An increase in parental investment during the breeding season

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.