Breeding timing and nest predation rate of sympatric scops owls with different dietary niche breadth

Breeding timing is one of the key life-history traits considered to be under strong stabilizing selection, such that offspring should be born when food resources are most abundant. Predation, however, may also affect the breeding timing because nest predation is a leading mortality for many species, although this possibility has been less considered. Here, we examined the possible effects of nest predation on breeding timing by comparing sympatric scops owls in a subtropical forest where only reptilian predators are present. The Japanese Scops Owl (Otus semitorques Temminck and Schlegel, 1844), a dietary generalist, bred one month earlier than the specialist Ryukyu Scops Owl (Otus elegans (Cassin, 1852)). The breeding timing of the Ryukyu Scops Owl matched with the emergence of their main prey species, but also matched with predator activity. Accordingly, the predation rate on eggs or nestlings was 7.5 times higher in the Ryukyu Scops Owl (13.9%; 21 out of 150 nests) than in the Japanese Scops Owl (1.9%; 1 out of 52 nests). Clutch size, on the other hand, was significantly larger in the Ryukyu Scops Owl than in the Japanese Scops Owl, possibly compensating loss from predation. Although alternative explanations still remain, our results suggest that the food generalist might have adjusted its breeding timing to avoid nest predation, whereas the breeding timing of the specialist might have been constrained by the availability of its main prey items.