Nest attendance, extended phenotype and social selection in multispecies colonies

Colonial species interact not only with conspecifics but often with other species nesting in the same site. The effect of conspecific traits have been measured recently with a multilevel selection analysis, but the effect of social traits of heterospecifics on individual fitness remain unquantified. We recorded nest attendance effort of two species, lesser kestrels (Falco naumanni) and jackdaws (Corvus monedula), nesting on the Gela Plain (Sicily, Italy). Both species are secondary-cavity nesters breeding in abandoned rural buildings where they form single-species or mixed-species colonies. By correlating reproductive success as a measure of fitness to conspecific and heterospecific nest attendance we revealed an asymmetric relationship where only lesser kestrels accrued fitness benefits by nesting with jackdaws in the same colony-housing site. Jackdaws, on the other hand, benefited from conspecific vigilance effort at their own nest, regardless of the attendance level of co-nesting lesser kestrels. In single-species colonies, stabilizing social selection coefficients revealed that the most favoured lesser kestrels were those living in groups with intermediate attendance, whereas, in mixed-species colonies, disruptive social selection coefficients altered the fitness surface so that the most successful kestrels were those associated with the highest attending jackdaw groups. Thus, in both cases, attendance efforts of focal individuals did not affect their own fitness, but their breeding performance depended on the social phenotypes of the individuals they associated with. Together with results from a larger scale investigation of the same study system, our findings illustrate the utility of applying a multilevel selection approach to social and behavioural traits in advancing hypotheses regarding ecological factors that may act as causal agents of selection.