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Breeding bird assemblages of hurricane-created gaps and adjacent closed canopy forest in the southern Appalachians

Greenberg, Cathryn H.; Drew Lanham, J.

We studied breeding bird assemblages in forest gaps created in 1995 by Hurricane Opal at the Bent Creek Experimental Forest in Asheville, NC. We hypothesized that forest gaps and adjacent closed-canopy forest would differ in bird density, richness, diversity, and relative abundances of some species. To test this hypothesis we censused breeding bird assemblages for 2 years in 12 gaps (0.1–1.2 ha) and 12 adjacent closed canopy controls using strip transects. Gaps had more coarse woody debris, shrub cover, brushpiles, and pit and mound microtopography than controls. Canopy cover was lower in gaps than controls, but remained high (69.4±2.1% versus 89.6±1.7%). Bird assemblage similarity was high. Total density and species richness of birds were higher in gaps than in controls, but species diversity did not differ between treatments. Shrub (primarily Carolina Wrens) and bark-foragers, and cavity shrub and canopy-nesters were more abundant in gaps than in controls. Densities of gap-associated (Indigo Bunting, Hooded Warbler, Carolina Wren) and edge (Eastern Towhee) species were more abundant in gaps. Abundance of interior species including Red-eyed Vireo and Scarlet Tanager were about equal in gaps and controls. Only Ovenbirds were more abundant in controls than gaps. Species that require larger patches of young second-growth forest, such as Prairie Warbler, and Yellow-breasted Chat, did not occur in gaps; but neither are they abundant in the Asheville basin. No Brown-headed Cowbirds were observed in gaps or controls. Unpublished data indicate that parasitism of artificial ground nests did not occur, and predation rates did not differ between gaps and controls. Juvenile birds and other evidence of breeding were observed more often in gaps than in controls, suggesting that gaps attract bird families for foraging and provide microsites that attract breeding pairs. Gap size was positively correlated with bird density, species richness, and diversity. This study suggests that small openings and interior edge habitat created by treefall gaps within a forested matrix do not adversely affect breeding birds as measured by the abundance of individual species or community indices. We suggest that canopy gaps increase avian diversity at a landscape scale by providing habitat patches for some species that require young, second-growth forest, and serve as magnets for recruitment and foraging.

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