Aggressive behaviour and nest-site defence during the breeding season in an island kestrel population

We examined the changes in the intensity of intra-specific aggression (ISA) in a kestrel (Falco tinnunculus canariensis) population throughout the breeding season, in order to evaluate the main hypotheses regarding ISA (defence of parentage, food, nest-site and offspring). Each pair was presented with a live caged adult male or female as a conspecific intruder. The intensity of ISA did not vary significantly during the breeding season, and nor did it correlate with offspring value, or food availability. Our island-dwelling kestrel population showed low fluctuation in ISA during the breeding season, and similar levels of aggression in both members of the pair towards male or female intruders. These findings support the nest-site defence hypothesis. Pairs with more exposed nest-sites showed increased ISA, unexplained by offspring value. We suggest that this behaviour is related with nest-site conspicuousness and vulnerability. The pairs displaying greater ISA also showed greater aggression towards humans, the main local predator. This may indicate that defensive behaviour evolved in the past when it was advantageous against predators, and became a trait. Compared with the non-island reference population, we only found differences in ISA during the pre-laying phase: lower intensity of aggressive behaviour between males, and higher between females.