Patch utilization by migrating birds: resource oriented? Ornis Scand

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 (1978-1980). 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 ("frugi-vores " 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 concen-trated 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. Abun-dance of foliage-gleaning insectivores was highly correlated with foliage density in both spring and autumn. Frugivore abundance was highly correlated with fruiting fo-liage density during autumn when they are frugivorous, but not during spring when they are insectivorous. Insectivores not relying on foliage insects or fruit were un-correlated with either index of resource availability. These same relationships hold even when examining gap sites only. Thus, migrants can be consistent in their se-lection of foraging sites and this consistency appears to exist when resource densities are markedly different among sites (patchy) but not when resources are more disper-sed