A METAPOPULATION APPROACH TO THE POPULATION BIOLOGY OF THE SONG SPARROW MELOSPIZA-MELODIA

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 re cent theory has shown the value of taking spatial variation in populat ion dynamics into account. In this theory, collections of local popula tions in habitat patches connected by dispersal are defined as metapop ulations. We review metapopulation models briefly and then use these i deas in a study of the Song Sparrow. Sparrows were studied on islands of varying size and degree of isolation from the North American contin ent. For analysis, these islands are grouped into three areas. Sparrow s on the isolated Mandarte Island usually reproduced and survived well over 17 years but suffered from catastrophic mortality in some winter s. After such events, the population recovered quickly without much im migration. Sparrows on smaller islets near Mandarte also survived and reproduced well during 4 years, and they did not experience severe mor tality in a winter in which those on Mandarte declined by over 90%. Th ese small islets frequently exchanged first year birds, but only one y earling bird moved between the small islets and the more isolated Mand arte. Sparrows at one of two sites on the larger Westham Island, near the continental mainland, survived less well than on the small offshor e islands. Reproduction on Westham Island was poor because of frequent nest failure and high levels of brood parasitism by Brown-headed Cowb irds Molothrus ater. Despite this poor reproduction and survival, the Westham population remained stable as a result of immigration. We inte rpret the dynamics of these sparrow populations in terms of H.R. Pulli am's source-sink metapopulation model.