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Birds in cities : a study of populations, foraging ecology and nest-sites of urban birds Weber, Wayne Carson

Abstract

The ecology of urban birds—populations, foraging ecology, and nest-sites—was studied between 1968 and 1970, mostly in Vancouver, British Columbia. Four census plots in Vancouver, each in a different habitat type, were studied year-round. Additional winter studies were made on two plots each in Sacramento, California and Ottawa, Ontario and on two additional Vancouver plots. Breeding bird densities on the Vancouver plots were lower than those in some other urban areas, but comparable to those in most non-urban habitats. Densities decreased with increasing urbanization. In winter, densities were generally higher than in the breeding season, and were much higher than those in non-urban habitats. Winter densities, unlike breeding densities, increased with greater urbanization. The availability of food is probably a major cause of the high winter densities. Sacramento and Ottawa both had lower winter densities than Vancouver. The species diversity of the urban plots was low at all seasons. This results both from a small number of species and a low equitability (i.e., dominance of the population by a very few species). On the Vancouver plots, species diversity was highest in the breeding season, owing to a higher equitability then. The latter probably results from territorial behaviour, which makes it less likely that one or two species will dominate the community. The urban bird populations were dominated—especially on the most urbanized plots—by House Sparrows, Starlings, and Rock Doves. An attempt is made to explain how the biological features of these three species make them especially well suited to the urban environment. Two types of observations—stopwatch observations and spotchecks —were used, in studies of foraging ecology. This phase of the study was confined to the Vancouver plots. An analysis of the stopwatch observations revealed that nearly every species had a distinctive foraging pattern. The only pair of species which had closely similar patterns were the Starling and the Robin. However, these two had quite different foraging methods and utilized different foods. The spotchecks, being discrete, were amenable to statistical testing. Tests were carried out to see whether the use of different microhabitats corresponded to their availability, and to check for differences between species. In both cases, only tests involving the Crested Mynah failed to show significant differences. This lack of significance is believed to be merely the result of a small sample size. Nest-sites were also studied. Interspecific differences in nest height and placement were demonstrated. A conspicuous feature was the almost total absence of nests near the ground. Cat predation and human disturbance are probably responsible for this. In closing, some general features of urban bird ecology are discussed. The importance of studying the foraging ecology of unrelated species, as well as of related ones, is stressed. While two related species with similar foraging ecology usually occupy different habitats, the same, is probably true of unrelated species to a lesser degree. The foraging patterns of urban birds may be expected to overlap more than those of non-urban birds, and nest-site availability may be particularly crucial to birds in cities.

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