Parental behavior and reproductive output in male-only cared and female-only cared clutches in the Eurasian Penduline Tit (<i>Remiz pendulinus</i>)

Diverse patterns of parental care, including uniparental care by either the male or the female, provide excellent opportunities to investigate how variation in social traits is maintained in wild populations. Coexistence of different parental strategies within the same population is expected when they exhibit similar cost-benefit ratios. We investigated one of the most diverse avian breeding systems and compared parental behavior and reproductive output between nests that are uniparentally cared for by male or female Eurasian Penduline Tits (Remiz pendulinus). In this small passerine bird, full care (incubation and brood care) is provided by the male only (7-18% of nests) or the female only (48-65% of nests). Additionally, a third of all nests are deserted by both parents as a result of intense sexual conflict over care. We show that parental effort is similar between male and female parents, and they achieve similar offspring survival after controlling for clutch-size differences. Females care for larger clutches than males (5.7 +/- 0.1 [SE] and 3.5 +/- 0.2 eggs, respectively), and they produce more nestlings (3.6 +/- 0.2 and 2.4 +/- 0.3 nestlings, respectively), although this difference is not significant at fledging of the chicks (3.1 +/- 0.2 and 2.4 +/- 0.3 young, respectively). Therefore, sex difference in care does not explain the bias toward female-only care in Eurasian Penduline Tits. We propose that the differential benefits of desertion and/or the differential costs of care for males and females facilitate the existence of both uniparental care types in the population. Received 18 April 2012, accepted 2 July 2012.