Size-mediated dominance and begging behaviour in Eurasian kestrel broods

Resource allocation from parents to their offspring can be modulated by inter-sexual size dimorphism. High dimorphism promotes differential costs in rearing male and female offspring and unequal competitive ability among siblings. We examined whether any of these types of biases occur in Eurasian kestrels, Falco tinnunculus, in which females are larger than males. We measured begging behaviour of nestlings during the second week after hatching to establish how parents respond to begging signals. In addition, we looked for possible costs of begging as a trading function with T-cell-mediated immune response or growth. Begging display was higher in nests suffering from food shortage. Parents (mothers in 98% of the cases) allocated more food to chicks begging more intensely. Female chicks obtained more food from parents than males, but only in nests in which parents provided the chicks with prey items small enough to be swallowed whole (without dismembering) by the chicks. No difference in begging calls was observed between male and female nestlings. However, female chicks were closer to the parent in non-dismembering nests. Begging display was not associated with T-cell-mediated immunity or growth. Our results show a clear response of food provisioning by parents to begging display of nestlings, and suggest that the advantage of female nestlings in food acquisition was due to their competitive superiority over male sibling nest-mates in scramble competition for monopolizeable prey.

Juan A. Fargallo, Toni Laaksonen, Erkki Korpimaki, Ville Poyri, Simon C. Griffith and Jari Valkama, Evolutionary Ecology Research, 2003, 5: 549-558