The blessing of having younger nestmates: the case of the Arabian babbler

Sibling rivalry and brood reduction are traditionally considered to derive from food shortage. The aim of this study was to check this in the case of the Arabian babbler (Turdoides squamiceps) and to enhance our understanding of the negative and positive effects of each nestling on its siblings. We carried out 2 experiments: 1) we added for 1 day a foreign nestling to 18 nests in which a nestling had died on the previous day and 2) we removed from the nest for 2h each individual from 10 four-nestling nests. We found that parents adequately adjusted their feeding rate to the introduction of a foreign nestling and that nestlings received less feeds when the removed nestling was smaller than them and more feeds when the removed nestling was larger than them. This finding corroborates Forbes’s (2007) hypothesis by which smaller, hungrier nestlings are effective in soliciting additional feeds, many of which are eventually appropriated by the larger nestlings when brought to the nest. Nevertheless, a game-theoretic analysis shows that if the 4 nestlings are close in size, in terms of net energy gains, the larger nestlings actually lose from the presence of their youngest sibling, due to the increased evolutionarily stable strategy level of aggression in the nest; only if the youngest nestling is sufficiently smaller than them, aggression gets sufficiently diminished so that these additional feeds translate into net energy gains. A medium extent of clutch asymmetry, preventing excessive aggression while sustaining the smallest chick, may thus be beneficial to all.