An Experimental Test of Central Place Foraging Theory in a Cooperatively Breeding Bird

Central place foraging, whereby individuals are constrained in their foraging by their need to return to a central place, is common in nesting birds. A key prediction of the central place foraging theory models developed by Orians and Pearson is that as individuals forage further from their central place, optimal load size will increase in order to maintain the same energetic profitability. While this prediction has received widespread support, to our knowledge, it is yet to be tested in a cooperatively breeding species in which multiple adults care for young. In this study, we experimentally tested this central place foraging theory prediction in wild cooperatively breeding Southern Pied Babblers (Turdoides bicolor) by supplementally feeding individuals with different numbers of mealworms (load sizes of 1, 2 or 3 mealworms), at different distances from their active nests. While babblers were more likely to return to the nest to feed young when given a larger load and when given food closer to their nest, the probability of returning to the nest to feed young at a greater distance did not differ with load size, thus revealing no support for the load size-distance relationship predicted by Orians and Pearson. However, the likelihood of babblers returning to the nest to feed young was affected by the chick:adult ratio, with birds more likely to provision if there were more chicks per adult individual, and was also influenced by the relationship between age and load size, with older birds significantly more likely to feed young when given three mealworms compared to younger birds given the same load size. Our results represent the first experimental test of central place foraging theory in a cooperatively breeding bird species and highlight the importance of considering sources of variation in provisioning behaviour.