Hierarchical relationships of breeding and wintering bird communities to tree-fall canopy gaps in southern bottomland hardwood forests
Bottomland forests are well known to support diverse avian communities, and these habitats are vital for the sustainability of local and regional bird populations. These forests are subjected to repeated disturbance events typically as a result of flooding cycles. The impacts of disturbance on birds in these habitats were examined by determining how birds respond to tree-fall canopy gaps at the community, population, and individual hierarchical scales on three geographically distinct bottomland tracts, all ≥80 years old and ≥300 ha in size. Relationships of canopy gaps to the avian community were investigated using point-count surveys during the 1996-1998 breeding seasons, and the 1997-1998 winter seasons. Study sites were located along the Cache River, AR, Coosawhatchie River, SC, and Iatt Creek, LA. Research at the population level (activity area distribution) and individual level (microhabitat use and foraging behavior) were conducted on the Cache River study area, using four insectivorous focal species: the Acadian Flycatcher (Empidonax virens ) (breeding season), Blue-gray Gnatcatcher (Polioptila caerulea ) (breeding season), White-breasted Nuthatch (Sitta carolinensis ) (breeding and wintering seasons), and the Golden-crowned Kinglet (Regulus satrapa ) (wintering season). Line transect surveys were conducted to map the distribution of focal species' activity areas with respect to distribution of canopy gaps. Individual birds were located and observed during the breeding and wintering seasons to determine microhabitat use and foraging behaviors in tree-fall canopy gaps versus canopied forest habitat. Numerous differences were observed between the community, population, and individual levels investigated in this study. At the community level, most bird species metrics were positively correlated with vegetative characteristics including canopy gaps on all three sites. However, on the Cache River study area, few relationships between canopy gap and activity area distributions were observed for any focal species. Interestingly, all species were observed more often in canopy gaps than canopied forest habitat, yet microhabitat use and foraging behavior only differed for the White-breasted Nuthatch during the winter seasons. Results suggest that this species utilizes gaps during the winter because of increased foraging opportunities. The Acadian Flycatcher used gaps as nesting sites, but no other explanations of gap use were determined for the other focal species. Results of this study are consistent with the concept that observed patterns of avian habitat relationships vary depending upon the scale at which the patterns are investigated. Employing a hierarchical research design in the investigation of disturbance impacts on avian communities can help in the understanding of how birds respond to disturbance. However, hierarchical designs are not a panacea for avian community research. Not all questions asked will have an answer. Rather, the incorporation of hierarchical designs can used as a tool to help elucidate patterns missed in community level studies, while providing more information and direction for future research.