Big groups attract bad eggs: brood parasitism correlates with but does not cause cooperative breeding

There has been great interest in how complex social behaviours such as cooperative breeding evolve and are maintained; however, it is still unclear what exact phenomena trigger the transition to cooperative breeding. Recent work in birds has suggested a number of factors associated with cooperative breeding, including environmental uncertainty and brood parasitism. One recent study found a correlation between brood parasitism and cooperative breeding, but it examined this relationship from a geographically restricted perspective. We investigated evolutionary correlations between brood parasitism and cooperative breeding at a global scale, including nearly half of all bird species and brood parasites. At a global level, we found a strong positive correlation between cooperative breeding and brood parasitism. However, when partitioned regionally, we found that the global pattern was driven exclusively by relationships within Africa and Australia, suggesting that any causal relationship in the transition to cooperative breeding is idiosyncratic. In addition, we found that even where a correlation was supported, transition rates between states were more consistent with cooperative breeding attracting brood parasitism, rather than brood parasites driving the evolution of cooperative breeding, weakening any hypothesized causal connection.