Song Similarity in an Isolated Population of Fox Sparrows (Passerella iliaca)

Abstract

The structure of song in island populations of birds has been the subject of a number of studies (see review in Miller 1982). Two different hypotheses predict how song might evolve in such populations. First, island populations may have more complex songs because they are subject to weaker selective pressures for species-specific signals than are mainland populations (Miller 1982). We refer to this as the song complexity hypothesis. Second, founder effects may constrain song structure (Nottebohm 1969, Thielcke 1973, Mundinger 1975, Mirsky 1976, Baptista and Johnson 1982, Lynch and Baker 1986); a population bottleneck could lead to reduced song variation because young birds are exposed to fewer kinds of songs and song elements (Baker and Jenkins 1987). We refer to this as the founder effect hypothesis. These two hypotheses are not necessarily incompatible, as a population could possess complex songs that are highly stereotyped within each bird and vary little among birds, or could possess simple songs that vary greatly from male to male. Furthermore, in a small population, periodic bottlenecks after low population levels, such as might occur after a season of high mortality, could function to re-establish song similarity. As a contribution to our understanding of song characteristics in island populations, we present data on the song structure of an insular population of Fox Sparrows (Passerella iliaca) in Nova Scotia, Canada. Fox Sparrows reach the southeastern limit of their breeding range in Nova Scotia, and breeding birds are largely restricted to scattered small offshore islands along the coast, where relatively few other passerine species breed. These populations are therefore isolated by the location of islands and the paucity of adjacent mainland populations. Fox Sparrows are largely migratory, but probably exhibit high fidelity to the island (see discussion).