Kin selection and reciprocation of biological services are distinct theories invoked to explain the origin and evolutionary maintenance of altruistic and cooperative behaviours. Although these behaviours are considered as non-mutually exclusive, the cost-to-benefit balance to behave altruistically or to reciprocally cooperate, and the conditions promoting a switch between such different strategies have rarely been tested. Here we examined the association between allofeeding, allopreening and vocal solicitations in wild barn owl (Tyto alba) broods under different food abundance conditions: natural food provisioning, and after an experimental food supplementation. Allofeeding was mainly performed by elder nestlings (hatching is asynchronous) in prime condition, especially when the cost of forgoing a prey was small (when parents allocated more prey to the food donor and after food supplementation). Nestlings preferentially shared food with siblings that emitted very intense calls, thus potentially increasing indirect fitness benefits, or the ones that provided extensive allopreening to the donor, thus possibly promoting direct benefits from reciprocation. Finally, allopreening was mainly directed towards older siblings, perhaps to maximize the probability of being fed in return. Helping behaviour among relatives can therefore be driven by both kin selection and direct cooperation, although it is dependent on the contingent environmental conditions.

We recorded the allofeeding, allopreening and vocal negotiation of entire barn owl broods in the natural condition using cameras and individual microphones. We recorded during two consecutive nights under different conditions of food abundance: one night under the natural food provisioning regimes provided by parents and a second one after an experimental food supplementation. The sex of each nestling was determined using molecular markers and age by measuring the left-wing from the bird’s wrist to the tip of the longest primary.