Summary
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 sequence 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 predictability of egg infertility. Therefore it remains undetermined whether these patterns represent adaptive strategies or real but nonadaptive artifacts.
Methodology
The study was conducted from May through July 1982 and 1983 on marsh-nesting populations of Red-winged Blackbirds located within 10 km of the Queen's University Biological Station at Lake Opinicon in eastern Ontario, Canada. All study sites were visited daily from the initiation of breeding through the breeding season. Nests were individually numbered when discovered (usually while being constructed) and checked on each subsequent visit. Eggs in each nest were numbered according to order of appearance using a waterproof felt pen. Because one egg is laid per day in this species, daily visits allowed accurate sequencing of entire clutches. If a nest was discovered after laying was initiated, those eggs already present were marked with a letter and subsequent eggs numbered according to sequence.