Matching times of leading and following suggest cooperation through direct reciprocity during V-formation flight in ibis

Cooperation in animals is an enigma because it contravenes the basic notion that evolution favors selfish genes that promote only their own well-being. Bird migration in organized V-shaped or echelon formations constitutes such a cooperation dilemma. We show that juvenile Northern bald ibis (Geronticus eremita) cooperate by taking turns and precisely matching times they spend in the advantageous trailing position and in the disadvantageous front position. This time matching is done on a pairwise level. Furthermore, we found evidence that the animals' propensity to reciprocate in leading has a substantial influence on the size and cohesion of the flight formations. This study shows that direct reciprocation can enable cooperation between animals in a natural context.