Territorial and Non-Territorial Subpopulations of Wilson's Warblers Return to Central California Breeding Grounds in Two Migratory Waves.

Early migratory return for territorial passerines is important, since earlier return results in better territories, and greater reproductive success. Many passerine studies have found that migratory return dates for first-breeding-season (SY) individuals was a week or less later than for older (ASY) birds. Many of both ASY and SY birds in these studies became territorial, although more SY than ASY birds became non-territorial floaters. In this study of Wilson's Warblers, however, nearly all individuals returning early and becoming territorial were ASY birds, and very few were SY birds. However, second waves of returning Wilson's Warblers, containing both SY and ASY birds, returned about 3-5 weeks after the first migratory waves, and no males in those second waves ever became territorial. No later-returning, second-wave female initially ever became territorial either, although some subsequently became replacement mates for earlier-returning males. Some male Wilson's Warblers were territorial, and others were non-territorial, during all of their study-area years. However, some males switched status over successive years, and migratory return when non-territorial always was several weeks later than when territorial. Findings from this study indicated, for some males during some years, that non-territoriality was more adaptive than was territoriality. This study's finding, of 3-5 week later migratory return waves for non-territorial, than for territorial, Wilson's Warblers, supports findings by Stewart (1973), that Wilson's Warblers return to breeding grounds in two migratory waves. Findings from both studies may provide empirical evidence supporting Kokko's (1999) hypothesis that migratory birds, not competing for territories, should return to breeding grounds later, on their "cost-minimizing dates."