Biparental incubation and allofeeding at nests of Sagebrush Brewer's Sparrows

E-ISSN： 1557-9263|86|2|153-162

ISSN： 0273-8570

Source： JOURNAL OF FIELD ORNITHOLOGY, Vol.86, Iss.2, 2015-06, pp. : 153-162

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Abstract

ABSTRACTFacultative male incubation and allofeeding are thought to be behavioral adaptations by which male songbirds maximize their fitness by reducing the energetic stress of their mates. However, few data are available for most species that exhibit these behaviors so the extent to which they might enhance male fitness remains unclear. Among North American sparrows, male incubation is known in four species, but the relative contributions of each sex have been estimated for only one species. We quantified biparental incubation and allofeeding in Sagebrush Brewer's Sparrows (Spizella breweri breweri) with 24‐h video surveillance of nests (N = 24) at two locations in northern Nevada. We detected biparental incubation at both sites (17 of 24 nests, 71%), and found that mean constancy (i.e., proportion of 24‐h period eggs were covered) was significantly higher at biparental nests than at uniparental nests, and mean recess duration (i.e., when eggs were uncovered) was significantly shorter. Incubation constancy at biparental nests in our study was the highest yet reported in the genus Spizella (range = 0.87–0.96), and constancy at uniparental nests (mean = 0.76, range = 0.71–0.81) was greater than that of congeners where female‐only incubation is thought to be the norm. Biparental incubation was more likely on colder days, but a comparison of models where incubation strategy and its interaction with ambient temperature were included as independent variables revealed that temperature was not the best predictor of constancy. Allofeeding occurred at very low frequency and only at biparental nests (11 of 20, 55%). Biparental nests with allofeeding had more incubation sessions per hour than biparental nests without allofeeding. The recipient usually left the nest immediately after being fed, and the feeding individual assumed the role of incubator. Our results suggest that some factor other than low ambient temperatures favors biparental incubation by Sagebrush Brewer's Sparrows. One possibility is that male incubation and the resulting increase in incubation constancy may better conceal nests and reduce the likelihood of nest predation. The low frequency of allofeeding at nests with biparental incubation in our study suggests that it serves some function other than improving female nutritional status or reducing activity levels at nests. Rather, allofeeding may serve as an intraspecific signal important for maintaining the social bond between mates.