Eleven years of field data showing that female tropical cuckoos resort to breeding parasitically if they are unable to breed cooperatively is reported in a study published online this week in Nature.

Cooperatively nesting birds are vulnerable to social parasites, which lay their eggs into host nests but provide no parental care. The greater ani (Crotophaga major), a tropical cuckoo, is unusual in that the females breed both cooperatively and parasitically, with some doing both. However, empirical evidence of when and why female anis deploy these different reproductive strategies has been lacking.

Christina Riehl and colleagues present 11 years of field research in Panama to examine the fitness payoffs of alternative breeding strategies in the greater ani. The authors found that most females in the population nested cooperatively at the beginning of the breeding season, but a minority of these subsequently acted as reproductive parasites if their first nests were destroyed. Females that bred parasitically tended to repeat the behaviour. The authors demonstrate that over the years, the fitness payoffs of both breeding strategies were approximately equal. Ani that never parasitized and bred strictly cooperatively laid larger clutches of eggs and raised more young from their own nests than those that both nested and parasitized, so their total reproductive output did not differ.

These findings provide insights into the breeding strategies of the greater ani, and demonstrate how populations can use both cooperative and parasitic reproductive tactics.