Abstract
We conducted winter censuses of two short-distance migrants, Hermit Thrushes (Catharus guttatus) and Yellow-rumped Warblers (Dendroica coronata), over seven years in five different habitats to determine whether their local abundances could be predicted by fruit pulp biomass. Sampled habitats were stands of upland and bottomland hardwood, loblolly pine (Pinus taeda), longleaf pine (P. palustris), and young (<10 years) longleaf pine. Hermit Thrush abundance, which was highest in bottomland hardwood habitats, was positively related to total dry mass of fruit pulp. Those results are consistent with the hypothesis that resource availability affects the local distribution of migrant passerines on their wintering grounds. Our results also indicate that bottomland hardwood habitats in the southeastern United States may be especially important to wintering Hermit Thrushes. Yellow-rumped Warbler abundance was correlated with ripe-fruit pulp dry mass of Myrica cerifera, a major source of winter food for that species. However, because M. cerifera pulp dry mass was confounded with habitat type, we could not distinguish the relative importance of fruit resources and habitat for Yellowrumped Warblers. Our results underscore the importance of fruit to wintering birds. However, the overall percentage of variation in winter bird abundance explained by differences in ripefruit biomass was modest, indicating that other factors are also important.
Geographic variation in song structure in the Hermit Thrush (Catharus guttatus)