Experimentally manipulated brood sex ratios: growth and survival in the black-headed gull (Larus ridibundus), a sexually dimorphic species

In sexually size dimorphic species, individuals of the larger sex often suffer from enhanced mortality during the nestling period. This has been attributed to higher nutritional requirements of the larger sex, which may render this sex more vulnerable to adverse food conditions. However, sex-biased mortality might not exclusively depend on the differences in food demand but also on other phenotypic differences, e.g., in competitiveness. Interference competition between the sexes and position in the laying sequence in particular may be essential components contributing to biased mortality. By creating synchronously-hatched unisex broods in the sexually size dimorphic black-headed gull, we specifically tested the effect of sex-specific food demand by excluding interference competition between the sexes as well as hatching asynchrony. To test the effect of egg quality, which varies with the position in the laying sequence, we composed each nest of chicks from eggs of all different positions in the laying sequence. All-male nests showed significantly enhanced mortality compared to all-female nests from the beginning of the development of the sexual size dimorphism onwards. This underlines the role of a higher food demand in biased mortality of the larger sex. In males but not females, asymptotic body mass and skeletal size were negatively associated with position in the laying sequence, while survival was not affected by position. As a consequence, sexual size dimorphism at the end of the nestling periodwas less pronounced compared to the natural situation. These data show that, although male growth is more sensitive to a decrease in egg quality, the higher mortality of last hatched chicks in natural nests is mainly due to hatching asynchrony and egg size but not egg content.