Infanticide in Relation to Individual and Flock Histories in a Communally Breeding Bird, the Mexican Jay (Aphelocoma ultramarina)

Abstract

Infanticide, the killing of eggs or nestlings, has been observed eight times during studies of the communally breeding Mexican jay from 1969-1980. In five of the six cases in which the robber was identified, both robber and owner of the victimized nest were from the same communal unit. In the remaining case the robber was a trespasser from a neighboring unit. Evidence from Mexican jays and anis suggests that infanticide among communal birds may be frequent in plural-breeding species, in which more than one pair per unit lay eggs. Infanticide in Mexican jays appears to be restricted to certain social units. Affected units are characterized by high rates of immigration, low reproductive success, and low relatedness. In most cases robbers were immigrants to the group in which infanticide occurred, and in no case was the robber related to its victim. In contrast to anis, infanticidal Mexican jays do not have higher reproductive success than do their victims. The same individual may be a victim of infanticide at one nest and a robber at another, and infanticide does not appear to be restricted to particular "rogue" individuals.