Patch utilization by migrating birds: resource oriented?

Use of gap (created by tree falls) and non-gap forest understory sites by migrating birds in central Illinois was studied during spring and autumn for three years (19781980). Fruit and understory foliage were concentrated in gaps. Birds that relied on these resources (foliage-gleaning insectivores, frugivores in autumn) used gaps more than non-gaps. Birds that fed on food other than fruit and foliage insects ("frugivores" in spring, other insectivores) did not use gaps more than non-gaps. Bird abundance varied markedly among gap and non-gap sites, potentially reflecting differences in site preferences. Site selection, as determined by bird abundances, was consistent (correlated) between years for birds that fed on items that were concentrated in gaps but not for birds that did not rely on these patchy resources. Foliage density is a measure of foraging substrates for foliage-gleaning birds to search. Abundance of foliage-gleaning insectivores was highly correlated with foliage density in both spring and autumn. Frugivore abundance was highly correlated with fruiting foliage density during autumn when they are frugivorous, but not during spring when they are insectivorous. Insectivores not relying on foliage insects or fruit were uncorrelated with either index of resource availability. These same relationships hold even when examining gap sites only. Thus, migrants can be consistent in their selection of foraging sites and this consistency appears to exist when resource densities are markedly different among sites (patchy) but not when resources are more dispersed.