Cooperative breeding in birds: the role of ecology

Theory predicts that cooperative breeding should only occur in species in which certain individuals are constrained from breeding independently by some peculiarity of the species' ecology. Here, we use comparative methods to examine the role of variation in ecology in explaining differences between taxa in the frequency of cooperative breeding. We address three questions. First, does the frequency of cooperative breeding vary at just one phylogenetic level, or across several levels? Second, are differences in the frequency of cooperative breeding among closely-related species correlated with ecology! Last, are ecological differences between ancient lineages important in predisposing certain lineages to cooperative breeding? We find that variation in the frequency of cooperative breeding occurs across all phylogenetic levels, with 40% among families and 60% within families. Also, variation in the frequency of cooperative breeding between closely related species is associated with ecological differences. However, differences in the frequency of cooperative breeding among more ancient lineages are not correlated with differences in ecology. Together, our results suggest that cooperative breeding is not due to any single factor, but is a two step-process: life-history predisposition and ecological facilitation. Low annual mortality predisposes certain lineages to cooperative breeding. Subsequently, changes in ecology facilitate the evolution of cooperative breeding within these predisposed lineages. The key ecological changes appear to be sedentariness and living in a relatively invariable and warm climate. Thus, although ecological variation is not the most important factor in predisposing lineages to cooperative breeding, it is important in determining exactly which species or populations in a predisposed lineage will adopt cooperative breeding. Key words: birds, comparative methods, cooperative breeding, ecological constraints, mating system.