Family living: an overlooked but pivotal social system to understand the evolution of cooperative breeding

Cooperative breeding occurs in several major animal phyla, predominantly in arthropods and chordates. A number of comparative analyses have focused on understanding the evolution of cooperative breeding, yielding mixed, inconclusive, and often phyla-specific findings. We argue that much of this ambiguity results from an erroneous classification of social systems into noncooperatively and cooperatively breeding species. The shortcomings of this assumption are apparent among birds where noncooperative species constitute a heterogeneous group: some species are clearly non–family living, with offspring dispersing at or shortly after nutritional independency, whereas other species form persistent family groups through offspring delaying their dispersal substantially beyond independency. Here, we propose an objective, life history–based criterion classifying noncooperative bird species into non–family living and family living species. We demonstrate that by using the family time (the time offspring remain with its parent/s beyond independence) and body size–scaled reproductive investment, we are able to differentiate 2 groups with contrasting life histories. Our classification matches seasonal environmental variation experienced by different species: family living species postpone dispersal beyond the onset of less favorable autumn conditions. We discuss the consequences of this new social system classification for evolutionary and ecological research, potentially allowing solutions to some of the most intriguing riddles in the evolutionary history of birds—and cooperative behavior itself.