Parental Care in the Wheatear Oenanthe oenanthe: Effects of Nestling Age and Brood Size

Abstract

Changes in different aspects of parental care of Wheatears with nestling age and brood size were studied over 4 yr in two Swedish breeding populations. Some broods were enlarged or reduced to increase the range of observed brood sizes. The proportion of time females spent brooding decreased with nestling age until day 8, but the mean duration of brooding bouts did not, implying that females reduced the frequency of brooding bouts as homeothermic capacity of the young improved. Females reduced mean bout duration with brood size but not the total time spent brooding. The different effects of nestling age and brood size, and the negative effect of females' feeding rates on brooding commitment suggest that the food requirements of the young may affect female brooding behaviour independently of the homeothermic capacity of the young. Feeding rates of males and females increased similarly throughout the nestling period and reached a plateau from day 9 to fledging at about day 14. Load size and prey size increased with nestling age, and the proportions of spiders and small caterpillars in the diet of nestlings decreased. Males delivered larger loads than females. The relationship between feeding rates and brood size during the plateau phase differed depending on whether manipulated broods were included in the analysis or not. For unmanipulated broods, feeding rates did not increase with brood size and there was thus a decrease in feeding rates per nestling with brood size. When including manipulated broods in the analysis, there was a decelerating positive relationship of feeding rates with brood size. When removing the small broods, no decrease in feeding rates per nestling with brood size was found in the joint analysis of all broods. Brood manipulations led to changes in feeding rates with respect to the normal pattern observed. Future studies of parental care should consider these effects of manipulations before deriving any definitive conclusion about brood size effects.