Agonistic behaviour prior to laying predicts clutch size in Eurasian kestrels: an experiment with natural decoys

Aggressive behaviour against conspecifics is considered a costly behaviour that forms part of reproductive investment. Previous studies have suggested that the intensity of agonistic behaviour may indicate individual quality since it exhibits a positive correlation with brood value. However, this relationship has been demonstrated after clutch completion, and by creating a nexus between efforts it has not been possible to disentangle other hypotheses that could explain such findings. Here, by presenting a trio of natural decoys (adult male, adult female and 1-year-old male) to Eurasian kestrels, Falco tinnunculus, we tested whether the aggressive behaviour to conspecifics prior to laying predicted posterior reproductive performance (clutch size). We also tested the effect of breeding stage and breeding density on this behaviour. In addition, we examined the effect of the presence of moulted versus unmoulted 1-year-old males on aggressive behaviour of breeders. Intrasexual agonistic behaviour, measured as attack intensity to the decoys, was positively correlated with clutch size in females, but negatively in males. In males, aggressive behaviour was lower during the female fertile stage than during previous stage, but in females, was similar over stages. Furthermore, agonistic behaviour increased with breeding density in females but not in males. We discuss these results in the context of sexual selection theory, and propose that the link between agonistic behaviour prior to laying and clutch size is via the connection to courtship feeding and that may be considered as an indicator of individual quality for both sexes.