Summary
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.
Methodology
We conducted the study in the vicinity of the Queen's University Biological Station in eastern Ontario. Song sparrows are one of the most abundant birds that breed in this region and display an extremely catholic choice of breeding habitat, including early successional fields, hedgerows, roadsides, marsh edge, gardens and lakeshore. Our main study area included the islands and shoreline of Lake Opinicon, a lake approximately 8 km long by a maximum of 1 km wide. Song sparrows held territories on nearly every small island, regardless of whether the vegetation was mature trees or shrub interspersed with bare rock. On the shoreline of the mainland and the two largest islands, territories were established primarily where the mature forest did not extend onto rocky points or was interrupted by human disturbance (cottages, lawns). In 1984 we also studied song sparrows in a large, early successional field 3 km from the lake.
Song sparrows arrived on the study area in mid April, began breeding by late April or early May and continued breeding until mid August. This allowed a pair of birds to breed successfully up to three times in a single season. From 1981 to 1984 we monitored the territorial and nesting activity of the song sparrows from late April to mid August. In the last 3 years, we banded approximately 80-90% of the territorial males on the lake early in the breeding season. Males were attracted into mist nets by playing tape-recorded song sparrow song. Since we could band only males that responded aggressively to a conspecific song, we considered that all the males we banded were territorial when banded.