Michener’s group-size paradox in cooperatively breeding birds

According to Michener's Paradox, most altruistic groups in nature
should be small, and large groups should not exist. This is because per
capita productivity is thought to decrease as groups get larger, meaning
that the share of indirect fitness available to each group member
declines, which favours dispersal. The empirical evidence for a decrease
in per capita productivity is contradictory, however, and limited to the
social Hymenoptera. I report that per capita reproductive success
decreased with increasing group size across 26 cooperatively breeding bird
species. Small groups comprising two or three individuals were the most
common (79% of 16,101 groups) and these had the highest per capita
reproductive success. This close fit between per capita reproductive
success and the distribution of group sizes in nature suggests that it may
indeed be difficult for large groups to evolve through indirect fitness
benefits alone.