The Relationship of Habitat Quality to Group Size in Hall's Babbler (Pomatostomus halli)

Abstract

A central problem in evolutionary biology concerns the ecological role of operational altruists (Brown 1975a), or helpers (Skutch 1961), as they are known in certain avian communal or cooperative social systems. Critical to the understanding of their role is determination of the factors controlling their numbers in a group. The present study approaches this question by examining some of the proximate factors that influence flock size in a species of bird with a communal social system (sensu Lack 1968, Brown 1975a). We believe that knowledge of proximate factors will help us to understand the ultimate factors that are responsible for the adaptive aspects of operational altruism in such social systems. The adaptive value of helpers has been discussed elsewhere (e.g., Brown 1974). Hall's Babbler (Pomatostomnus halli) was favorable for this study because its flocks are distinct and the members of a flock readily counted. The species occurs in relatively open habitat so the flocks are easy to find and follow at a distance with minimal disturbance to the birds. Babblers of the genus Pomatostomus occur throughout Australia in suitable habitat. The behavioral ecology of P. temporalis has recently been studied by Counsilman (1974) and King (1974). Babblers in the genus Turdoides have been studied by Zahavi (1974), Gaston and Perrins (1974) and Gaston (1976). In P. temporalis, fertilized eggs are produced by a single pair that is aided in various other aspects of the care of the young by helpers (King 1974). The flock consists of the pair plus the helpers. Limited evidence suggests that this is also true in P. halli (Balda and Brown 1977). Using behavior of the babbler flocks to indicate their order of preference for various types of habitat, we found three vegetational indices of home range quality. We used these to test for correlations between home range quality and flock size.