COOPERATIVE BREEDING IN BIRDS - A COMPARATIVE TEST OF THE LIFE-HISTORY HYPOTHESIS

In approximately 3.2% of bird species individuals regularly forgo the opportunity to breed independently and instead breed cooperatively wit h other conspecifics, either as non-reproductive 'helpers' or as co-br eeders. The traditional explanation for cooperative breeding is that t he opportunities for breeding independently are limited owing to pecul iar features of the species' breeding ecology. However, it has proved remarkably difficult to find any common ecological correlates of coope rative breeding in birds. This difficulty has led to the 'life history hypothesis: which suggests that the common feature of cooperatively b reeding birds is their great longevity, rather than any particular fea ture of their breeding ecology. Here, we use a comparative method to t est the life history hypothesis by looking for correlations between li fe history variation and variation in the frequency of cooperative bre eding. First, we find that cooperative breeding in birds is not random ly distributed, but concentrated in certain families, thus supporting the idea that there may be a common basis to cooperative breeding in b irds. Second, increases in the level of cooperative breeding are stron gly associated with decreases in annual adult mortality and modal clut ch size. Third, the proportion of cooperatively breeding species per f amily is correlated with a low family-typical value of annual mortalit y, suggesting that low mortality predisposes cooperative breeding rath er than vice versa. Finally, the low rate of mortality typically found in cooperatively breeding species is associated with increasing seden tariness, lower latitudes, and decreased environmental fluctuation. We suggest that low annual mortality is the key factor that predisposes avian lineages to cooperative breeding; then ecological changes, such as becoming sedentary, further slow population turnover and reduce opp ortunities far independent breeding. As the traditional explanation su ggests, the breeding habitat of cooperatively breeding species is satu rated, but this saturation is not owing to any peculiar feature of the breeding ecology of cooperative breeders. Rather, the saturation aris es because the local population turnover in these species is unusually slow as predicted by the life history hypothesis.