The reproductive success of secondary females relative to that of monogamous and primary females in Red-winged Blackbirds

We use two approaches to compare the reproductive success of secondary (S) females to the reproductive success of monogamous and primary (M/P) females in two populations of the polygynous Red-winged Blackbird Agelaius phoeniceus. One approach is to calculate x, the mean reproductive success of all S females divided by the mean reproductive success of all M/P females. Estimates of x are corrected for seasonal declines in reproductive success of MIP females when necessary. Considering all nests, the mean x is 0.98 for two years in a Washington population and is also 0.98 for four years in an Indiana population. Considering only successful nests, and pooling data over years, x is 0.98 for Washington and 0.96 for Indiana. A second approach is to perform pair-wise comparisons of the reproductive success of each S female to the reproductive success of those MIP females that choose unmated males available to, but not chosen by, the S female. In both redwing populations, individuals more often than not have higher success breeding as S females than they would have breeding as M/P females with available unmated males. The implications of these results for polygyny models are discussed.