Distinct patterns of food offering and co-feeding in rooks

Food sharing is widespread in animals and has received considerable attention because of its apparently altruistic nature. Sharing food includes different behaviours, notably offering food and co-feeding. In primates, food sharing between unrelated animals can largely be explained by reciprocity and harassment avoidance, while kin selection explains most instances of food sharing in cooperatively breeding birds. 'Costly signalling' has also been put forward as an explanation of food sharing in birds. A recent study on jackdaws, Corvus monedula, suggested that the costly signal of food sharing, notably offering food between juvenile birds, may play an essential role in the formation of pair bonds. We analysed food-sharing patterns in a group of juvenile rooks, Corvus frugilegus and found differences between the two modes of food sharing: offering food and co-feeding. Food offering was affected by dominance relationships and by gender, whereas co-feeding was reciprocated and occurred mainly between nestmates. We conclude that offering food and tolerating co-feeding have different functions in rooks: food offering acts as a costly signal directed to all members of the group whereas patterns of tolerating co-feeding were in line with both the reciprocity and pair-bonding hypotheses. (C) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.