SPATIAL DISTRIBUTIONS OF TERRITORIAL AGGRESSIVENESS IN OYSTERCATCHERS, HAEMATOPUS-OSTRALEGUS L

In two habitats supporting different densities of breeding oystercatchers, Haematopus ostralegus L., the intensity with which these birds defended central and peripheral parts of their territories was compared. Territorial birds in both study areas were equally aggressive to intruders landing close to central nests, but birds breeding at the higher density were more aggressive to intruders near the edge of territories than were the low density birds. In the more densely settled area, territories were smaller and more often contiguous with other territories, territorial settlement was more synchronous, and the frequency of intrusions by apparently non-territorial oystercatchers was higher. These findings suggest that the level of conspecific pressure for territorial space was an important proximate factor contributing to the marked difference in mean territory size in the two areas. © 1979.