Summary
Red-winged Blackbird (Agelaius phoeniceus) offspring sex ratios at hatch were determined for 97 clutches suffering no egg loss or infertility and produced an overall sex ratio of 1 male: 0.94 females.
Offspring sex ratios varied seasonally, with early and late initiating females producing more female offspring and mid-season initiators producing more males. While this pattern is in keeping with the assumed pattern of seasonal resource abundance, the sex ratio biases seen were at best only slight.
Using two measures of male quality, no support was found for the hypothesis that females should preferentially produce sons when mated to high quality males. The data did suggest, however, that females may reduce the variance in offspring sex ratios in larger harems. Some alternative interpretations are presented.
Methodology
The study was conducted from May through July 1981 at the Queen's University Biological Station in eastern Ontario. All study areas were Typha marshes located within 30 km of the Biological Station. Each area was searched thoroughly for nests at least twice a week throughout the breeding season with most nests being found during the nest-building or egg-laying stage. Territory boundaries were determined through repeated observations of male behavior (song posts, interaction with other males, etc.) with territory area subsequently computed from aerial photographs. Harem size was defined as the number of females necessary to account for the maximum number of synchronously active nests in a territory during the breeding season. Nests were monitored through incubation)n and the eggs or nestlings collected just prior to or imnediately after hatching. Eggs and nestlings were preserved in formalin for later sexing by necropsy. Nests suffering partial predation during incubation or those having one or more infertile eggs were not included in the analysis although this only accounted for 8 of 105 clutches collected. In doing so I have assumed that both egg predation and infertility are independent of the sex of an egg.