Influence of Resource Abundance on Use of Tree-Fall Gaps by Birds in an Isolated Woodlot

Abstract

Abstract The occurrence of birds in forest understory and tree-fall gaps during spring and fall migration periods was determined in an isolated woodlot. We used mist-net captures to test the hypothesis that birds are attracted to gaps because of higher resource levels. We captured 1,010 birds (74 species) in spring and 458 (44 species) in fall. Total captures and captures per net were higher (P < 0.001) in gaps during spring and fall. Mean number of species per net was higher in gaps (P < 0.001) during both seasons, but total species in gaps (69 spring, 43 fall) was not significantly higher than in forest understory (60 spring, 28 fall). Of 44 species represented by adequate sample sizes (n > 5) in spring, 9 were significantly (P < 0.05) more common in gaps and 2 were more common in forest understory. Nine of 17 species were captured more often (P < 0.05) in gaps during fall. During spring, flycatchers, ground insectivores, foliage insectivores, and granivore-omnivores were captured more frequently (P < 0.05) in gaps. Flycatchers showed no difference in fall, but other trophic groups, including frugivores, were captured more frequently (P < 0.05) in gaps than in forest understory sites. Bark foragers showed no statistical preference for gaps or forest understory in spring or fall. Total species per net and total captures per net correlated positively (P < 0.05) with density of foliage in the lower canopy and negatively with density of upper canopy foliage in both spring and fall. Total species and captures correlated positively (P < 0.05) with insect abundance in spring and with fruit abundance in fall. Foliage insectivores correlated positively with low canopy foliage and insect abundance in both spring and fall. Captures of frugivores correlated with fruit abundance in fall. These data support the hypothesis that birds are attracted to tree-fall gaps because of higher resource abundance and provide further evidence of the importance of habitat heterogeneity to the structure and composition of bird communities.