Acquisition of Predator Information By Active and Passive Mobbers in Ring-Billed Gull Colonies

Abstract

AbstractMobbing flocks of ring-billed gulls consisted to two groups: individuals whose nests were near to and threatened by the predator (active mobbers) and others from more distant parts of the colony (passive mobbers). This study examined whether either group can learn about the predator from their mobbing experience. The active mobbers quickly habituated to a human intruder near their nests when the intruder was inactive and empty-handed. Conversely, the gulls responded with a greater flock size and flight distance and fewer swoops when the intruder was holding an adult gull. These response patterns persisted even after the adult gull was removed and the human intruder reappeared alone. When the human intruder held a chick, the gulls approached more closely and swooped more than when the intruder held an adult gull. To examine whether passive mobbers learned anything about the human intruder during the earlier tests, the intruder (now empty-handed) moved to a new location 50 m away and repeated the test. The gulls at these sites, also were significantly less wary than naive gulls of the human intruder following tests in which the intruder was empty-handed, and more wary following tests in which the intruder held an adult gull. Because the earlier test sites were situated so that they could not be seen from the ground at the 50 m sites, a gull at the latter sites could only have learned details about the intruder if it or some of its neighbors had become mobbers. Gulls whose nests were 100 m from an owl model or human intruder learned less about it than gulls at 50 m: perhaps because fewer gulls became passive mobbers as the distance between them and the owl or intruder increased. Costs of becoming a passive mobber include an energetic and time cost and some risk of personal injury due to mid-air collisions with other gulls. The number of passive mobbers increased on breezy days perhaps because strong updrafts on those days reduced the energetic costs of flight within the colonies (and hence of being a mobber). These results support the hypothesis that the gulls' predator-attraction behavior allows them to learn about predators from watching predator-conspecific interactions. The opportunity to witness such interactions and learn about predators may be an important secondary benefit of coloniality for these birds.