Parental care of nestling and fledgling American robins, Turdus migratorius, was studied in eastern Ontario. There was no evidence of brood division prior to fledging but brood division was the predominant pattern after fledging. Females initiating second nests reduced their care of fledglings sooner than males. Once incubation began at second nests, males provided all the care for fledglings from the previous nest. A consequence of overlapping first and second nesting efforts is that care of fledglings from the first nest allows males little opportunity of mate guarding when their mates are initiating the second nest. Reduced opportunity for mate guarding is probably exacerbated by the wide dispersal of fledglings at the time of egg laying at second nests. In spite of the apparent reduced opportunity for mate guarding, males fed nestlings in second nests as much as those in first nests. This result suggests that females initiating second nests may refrain from extra-pair copulations if their mates have proven themselves by the success of the first nest, because confidence of paternity ensures male parental care at the second nest. Information from the literature suggests that the mixed strategy by females of being willing to engage in extra-pair copulations when initiating first but not second broods may occur in other species with overlapping broods.
We studied American robins breeding at the Queen's University Biological Station in eastern Ontario from late April to mid-August 1987 and 1988. Although there are open lawns and roads around the station buildings, habitat in the study area is predominantly mature deciduous forest.
We caught and individually colour-banded all adult robins using mist nets during the spring of both study years. We determined adult gender by checking for the presence of a cloacal protuberance, a characteristic of breeding males. Gender was corroborated by wing chord measurements (generally more than 130 mm in males, and less than 130 mm in females), plumage assessment (male breast plumage is generally brighter than that of females) and detailed observations of breeding pairs (in addition to egg laying, we only observed females building nests and incubating).