Comparative ecology of Australian corvids. VI. Why five species?

Abstract

The five Australian Corvus species are the only members of the genus with white eyes and probably stem from a common white-eyed ancestor. Two species-groups are recognized; the crows (C. orru and C. bennetti) mainly inhabit the tropics and arid interior, whilst the ravens (C. coronoides, C. mellori, and C. tasmanicus) are basically temperate southern species although one (C. coronoides) has spread north recently, probably following pastoral settlement. C. tasmanicus on the mainland is regarded as a relict population retreating as its forest refuges are felled and C. coronoides takes over the altered habitat; in Tasmania it has no competitors. Congeneric species-pairs of crows (C. orru with C. bennetti) and ravens (C. coronoides with C. mellori) coexist sympatrically in certain areas; each pair consists of a territorial resident and a nomad. The smaller nomadic species minimize competition with the larger residents by: (1) nesting semi-colonially in small trees and shrubs; (2) reducing the length of their nesting cycie; (3) leading fledglings into feeding flocks soon after they leave the nest, thereby ending their dependence on a breeding territory; (4) spending three-quarters of the year exploiting seasonal abundances of food, frequently in habitats unsuitable for breeding; (5) showing a greater flexibility in the timing of breeding, which enables them to respond to unusual seasonal conditions and to breed at times other than the spring; (6) eating foods different from those eaten by the larger birds. A pattern of speciation is suggested to account for the evolution of five Corvus species from an original white-eyed colonizing stock.