Brood parasitism and the evolution of cooperative breeding in birds part II

To confirm their suspected correlation between cooperative passerines and parasitic species they used phylogenetic comparisons of the fauna in the aforementioned areas. Their results showed that a large percentage of the species targeted by parasitic individuals in these areas were indeed cooperative breeders. Their theories were confirmed after comparing the growth rates of cuckoo (parasitic) chicks in biparnal and group care settings. In larger groups (3+) cuckoos grew at a faster rate with decreased levels of death due to smaller predation rates.  However, these potential benefits were seldom realized as larger groups were able to defend from parasitism more efficiently. Finally, they determine that cooperative behavior is driven by their increased reproductive success secondary to their defensive measures.