Summary
The intensity of nest-defence aggression by female redwinged black birds (Agelaius phoeniceus) with eggs decreased late in the breeding season, while aggression at this time increased amongst females defending broods. High predation pressure late in the breeding season decreased the probability of survival of the late nesting attempts, so that females with young had a much greater chance of success than females with eggs at this time. The observed changes in aggressive responses are predicted by the theory of parental investment.
Methodology
Redwinged blackbird nests were monitored from late May to early July 1977 in two small marshes on Lake Opinicon, approximately 45 km north of Kingston, Ontario, Canada. Clutch and brood manipulations were conducted for a separate study (Robertson & Biermann 1979). A nest predator was simulated by a 70-cm long black-and-green rubber snake wrapped around the nest-supporting vegetation with its head in the nest cup. Water snakes (Natrix sipedon sipedon) are common marsh predators (Robertson 1972; Clark & Robertson 1979) that female redwinged blackbirds can occasionally deter from destroying an entire nesting attempt. Behaviours were scored based on the distance of the females from the nest, calling, movement near the nest, and attacking. A scoring scheme was devised to rank the females' aggression toward the snake, incorporating both the behavioural acts performed and the duration of the acts during the first 5 min after the females returned to their nests.