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Vocal begging and locomotor activity are modulated independently in Bank Swallow (Riparia riparia) nestlings

ABSTRACT Nestlings of birds breeding in closed nests compete for food by both vocal begging and competing for a position near the nest entrance (locomotor activity). The latter strategy can be particularly effective in nests with long entrance tunnels, where nestlings can intercept feedings from incoming parents. However, little is known about whether the intensity of locomotor activity is related to food availability in the same way as the intensity of vocal begging. We hand-fed Bank Swallow nestlings (Riparia riparia) in the laboratory under four feeding regimes: without restriction (AL), or facing different levels of food restriction (in order of increasing degree: FR1, FR2, FR3). Compared with group AL, the intensity of vocal begging was elevated in group FR1 and did not increase further in the FR3 group, whereas only the FR3 nestlings showed higher locomotor activity (quantified as the frequency of crawling to the nest tunnel). The FR3 group was also the only group where reduced food intake slowed wing development. We conclude that vocal begging and locomotor activity were modulated independently by the level of food intake. We hypothesize that in young Bank Swallow nestlings locomotor activity is a more effective but also more expensive means of soliciting food than vocal begging and therefore is used only by the neediest of nestlings (e.g., those with impeded wing development). Our results illustrate the interplay between different components of complex begging displays serving to convey information about individual needs and competition with nestmates.