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

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