recommended survey methods for Bicknell's thrush. Wilson Bull. 108(4):639-650.

Wallace, G. 1939. Distribution and migration of Bicknell's thrush. Proc. Boston. Soc. Nat. Hist. 41(6):211-402