Brood Sex Ratio In The Common Kestrel (Falco Tinnunculus)

Sex allocation and sex ratio adjustment have been emphasised in bird ecology. Fisher's theory predicts equal sex ratios at the end of parental care if the cost associated with raising each sex of offspring is equal. However, sex ratios have important evolutionary consequences and are often biased by many factors. In raptors, which display different degrees of reversed sexual dimorphism (RSD: females the larger sex), sex ratios bias are often reported. Here, we investigated the primary and secondary offspring sex ratio of a medium-sized falcon, the common kestrel Falco tinnunculus in two consecutive years, This species exhibits RSD, this allow us to test: (1) whether parental kestrels can skew the sex ratio of their offspring; (2) if they can, which strategies (i.e., date of breeding, brood size, laying sequence, egg mass, hatching sequence) the parental kestrels adopted to adjust their reproductive effort. Using molecular methods, we sexed 105 chicks from 23 nests at hatching and followed them to fledging stage to evaluat the secondary sex ratio, among these chicks, we used only 81 chicks from 15 clutches to evaluat the primary sex ratio (for these 15 clutches with complete eggs, all eggs were hatched, no mortality occurred during the nestling period, if mortality occurred, we collected dead nestlings'body tissues and sexed them). Our results showed that the primary and secondary offspring sex ratios in population level were 0.93:1 and 0.75:1(♂:♀), respectively, although there was a tendency for a skew towards daughters but they both did not depart from parity. Simultaneously, in broods level, the primary offspring sex ratio did not depart from parity, the secondary sex ratio was significantly female-biased.General linear mixed models (GLMMs) calculated and indicated that clutch size(F=15.763,P=0.046) and egg mass(F=15.261,P=0.047) and interaction between them (F=15.261,P=0.047)have significant effects on the primary offspring sex ratio at broods level. But the proportion of male eggs in broods changed non-linearly with clutch size. The proportion of males at broods with 5 eggs is maximum ,while the proportion of males at broods with 6 eggs is minimum.However ,we did not find any effects of clutch size ,egg mass and laying date on secondary sex ratio. We thought the female-baised secondary sex ratio was due to male-baised mortality. Our result showed that F. tinnunculus did not adjusted sex ratio of offspring by laying sequence or hatching sequence ,We just found last laid eggs and last hatched chicken were more likely to be males, which may be a valuable mechanism of parental control of sibling competition,because our result showed that eggs bearing male embryos were heavier than eggs bearing female embryos,although the difference between them was not significant(females,22.897±0.199;males,23.149±0.222;t = -0.847, p = 0.400).