Aggressive Neglect as a Factor in Interspecific Competition in Birds

Abstract

IN their comments on an earlier paper of mine (1959), on the subject of competition between species in the Moluccan Islands, Hutchinson and MacArthur (1959) coined the phrase aggressive neglect to describe the tendency of one species to neglect its nest or young owing to the release of excessive aggressive behavior in the presence of a second species. This aggressive behavior with consequent limiting of the reproductive rate has been suggested by myself (t.c.) as possibly having survival value in cases of interspecific competition. According to the Volterra-Gause principle, two species may not occupy identical niches. And yet under certain circumstances, such as the case of the Asian-derived sunbirds and the Australian-derived honeyeaters in the Moluccan and New Guinea islands, it appears as if these species were in competition, not complete, in the absolute sense, but to a degree where the presence of one appears to affect the other. One evidence of this is the fact that on small islands throughout the area one species or the other may occur but not both. Thus in such confined situations where the total available biotope is highly compressed, competition has gone in favor of Nectarinia, the sunbirds, or Myzomela, the honeyeaters (see Figure 1). On larger islands such as on Batjan in the Moluccas where I made my study,1 small habitat preferences in addition to behavioral differences may then allow these species to co-occur. In this connection additional observations may be of interest. Both on Batjan and on Halmahera Island I observed an Asian-derived species, a moderate-sized, oliveyellow-colored bulbul, Hypsipetes affinis. This bulbul, which seems closely allied to species found in Sanghir and the southern Philippine Islands to the northwest, was found by us in small parties in a variety of habitats varying from cut-over scrub and garden patches to heavy evergreen forest, ranging from sea level up to at least 3,300 meters. In our experience the species showed no special habitat preference, being found indiscriminately on the edges of human habitation or in undisturbed forest. Birds in breeding condition tended to be in pairs and presumably held territories, although I was unable to determine the size or composition of these. Out of the breeding season, groups or small flocks, perhaps family parties, numbering up to eight individuals or more. were found in the forest, occasionally attracting other species such as