The function of early onset of nocturnal incubation in Red-winged Blackbirds

We examined variation in the onset of nocturnal incubation by female Red-winged Blackbirds (Agelaius phoeniceus). Many females began nocturnal incubation after laying the prepenultimate egg, which resulted in asynchronous hatching. This pattern of incubation is consistent with a model that predicts hatching asynchrony when the ratio of nest predation in the egg stage to predation in the nestling stage is near unity. Using 13 years of nest data, however, we found no significant relationship between the natural degree of hatching asynchrony and fledging success, and a significant negative relationship between hatching asynchrony and nestling quality. Therefore, we considered three hypotheses to explain the early onset of nocturnal incubation in this species: (1) increased egg viability, (2) protection of clutches from predators, and (3) protection of clutches from brood parasitism by Brown-headed Cowbirds (Molothrus ater). We found no evidence that early onset of nocturnal incubation increased egg viability. Incubation was not related to ambient temperature or precipitation, and early onset of incubation was not associated with increased hatching success. Nocturnal incubation was similar at depredated and nondepredated nests, which does not support the hypothesis that nest predation selects for early onset of incubation. Onset of nocturnal incubation occurred significantly later at nests parasitized by cowbirds than at unparasitized nests, however, in one of two years. These data suggest that protection against cowbird parasitism favors early onset of nocturnal incubation in Red-winged Blackbirds.