A Selfish Herd of Martins

Hamilton's (1971) "selfish herd" model demonstrated that an individual can benefit from group membership because of reduced probability that it will become the victim of predator attack. Basically, he showed that an individual can increase its safety by diminishing the distance between itself and conspecifics (alternative targets for predators); that is, individuals achieve safety by crowding. We present evidence that night-roosting birds show preferences for being on the "inside" of flock structure, as predicted by the hypothesis. We studied a night roost of Gray-breasted Martins (Progne chalybea) in downtown Manzanillo, Colima, Mexico (19ø2'N, 104ø21'W) on the evenings of 9 and 10 January 1982 and 4, 5, 6, and 8 January 1985. Approximately 10,000 birds roosted nightly on utility wires over city streets in the vicinity of the downtown square. During the 1982 study observers were scattered over the roosting area, and in 1985 2 observers concentrated on approximately 3,000 birds within 10-20 m of a park bench. The birds were distributed evenly along 14 main powerlines and several smaller lines 7-10 m above the street. An ob-