Male and female offspring can differ in their susceptibility to pre-natal (e.g. egg quality) and post-natal (e.g. sib–sib competition) conditions, and parents can therefore increase their individual fitness by adjust- ing these maternal effects according to offspring sex. In birds, egg mass and laying/hatching order are the main determinants of offspring viability, but these effects can act differently on each sex. In a previous study, relatively large last-laid (c-)eggs of yellow-legged gulls (Larus michahellis) were more likely to carry a female embryo. This suggests compensatory allocation of maternal resources to daughters from c-eggs, which suffer reduced viability. In the present study, we supplemented yellow-legged gulls with food during the laying period to experimentally test whether their nutritional conditions were responsible for the observed covariation between c-egg sex and mass. As predicted, food supplementation enhanced female c-eggs’ mass more than that of male c-eggs. Thus, this experiment indicates that mothers strategically allocated their resources to c-eggs, possibly in order to compensate for the larger suscepti- bility of daughters to hatching (and laying) order. The results also suggested that mothers decided on resource allocation depending on the sex of already ovulated c-eggs, rather than ovulating ova of either sex depending on food availability.

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