Sex Ratios of Red-Winged Blackbirds by Egg Size and Laying Sequence

Abstract

Egg sex, size, and laying sequence were determined for 44 three-egg and 129 four-egg Red-winged Blackbird (Agelaius phoeniceus) clutches collected in 1982 and 1983. The proportion of males decreased with laying seq. uence in three-egg clutches and increased with laying sequence in four-egg clutches. In both three- and four-egg clutches, egg size increased with laying sequence. However, egg sex was independent of egg size for both clutch sizes, indicating that the sex-sequence and size-sequence patterns are independent of each other. No support was found for the hypothesis that these patterns represent adaptive strategies associated with hatching asynchrony and clutch reduction or with the predict- ability of egg infertility. Therefore it remains undetermined whether these patterns repre- sent adaptive strategies or real but nonadaptive artifacts.