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The reproductive cost hypothesis predicts that parents with low investment capacity (e.g., those in poor condition) should produce the larger sex (usually males) less often so as to minimize the risk of current reproductive failure and/or a decrease in their residual reproductive value. The study aimed to test whether offspring sex ratio bias in replacement clutches of the Mew Gull (MG;  Larus canus ) and Black-headed Gull (BHG;  Chroicocephalus ridibundus ) exposed to different food conditions was in agreement with predictions of the reproductive cost hypothesis. Food availability was experimentally modified before egg laying. Our experimental treatment affected hatching sex ratio, being also significant the effect of laying order and species identity. For both gull species, parents that were provided with supplemental food had more sons at hatching than unfed (control) pairs. This was particularly so if they were fed until production of a replacement clutch. This gradient in hatching sex ratio was much more pronounced in MG than in BHG. In both gull species, sons hatched more often from first laid (A) than from second (B) or third-laid egg (C). Five days after hatching, the sex ratio of surviving chicks was dependent mainly on the experimental treatment and egg sequence. Unlike the time of hatching, there was no important effect of species × treatment interaction. In both gull species, the brood sex-ratio was clearly male-biased in parents receiving supplemental feeding. However, differences between experimental groups were less extreme than at hatching. Chicks hatched from C-egg had much lower survival than chicks from the first two eggs (A- and B-egg), irrespective of sex, treatment, or species. Supplementary feeding was associated with improved chick survival for both gull species, although this effect was dependent on chick gender. Surprisingly, in the case of control broods (i.e., broods whose parents did not receive supplementary food), male chicks survived better than female chicks, but the opposite was true for broods in which parents were fed. There was an overall tendency for higher survival of BHG chicks than MG chicks. During the early-chick period, competition between siblings and the related differentiated mortality of chicks from A-, B-, and C-eggs, constituted a mechanism which shaped the brood sex ratio bias. In this context, differences in the sex ratio bias in eggs of different rank (A, B, C) at hatching were important. The results appear to fulfil the expectations of the reproductive cost hypothesis. For the species with greater sexual dimorphism (MG), a relationship between the condition of parents and the expected bias of sex ratio was visible only at hatching, which may indicate that this feature is not significant in shaping the sex ratio bias in MG.

journal of ornithology

Effect of food availability on offspring sex ratios in replacement clutches of Mew Gulls (Larus canus) and Black-headed Gulls (Chroicocephalus ridibundus) in the highly unstable environment of the Vistula River