| The spatio-temporal variation of the distribution of bird species was analyzed at multiple scales. At the local scale, I examined the effects of drought on community structure of birds breeding across a size gradient of aspen patches in east-central Utah. Leaf biomass, which limits the available energy to species at higher trophic levels, was lower during the drought. As a result, avian species richness, species diversity and average abundance declined. The density of breeding birds also declined, but losses were disproportionately recorded in smaller aspen patches. The drought caused a reduction of common species and a loss of many locally rare species.; Neotropical migrants (NTMs), showed significant declines during the drought. Five of seven species lost from area were NTMs. Smaller aspen patches did not house rare species, and during times of environmental stress appeared to lose a disproportionate number of individuals. A combination of habitat fragmentation and more frequent periods of environmental stress, as some climate models predict, could result in dramatic alteration of avian community structure in some montane habitats.; At the continental scale, I investigated the relationship between abundance and geographic range fragmentation of North American landbirds. I used six measures of fragmentation, based upon fractal geometry and geostatistics, and grouped birds by three ecological characteristics. I estimated the amount of demographic and areographic fragmentation of each species' range. All three ecological variables, migratory strategy, breeding habitat, and trophic level, were found to strongly influence how species distribute themselves across their geographic ranges. A better understanding of how and why the distribution of abundance varies between species will aid in the preservation of biodiversity.; Next, I examined the distribution of an avian brood parasite, the Brown-headed Cowbird (Molothrus ater), with respect to the distribution of its potential hosts. First, I created maps of spatial patterns of abundance for 210 species of passerines. Overlaying each species map on the Brown-headed Cowbirds' map, I calculated the proportion of each species' geographic range that was common with cowbirds. It was found that cowbirds concentrate their parasitism efforts on a nonrandom sub-sample of North American passerines, but that strategy varies across the landscape and from one habitat to another. This is evidenced by the fact that the group of bird cowbirds most commonly parasitize are less abundant than other species. Host species that have experienced negative population trends over the past 30 years also had more of their geographic range in common with cowbirds than species not declining. This study suggests that even at the continental scale a hosts' breeding ecology and life-history traits strongly influence the distributional pattern of the cowbird. |
