Seasonal Variation in the Sex Ratio of Marsh Harrier Circus aeruginosus Broods

Analysis of the sexes of 2260 nestlings in 735 marsh harrier broods revealed an overall excess [sex ratio (SR) = 54.8%] of males, and a significant increase in the proportion of males with progressive laying date (d = day of the year): In [SR/(1-SR)] = -1.286 + 0.013 d. 2. We argue that it is likely that these deviations from parity are determined at ovulation rather than by differential mortality ot the sexes. This argument is based on two facts. First, the seasonal trend was most pronounced among the largest and oldest two nestlings per brood. These corresponded with the initial two eggs in 95% of the cases in a subset of 61 nests followed closely. Secondly, these initial eggs suffered least mortality. 3. We speculate that marsh harriers may benefit from producing more daughters early in the season if the probability of females breeding as yearlings declines with progressive date of birth.