Site infidelity in song sparrows

In each of 4 years of study of a breeding population of song sparrows (Melospiza melodia) approximately two-thirds of the males holding territories at the beginning of the breeding season had abandoned those territories before the end of the breeding season and overall 83% of those males were never seen again. Males were more likely to abandon a territory following a nest failure but some individuals left after a success and some remained after a failure. Indirect evidence suggested that territory quality also influenced a male's decision to stay or go. Data on nesting success suggested that both birds that stay or those that move benefit from their actions. Between years, males showed the same lack of site fidelity, but were more likely to return to a territory on which they had been successful. Females also showed little site fidelity within breeding seasons, and although they abandoned territories for the same reasons as males, they appeared to move independently of their mate. Only one of 146 birds banded as a nestling was ever resighted as an adult. Together, the low adult site fidelity and low natal philopatry indicate a much higher level of outbreeding than has been estimated previously for this species. We discuss the implications of these results and propose that, rather than being an evolved attribute to maintain an optimal level of inbreeding, site fidelity is a conditional strategy related to intraspecific competition for territories.