COMPETITION AND BLACKBIRD SOCIAL SYSTEMS

The principle of competitive exclusion, variously known as Gause's Law, the Volterra-Gause Principle, or Grinnell's Axiom, has emerged as an important ecological generalization notwithstanding the reservations of some ecologists (Cole, 1960) and the active antagonism of others (Andrewartha and Birch, 1954). The evidence in support of competitive exclusion, derived from six main sources, strongly suggests that interspecific competition has had an important influence on the evolution of contemporary community structure despite the apparent relative abundance of resources with respect to the sizes of the populations utilizing them (Hutchinson, 1957). Unfortunately, most of the supporting evidence is indirect. Closely related sympatric species have attracted the most attention, and whenever they have been carefully investigated, important ecological differences have been discovered (Lack, 1944, 1945, 1946; Carpenter, 1952; Diver, 1940; MacArthur, 1958; Gibb, 1954). However, since identical species are theoretically unlikely, differences are to be expected whether competition is manifest or not. This evidence would, therefore, be less convincing if it were not for evidence of the second type, namely, that closely related species may be more different morphologically and ecologically in areas of sympatry than in areas of allopatry (Lack, 1947, 1949; Vaurie, 1951; Brown and Wilson, 1956). A third source of evidence is the structure of the incomplete communities of isolated mountains and remote islands, where those few species present nearly