Wood Thrush Habitat Model
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Draft Date:
February, 2001

Species:
Wood thrush, Hylocichla mustelina

Use of Study Area Resources:
Reproduction, throughout study area. The wood thrush breeds from northern Gulf of Mexico to southeastern Canada and winters in southern Mexico through Panama.

Habitat Requirements:
Cover. The wood thrush breeds in cool mature, lowland, mixed or more typically, deciduous forests, particularly mesic to damp woodlands with an abundance of saplings, often near swamps or water (Kendeigh 1948, Dilger 1956, DeGraaf and Rappole 1995). It prefers a shrub sub-canopy layer, shade, and an intermediate soil moisture regime (Morse 1971, Bertin 1977, Roth et al. 1996). Hoover and Brittingham (1998) found that nests in forested area of Pennsylvania were associated with greater densities of trees, greater canopy closure, higher density of shrubs, and taller shrub height. Wood thrush nests are built in trees or shrubs; nests are made of herbaceous stems, leaves, grasses ,and mud (Roth et al. 1996).

Foraging. Wood thrushes forage on the ground, probing loose soil, gleaning insects from the leaf litter, or taking fruit from ground vegetation. They feed mostly on insects and insect larvae, such as beetles, flies, ants and caterpillars, as well as isopods and millipedes; fruits are also taken as they become available, including spicebush, elderberry, blueberry, holly, pokeweed, Virginia creeper, dogwood, and Jack-in-the-pulpit (Roth et al. 1996).

Area. The wood thrush is somewhat tolerant of forest fragmentation. It may be found in habitat patches of 1 to 5 ha (Robbins et al. 1989, DeGraaf and Rappole 1995) or even smaller. Friesen et al. (1999) did not find that nesting success was related to distance from forest edge or forest size in a highly fragmented landscape in Ontario. However, Weinberg and Roth (1998) observed just this effect in tracts from 0.2 to 15 ha in Delaware. That is, thrushes nested in even very small tracts, but these nests were unsuccessful, primarily as a result of high rates of nest depredation. Robbins et al. (1989) calculated that the optimum area for wood thrush breeding was 500 ha, and the minimum suitable area was 1.0 ha (50% of the frequency of occurrence of larger tracts). Robbins (1979) estimated 100 ha as the minimum area required to support a viable breeding population. Hoover et al. (1995) found that wood thrushes in a wide range of tract sizes in Pennsylvania had nesting success less than 50% in tracts < 80 ha. Moreover, "core area" (remote from the forest edge) appeared to be important to nesting success.

Disturbance.  Wood thrushes make use of suburban habitats (Dilger 1956), but are significantly less common near paved road or powerline edges (Roth et al. 1996).

Model:
Habitat was based on vegetative cover (see table, below), patch size, distance from edge, and moisture regime. Patches of suitable forest types of 150 ha or larger were regarded as optimal (1.0); 80 to 150 ha was scored 0.7; 1 to 80 ha was scored 0.3; any smaller = 0.

The edges of forested patches were regarded as less valuable than forest interiors. Scores for habitat within 60 m of non-forested covers were multiplied by 0.5.

Hydric soils were selected from digital USDA/NRCS soils maps. Sites having poorly drained (hydric) soils, or within 60 m from ponds or streams, were regarded as more favorable than drier sites; scores for the latter were multiplied by 0.5.
NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest 1.0
Upland coniferous forest
Upland mixed forest 1.0
Grassland
Upland scrub/shrub
Cultivated
Developed
Bare ground
PEM, L2EM Lake/pond, emergent vegetation
PFOcon Palustrine forest, conifer
PFOdec Palustrine forest, deciduous 1.0
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, R1US Estuarine, riverine 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

Overall habitat suitability was calculated as the product of the cover, area, edge, and moisture scores.

Model Testing: The wood thrush occurrences along Breeding Bird Survey routes throughout the study area were used to test the habitat map. We compared the presence of habitat near a random set of 797 upland points to that for Breeding Bird Survey stops at which wood thrushes were observed in 1997 or 1998. Of the 475 sites with birds, 474 had mapped habitat, while 768 sites out of the 797 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model does indicate localities useful to wood thrushes. When just the higher scored habitats were tested (e.g., 0.3, 0.5, 0.7 habitat suitability levels), the level of significance increased, respectively. This stronger association for the higher scored habitats indicating that the relative habitat scoring was valid.

Sources:
Bertin, R.I. 1977. Breeding habitats of the Wood Thrush and Veery. Condor 79:303-311.

DeGraaf, R.M. and J.H. Rappole. 1995. Neotropical Migratory Birds. Comstock Pub. 676 pp.

Dilger, W.C. 1956. Adaptive modifications and ecological isolating mechanisms in the thrush genera Catharus and Hylocichla. Wilson Bull. 68:171-199.

Friesen, L., M.D. Cadman and R.J. MacKay. 1999. Nesting success of neotropical migrant songbirds in a highly fragmented landscape. Conserv. Biol. 13(2):338-346.

Hoover, J.P. and M.C. Brittingham. 1998. Nest-site selection and nesting success of wood thrushes. Wilson Bull. 110(3):375-383.

Hoover, J.P., M.C. Brittingham and L.J. Goodrich. 1995. Effects of forest patch size on nesting success of wood thrushes. The Auk 112(1): 146-155.

Kendeigh, S. C. 1948. Bird populations and biotic communities in northern lower Michigan. Ecology 29:101-114.

Morse, D.H. 1971. Effects of the arrival of a new species upon habitat utilization by two forest thrushes in Maine. Wilson Bull. 83:57-65.

Robbins, C.R. 1979. Effect of forest fragmentation on bird populations. Pp. 198-212 in Management of Northcentral and Northeastern Forests for Non-game Birds (R.M DeGraaf and K.E. Evans, eds.). USDA Forest Serv. Gen. Tech. Rep. NC-51.

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.

Roth, R.R., M.S. Johnson and T.J. Underwood. 1996. Wood Thrush. In The Birds of North America, No. 246 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, D.C.

Weinberg, H.J. and R.R. Roth. 1998. Forest area and habitat quality for nesting wood thrushes. The Auk 115(4):879-889.