A metapopulation approach to the population biology of the Song Sparrow Melospiza melodia

Abstract

In this paper, we describe spatial variation in the demography of the Song Sparrow Melospiza melodia. Long‐term population studies of birds have generally considered only single sites in optimal habitat, but recent theory has shown the value of taking spatial variation in population dynamics into account. In this theory, collections of local populations in habitat patches connected by dispersal are denned as metapopulations. We review metapopulation models briefly and then use these ideas in a study of the Song Sparrow. Sparrows were studied on islands of varying size and degree of isolation from the North American continent. For analysis, these islands are grouped into three areas. Sparrows on the isolated Mandarte Island usually reproduced and survived well over 17 years but suffered from catastrophic mortality in some winters. After such events, the population recovered quickly without much immigration. Sparrows on smaller islets near Mandarte also survived and reproduced well during 4 years, and they did not experience severe mortality in a winter in which those on Mandarte declined by over 90%. These small islets frequently exchanged first year birds, but only one yearling bird moved between the small islets and the more isolated Mandarte. Sparrows at one of two sites on the larger Westham Island, near the continental mainland, survived less well than on the small offshore islands. Reproduction on Westham Island was poor because of frequent nest failure and high levels of brood parasitism by Brown‐headed Cowbirds Molothrus ater. Despite this poor reproduction and survival, the Westham population remained stable as a result of immigration. We interpret the dynamics of these sparrow populations in terms of H. R. Pulliam's source‐sink metapopulation model.