AN EVOLUTIONARY EXPLANATION FOR SEASONAL TRENDS IN AVIAN SEX-RATIOS

We present an extensive set of data for five species of raptorial bird s to demonstrate that some raptor species produce an excess of daughte rs early in the season and an excess of sons in late nests, while othe rs show the reverse. By means of a simulation model we investigate an evolutionary explanation for this phenomenon in terms of sex-specific differences in the relation between age at first breeding and date of birth. The model predicts that that gender should be produced first in the season whose age of first breeding is more strongly accelerated b y an early birth date. We argue that this tends to be the male gender in raptor species, such as the common kestrel (Falco tinnunculus), whi ch tend to breed early in life, while it is the female gender in large r species with later onset of breeding, such as the marsh harrier ( Ci rcus aeruginosus). The empirical evidence is qualitatively consistent with this hypothesis. Our model is quite general in that it makes no a ssumptions about the mechanism (primary sex-ratio bias at egg laying o r secondary sex-differential mortality before fledging) by which the b ias is generated. Yet it is able to create quantitative predictions fo r species where sufficient demographic and life-history data are avail able. From the available data set in the common kestrel we derive a qu antitative prediction for the seasonal trend in brood sex ratio. The o bserved trend is in good agreement with this prediction.