Vigilance during food handling by Oystercatchers Haematopus ostralegus reduces the chances of losing prey to kleptoparasites

Oystercatchers Haematopus ostralegus periodically pause while handling mussels Mytilus edulis to make visual scans. This paper presents evidence that scanning is associated with the high incidence of intra-specific food stealing among mussel-eating Oystercatchers. Scanning increased in frequency as bird density - and the risk of being attacked for mussels increased and the duration of attacks decreased. Additionally, among a sample of individually marked adults, the aggressive dominant birds spent half as much time scanning as the less aggressive subdominants and were also less likely to be attacked. Whereas detecting an attack made no difference to the success with which the dominants defended their mussels, subdominants increased their chances of retaining the mussel if they detected and carried the mussel away from the approaching attacker. The extra time which the less aggressive birds spent in vigilance seems best understood as a tactic for reducing food loss to kleptoparasites.