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Bird communities in relation to the structure of urban habitats Lancaster, Richard K.


This study is an investigation of the hypothesis that bird species' abundances and other characteristics of bird communities in urban areas are a function of habitat structure. To this end, eight study plots of approximately eight hectares each were selected in Vancouver, B.C., and described in terms of area covered by different classes of foliage, buildings, and other man-made features of the habitat. In addition, the amount of food intended for birds provided by people was estimated. The range of habitats represented by the plots depicted a gradient of urbanization; one extreme was a downtown commercial district and the other was a woodland. Birds were counted at regular intervals on these plots for one year. Bird species diversity (H'), the number of species, and the number of foraging guilds decreased with decreasing vegetation cover, especially during spring migration and summer breeding seasons. The number and diversity of bird species is not influenced by the diversity of man-made structures. In most plots, bird species diversity was lower in summer than in winter. Seasonal changes in bird species diversity were not related to urbanization. However, changes in number of species were greater in habitats with more vegetation. Evenness of species' abundances did not change greatly with season in most plots, but declined noticeably with urbanization. A few species of birds were most abundant throughout the urban environment; these were the Rock Dove (Domestic Pigeon), Starling, House Sparrow, and American Robin. The degree of numerical dominance by the first three of these species generally increased with urbanization in all seasons. Omnivorous species were the most abundant type in all urban habitats, in all seasons. There was a decrease in the relative abundance of insectivores in winter (many of these were migratory). Differences among plots in total bird density is not related to the amount of vegetation cover. However, human residents of suburban plots provided substantial amounts of food for birds, and this is thought to have had a pronounced effect on total bird density, especially in the winter. Bird densities in most habitats were highest in the winter, when some species appeared to be distributed in response to the availability of food provided by man, others in response to the amount of natural food available. It is hypothesized that resident bird species respond directly to essential factors (e.g. food) to a greater extent than migrants. Migrants are generally believed to be using habitat characteristics that are cues for these factors, in their selection of habitat.

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