Secondary sex ratio adjustment in red-winged Blackbirds (Agelaius phoeniceus)
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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.
KeywordsInfertility Alternative Interpretation Female Offspring Male Quality Resource Abundance
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