Structure Of Bird Assemblages Research Articles

Interactive effects of land use and other factors on regional bird distributions

Aim We assessed the independent and interactive effects of land use and other factors on regional distributions of lake shore bird assemblages. Location Analyses were restricted to lakes of the northeastern United States (New England, New York, New Jersey). Methods Lake shore bird and habitat data were collected from a regional sample of 186 lakes between 1991 and 1995. Local and regional gradients in environmental attributes of the lakes and their watersheds, and in bird species composition, were characterized using detrended correspondence analysis. Bird assemblages were also characterized with respect to species diversity, total bird abundance, and absolute and proportional abundance for neotropical migrants, omnivores and tree foragers. The assemblage measures were compared with the environmental gradients and with the environmental variables from which those gradients were derived using correlations, linear regression and bivariate plots. Results Anthropogenic variables (e.g. human density) and nonanthropogenic variables (e.g. climate, forest composition, lake productivity) were highly confounded regionally, reflecting the influence of climate and geomorphology on land use and, in turn, the influence of land use on the environment. Moving south along the regional gradient to watersheds with more intensive land use, changes in bird assemblage structure were consistent with anthropogenic effects on the avifauna attributable to forest fragmentation: total bird abundance increased, fragmentation-tolerant species were encountered more frequently, the proportional abundance of omnivores increased and the proportional abundances of tree foragers and neotropical migrants decreased. Bird diversity, as assayed using rarefaction, peaked at an intermediate position along the regional gradient, as did the absolute abundances of tree foragers and migrants. Main conclusions We propose that some community attributes peaked at an intermediate position along the regional gradient as a result of the interactive effects of land use and other factors on this region's avifauna, with broad-scale anthropogenic factors constraining assemblages on the urbanized lakes, and climate or other nonanthropogenic factors constraining assemblages on the pristine lakes. Anthropogenic and nonanthropogenic factors of similar scale and magnitude thus acted in concert to shape regional bird distributions.

Bird assemblages of an Iberian cereal steppe

The cereal steppes of Castro Verde (Portugal) support many bird species of high conservation value on which agricultural practices can have significant impacts. The aim of this study was to describe the seasonal variations in bird assemblages using the agricultural landscape mosaic and to explore the potential ecological implications of changes in land use on bird populations. Bird densities and habitat variables were measured in four habitat types (cereal fields, fallow land, stubbles, and ploughed land) during 1997/1998, and detrended correspondence analysis used to summarise information on the structure of bird assemblages across habitats in the winter, breeding and post-breeding periods. Pluvialis apricaria L., Vanellus vanellus L., Alauda arvensis L. and Anthus pratensis L. were abundant during winter. During the breeding season, bird assemblages were quite distinct across land use types, with species typical of cereal fields (e.g. Miliaria calandra L.), fallow land (e.g. Melanocorypha calandra L.) and ploughed land (e.g. Anthus campestris L.). After the breeding season, the overlap in bird assemblages increased again, with M. calandra, M. calandra L. and Ciconia ciconia L. being common. Species used different habitats along the year, showing the importance of maintaining the traditional agricultural landscape for preserving the diversity of bird species. The potential impacts of several types of agricultural changes (e.g. agricultural intensification, agricultural abandonment and increase in stocking levels) on the bird populations were discussed on the basis of the results obtained. Any change in the current farming system will probably lead to a decrease in steppe bird diversity.

Hierarchical correlates of bird assemblage structure on northeastern U.S.A. lakes.

We investigated the factorsstructuring lake shore bird assemblages of thenortheastern U.S.A. using data collected from 158 lakesbetween 1992 and 1994. The assemblage data wereaggregated and standardized to produce assemblagecompositions consisting of proportions of individualsemploying different foraging, dietary, and migratorystrategies. The assemblage data were then re-expressedusing compositional analysis techniques and subjectedto regression tree analysis to identify environmentalcorrelates over a range of scales. Regionally, humandensity in the watershed was the most importantpredictor for the foraging, dietary, and migratorycompositions. A combination of anthropogenic andnon-anthropogenic factors likely contributed to thesebroad-scale associations because land use was largelyconfounded with climate and geomorphology. Morelocally, associations with lake shoreresidential-urban development were identified as beingimportant for the foraging and dietary compositions,as were associations with lake shore wetlands, butonly contingent there being little human developmentpresent locally and regionally. Assemblages wereassociated with increasing local and regional humandevelopment such that: hawking, aerial foraging, andground gleaning individuals increased relative tohover-and-gleaners, foliage gleaners, and barkgleaners; omnivores increased relative toinsectivores; and residents increased relative toneotropical migrants. The observed changes in birdcommunity structure were consistent with declines inforest interior species relative to edge species inresponse to forest fragmentation and suggest thatanthropogenic factors have played a prominent role instructuring lake shore bird assemblages of this region.

A Morphological Comparison of Island and Mainland Assemblages of Neotropical Birds

serine birds, composed of assemblages from 8 distinct habitats replicated over 5 areas located in Panama, Trinidad, Jamaica, St. Lucia, and St. Kitts. The structure of morphological space was very similar among the 5 localities, despite widely varying numbers of species, genera and families. The level of species packing, the evenness of species packing, and the volume of morphological space occupied in a community varied with locality and habitat, but locality explained a much higher percentage of the overall variation than did habitat for all 3 parameters. The correlation of the level of species packing with the number of species was not different from an expectation based on random assembly within each locality, though the overall locality effects were different from such an expectation. The volume of space occupied by a community was correlated with the number of species, in contrast to the expectation from random assembly, at nearly every level of analysis. The correlations of the evenness of species packing with number of species were inconsistent with the expectation from random assembly only in some cases. The extraordinary evenness of species packing on the small islands and the greater morphological distance between nearest neighbor species on those islands suggest that species interactions may prove more important in these areas than elsewhere for determining the community structure of passerine bird assemblages.