Larid Site Tenacity and Group Adherence in Relation to Habitat

Abstract

WHILE conducting extensive banding studies of the Cape Cod, Massachusetts colonies of Common Terns (Sterna hirundo), 0. L. Austin, Sr. developed an overall theory concerning the roles of various interacting factors in controlling the numbers and colony distribution in his region (Austin 1940, 1942, 1944, 1945, 1946, 1947, 1949, 1951; Austin and Austin 1956). The Austins' colonies were situated in relatively but not completely stable habitats that usually allowed successful nesting, but were susceptible to occasional flooding or predation. They found that individual terns tended to return to the same colony site or even nest site year after year, a tendency that increased with age of the bird. Similarly individuals showed an age-increasing tendency to nest with the same neighbors year after year, even when the colony or subcolony had to shift from the usual colony site. These tendencies to nest repeatedly on or close to the same site and among the same neighbors are termed site tenacity and group adherence respectively. Thus, as a tern grew older, it had a greater tendency to nest in both familiar physical surroundings (site tenacity) and familiar social surroundings (group adherence). In general site tenacity undoubtedly has selective advantage in reducing susceptibility to predation and other damaging factors by familiarizing the bird with its surroundings and by allowing it to return to sites of previously successful nestings. This reduction of danger by familiarity of surroundings is frequently included among the functions of territoriality (Hinde 1956, Klopfer 1969). In slowly changing habitats, site tenacity may also help in pioneering new habitats (see Hilden 1965: 65 ff). Too great a development of site tenacity would be disadvantageous if it promoted the continued use of poor sites or of sites that changed rapidly or deteriorated. Thus the degree of site tenacity developed in a population may reflect the stability of nesting habitat, stable here used in the sense that many generations could be raised in the same location with little if any increase in danger to the offspring. Group adherence may appear largely fortuitous in a colony of high site tenacity, but does appear to be a separate attribute. This is indicated by the fact that when the Austins' colony sites became temporarily unsuitable, the members reappeared elsewhere in a group as a subcolony of a larger colony, rather than randomly dispersed within one nearby colony. Presumably this group adherence is based on individual recognition, known