Section 18
Chapter 17,785

Barn swallow mobbing: self-defence, collateral kin defence, group defence, or parental care?

Shields, WM.

Animal Behaviour 321: 132-148


ISSN/ISBN: 0003-3472
DOI: 10.1016/s0003-3472(84)80331-0
Accession: 017784872

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Stimuli (human and stuffed owl) were presented to a marked population of barn swallows (H. rustica) during the 1980-1982 breeding seasons at the Cranberry Lake Biological Station, Saint Lawrence County, New York USA . In experiment 1, the stimuli were presented at various distances from active nests during various stages of the reproductive cycle. Both the probability and intensity of mobbing varied during the breeding season, increasing with the probable reproductive value of young of the year and the degree of danger posed to them. Mobbing group sizes varied positively with local nest densities. In experiment 2, mobbing group structure was analyzed in greater detail. Barn swallow mobbing groups usually contained active mobbers (those that emitted mobbing calls, and approached the stimulus closely, < 2 m, or even hit it) and passive mobbers (which were silent and flew in circles at greater distances, 2-10 m, from the stimulus). Passive mobbers were a random sample of the local population with respect to sex, age, nest location and reproductive status. This pattern is consistent with a hypothesis that passive mobbing, the less risky type, is a form of self-defense that reduces an actor's chance of being eaten, probably by providing information about the identity or probable behavior of potential predators. Active mobbers were not a random sample of the source population. Mated birds and especially parents with nestlings were over-represented; non-breeders, juveniles and incubators rarely mobbed actively. The seasonal changes in mobbing and the identity of active and passive mobbers are inconsistent with hypotheses that mobbing is a form of cooperative group defense or altruism conditioned by reciprocity or kin or group selection. The data are consistent with other hypotheses which propose that mobbing benefits the mate or the young.

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