Begging form and growth pattern of nestlings correlate with parental food-allocation patterns in the Horned Lark (Eremophilaalpestris)

Abstract

Life-history theory assumes that selection favors parents that can maximize their reproductive success via behavioral strategies. As brood size determines the reproductive value of each nestling, parents may adjust their food-allocation patterns according to brood size. We test this assumption in the Horned Lark (Eremophila alpestris (L., 1758)). Our findings revealed that nestling begging forms varied with brood size, by gaping in one-chick broods and postural activity in two- and three-chick broods. Accordingly, parental food-allocation patterns differed in different-sized broods. In one-chick broods, parents increased feeding rates with the gaping duration of nestling. In two-chick broods, parents did not change food-allocation patterns according to nestlings’ begging. In three-chick broods, however, they fed later-hatched nestlings more even when early-hatched nestlings begged more intensely. Horned Larks exhibited obvious sexual differences in parenting style and ability, which resulted in nestlings from two- and three-chick broods changing their begging intensity according to the sex of the provisioning adult. Furthermore, nestling growth pattern diverged with brood sizes, with body mass growing faster in one-chick broods than in two- and three-chick broods. Growth rate of beak gape and tarsus length did not differ significantly among brood sizes, but beak gape was larger and tarsus length was shorter in one-chick broods than in larger broods at fledging. Our results thus support the idea that parents may use food allocation to regulate sibling rivalry, which in turn cause nestlings to beg food in different forms and grow in different patterns so that their reproductive success can be enhanced.