Inadequacy of activity time as a niche difference: the case of diurnal and nocturnal raptors.

It has been assumed that differences in activity time of diurnal (Falconiform) and nocturnal (Strigiform) raptors reduce competition for food by giving them access to different prey resources. Consequently, dietary similarity should be greater between synchronously active raptors (either diurnal or nocturnal) than between asynchronous ones (diurnal versus nocturnal). Using five collective raptor assemblages I tested the hypothesis that the statistical distribution of food overlaps between synchronous raptors (Falconiform x Falconiform and Strigiform x Strigiform combined) should be composed of larger figures than that obtained from asynchronous pairs (Falconiform x Strigiform). Both a conservative (Kolmogorov-Smirnov test with Chisquare approximation) and a powerful (Mann-Whitney U-test) nonparametric procedure were used to deal with the problem of the degrees of freedom to be assigned to non-indepedent observations such as overlaps. The tests consistently failed to reject the null hypothesis, thus pointing to the inadequacy of temporal segregation as a means to partition prey resources between diurnal and nocturnal raptors. I propose that this can be accounted for by two factors, acting singly or in combination: a) that both Falconiforms and Strigiforms extend their hunting activities to crepuscular hours thus sharing prey of that activity period; b) that reputedly diurnal and nocturnal prey become vulnerable to predators of asynchronous hunting time by extending their activities in the field. Based on records of interspecific territoriality, prevation, and interference competition I propose that the evolution of nocturnality in Strigiforms might be more clearly related to interference interactions rather than exploitation competition with Falconiforms.