Seasonal Patterns of Nesting Mortality in the Red-Winged Blackbird

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Abstract
The seasonal distribution of egg and nestling mortality was studied in the red-winged blackbird. The relative contribution of individual sources of mortality changed during the season. Deaths from nest abandonment and unhatched eggs decreased in relative contribution to the total mortality while predation and nestling starvation increased. The mean progeny lost per nest per day was calculated for eggs and nestlings through the season. The rate of egg mortality remained relatively constant with a seasonal mean of 0.0812 egg losses/nest per day. The rate of nestling mortality increased seasonally yielding a significant linear regression with the number of days beyond the onset of egg-laying in the colony. Clutch size decreased significantly from 3.7 eggs/nest at the beginning of the season to about 3.0 eggs/nest at the end of the egg-laying period. Based on the mean rate of egg mortality and the regression relating the rate of nestling mortality with season, the expected cumulative mortality was calculated for nests begun on each laying date of the season. This increasing function approximated clutch size at about the time when egg laying ceased in the colony. Because many individuals began nesting late in the season when chances for success were small, there appeared to be little selective pressure against late nesting attempts. Increasing nestling mortality with seasonal advance may be an important ultimate factor in establishing the termination of the breeding season.