Nest aggregation and reproductive synchrony promote Lesser Kestrel Falco naumanni seasonal fitness

Several factors promote coloniality by enhancing the fitness of colony members. In birds, spatial proximity among nests, breeder abundance and reproductive synchrony have been proposed as primary factors responsible for enhanced colonial defence and foraging success, which, in turn, enhance reproductive success. Whether these factors function synergistically or antagonistically remains, however, an open question due to the absence of an integrated analysis of their effects on fitness. We studied a large population of the Lesser Kestrel, Falco naumanni, a facultative colonial species, breeding in colonies of different sizes in their typical pseudo-steppe habitat. We quantified both the singular and interactive effects of nest distance, breeder abundance and reproductive synchrony on kestrel fitness measured as the time to survival of eggs to hatching and nestlings to fledging. Egg survival increased as reproductive stages became more synchronous with the timing of colony breeding, whereas nestling survival benefited from a higher synchrony with most nests in the entire population. Nestling survival was also positively affected by the interaction between nest distance and breeder abundance. Our results suggest that the presence of additional breeders in the colony is not sufficient per se, to trigger colonial advantages, but instead, that synchronised reproduction among multiple breeding pairs nesting in close spatial proximity is necessary to realise those benefits. Our findings provide a novel perspective for future investigations that explore the mechanisms underlying fitness variation among Lesser Kestrel colonies and group-living species in general