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This chapter examines the taxonomic relationships of the genus Passer, as well as the species boundaries of the House Sparrow with particular reference to the taxonomic position of the “Italian” Sparrow (a well-marked population thought to be the result of hybridization between House Sparrows and Spanish Sparrows). The geographic distribution of the House Sparrow is also described, including the history of introductions of the species. The rate of range expansion following introductions is also discussed.

This chapter presents two studies that have examined annual variation in dispersal in relation to climate. The first example, a long-term study on the dipper Cinclus cinclus, shows that fluctuations in winter temperature affect immigration, which in turn is an important contributor to fluctuations in population size. The second study addresses how the effect of climate on dispersal varies spatially and may depend on the quality of the natal habitat in a metapopulation of house sparrows Passer domesticus. There is a vast body of empirical work demonstrating that dispersal may vary considerably within species, often in response to environmental cues. Despite such apparent plasticity of dispersal, however, little is known about how the dispersal process is influenced by climate, especially in vertebrates.

Several species of small seed-eating farmland birds resident in Britain have shown marked population declines over the last 50 years. In most cases, shortage of food during winter and early spring has been implicated. However, some large seed-eating species have increased over the same period. This chapter explores the possible influence of two phenomena that have changed markedly over that period, which may have received insufficient research attention. The first is the increase in the release of captive-reared gamebirds by the commercial shooting industry, and the second is the loss of sources of small seeds at low spatial density, provided historically by weedy overwinter stubbles, and the increase in sources of larger seeds at high density, provided primarily by gamebird feed hoppers and game cover plots. The analyses suggests that the scale of gamebird release is such that the biomass of released gamebirds now greatly exceeds the combined autumn biomass of native seed-eating species, and changes in the density of pheasants Phasianus colchicus released annually show close temporal correspondence with the declines in small seed-eating birds. The chapter also suggests that weedy overwinter stubbles historically provided seeds at a density where small bird species could meet their daily food needs, but large species could not. The shift from low-density to high-density seed sources may have benefited large species to the detriment of small species. Conservation initiatives designed to provide small seeds at low spatial density might be effective in restoring the previous balance between small and large species.