Comparative demography of treecreepers: evaluating hypotheses for the evolution and maintenance of cooperative breeding

Despite the long history of research on cooperative breeding, few comparative studies have been undertaken to test hypotheses for the evolution and maintenance of delayed dispersal using data from both cooperative and noncooperative species. We tested predictions about demographic differences between cooperative and noncooperative species based on four hypotheses for the evolution of delayed dispersal: the ecological constraints hypothesis, the life history hypothesis, the broad constraints hypothesis and the benefits-of-philopatry hypothesis. We controlled for phylogeny and habitat by comparing sympatric populations of two Australasian treecreepers (Climacteridae): the cooperatively breeding brown treecreeper, Climacteris picumnus, and the pair-breeding white-throated treecreeper, Cormobates leucophaea, which has lost cooperative breeding despite phylogenetic conservation of the trait in most of the treecreeper lineage. Our data failed to support nearly every prediction of the constraints and life history hypotheses. The two species had similar levels of constraints and similar life histories, and any significant differences were in the opposite direction to those predicted. However, the cooperative species was characterized by significantly higher variance in reproductive success as a result of group size effects, supporting the benefits-of-philopatry hypothesis as an explanation for the maintenance of delayed dispersal. Our comparisons also suggest that the benefits parents derive from group living determine whether or not they tolerate offspring on the natal territory. We therefore propose an antipredator tactics hypothesis in which delayed dispersal will initially evolve or be lost in particular taxa depending on the relative costs and benefits of vigilance and active defence versus concealment-based antipredator tactics.