Offspring-defence behaviour towards a human intruder in three species of gulls: Larus marinus, L. argentatus and L. canus

Abstrakti

I studied the behaviour of three species of gulls (Great Black-backed, Herring and Common Gull) when I entered breeding colonies, or in the case of the Great Black-backed, the territories of solitary pairs. When rearing chicks, all threespecies reacttowards the intruder by circling above the intruder (passive mobbing), or swooping down on him (active mobbing) . Direct attacks usually end before the gull touches the intruder, but some pairs attack fiercely, striking with their feet . Herring and Common Gulls also performed a "distraction display" involving low flight abovethe sea with (sometimes) trailing feet . The results suggesta change in behaviour between the incubation and the chick rearing period, during incubation many pairs seemingly ignore the intruder, or remain inconspicuous . Great Black-backs never landed on the water during disturbances . Great Black-back pairs seem to be attackers more often than pairsof the other twospecies. In colonies, Herringand Common Gull pairs attack at their territories, and only a fraction of all pairs are attackers. I found no indication of "communal defence" . I suggest that in large colonial species at least there is a benefit in returning synchronously, and that the gain from flocking for passive mobbers is that they can protect their offspring from neighbours by landing synchronously with the other colony members once the disturbance is over.