Summary
Several aspects of nest defence behavior were investigated in song sparrows (Melospiza melodia) in eastern Ontario. Two independent tests were made of the hypothesis that the increase in nest defence observed through a nesting attempt is due to the birds becoming familiar with the nest threat, rather than because the nest contents increase in value to the parents. Neither test supported the hypothesis. As predicted by life history theory for species with age-independent mortality, males did not defend their nest more vigorously as they become older. Parents defended their nests less vigorously through the breeding season, contrary to the expected pattern of increased nest defence in response to declining renesting potential. This result may be attributable to a decline in offspring value through the breeding season. Nest defence behavior of mated individuals was positively correlated, independent of factors such as offspring age, renesting potential and brood size. From this result it is proposed that a source of variation in nest defence behavior may be individuals basing their own response on their mate's response in a positive feedback fashion. Males defended nests less vigorously than females, consistent with the expectation that males have lower certainty of parentage in the offspring. It is proposed that variation in paternal uncertainty could contribute to the unexplained variation reported in nest defence studies.
Methodology
I conducted this study at the Queen's University Biological Station in eastern Ontario, Canada between late April and mid-August, 1981--1984. All breeding birds used in this study nested either on islands or along the shore of Lake Opinicon (for details of the study population see Weatherhead and Boak (1986)). All males used in this study were individually color-banded. Because of difficulty in catching females, most were unbanded. Occupied territories were searched for nests from late April through early August, the period that includes the initiation dates of all nests found during the study. The majority of nests were found during egg laying or early incubation, although some were found later in incubation or during the nestling period. Once found, each nest was visited every 2-4 days to check its contents and, if still active, to conduct a nest defence trial (see below). The only nests used for this study were those for which clutch initiation was either observed or could be determined from hatching or fledging dates.