Food supply differentially affects sibling negotiation and competition in the barn owl (Tyto alba)

In contrast to most birds, nestling barn owls (Tyto alba) vocalise not only when parents are at the nest but also in their absence. Calls produced in their absence have been shown to facilitate sibling negotiation over the im pending food resource. Since nestlings vocalise more vigorously in the pres ence of parents, they may be calling not to negotiate resources but to comp ete amongst each other over parental food distribution. A critical issue is to determine whether offspring need differentially affects sibling negotia tion and sibling competition, that is vocalisation in the absence and prese nce of parents. To answer this question, I manipulated the food supply of 2 6 broods by adding or removing prey items. In the absence of parents, food- added broods vocalised at a significantly lower level than food-removed one s. In contrast, once a parent arrived at the nest, the vocalisation level w as not significantly related to the manipulation of brood food supply. This suggests that in the absence of parents, it is more important for food-rem oved nestlings to vocalise intensely, and that in their presence, the relat ionship between begging and offspring need tends to vanish. In other words, brood food supply may affect sibling negotiation to a larger extent than s ibling competition.