"Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs"The Proceedings of the National Academy of Sciences (PNAS) recently published an article by Dr. Jeff Hoover of the Illinois Natural History Survey who, along with Dr. Scott Robinson of the University of Florida, found further proof that parasitic birds engage in mafia-like retaliation activities to encourage hosts to accept their eggs. Nearly 100 bird species, notably cuckoos and cowbirds, are brood parasites that lay their eggs among unsuspecting hosts. In these situations, many scientists wonder why the hosts end up raising a chick that is obviously not their own. Some speculate that brood parasites enforce acceptance by destroying the nests of hosts that reject their eggs. While such behavior has been reported in one cuckoo species, controlled studies to confirm this have not been done.
The scientists developed a trial where they could control access of a parasite (brown-headed cowbird) to host's (prothonotary warbler) nests, then manipulated cowbird egg rejection to see the consequences. When they removed cowbird eggs from warbler nests, they observed that 56% of these "rejecter" nests were subsequently ransacked compared to only 6% of "accepter" nests where they did not remove cowbird eggs. If cowbird access was denied, then no nests were damaged, eliminating other predators as a potential cause of nest failure.
Interestingly, non-parasitized nests were ransacked 20% of the time, suggesting cowbirds also farm for hosts. Farming occurs when cowbirds destroy nests that they find that are too far along to be parasitized. The host builds a new nest (i.e. a "renest") that then provides a new parasitism opportunity for the cowbirds. Hoover and Robinson found that 85% of warbler "renests" were parasitized by cowbirds, supporting this idea.
In the end, "rejecter" warblers produced fewer offspring than "accepters", which shows that brood parasites make accepting their eggs a better evolutionary proposition for the hosts. Theirs is the first study to test for mafia behavior in a brood parasite by controlling access of the parasite to host nests following rejection or acceptance of the parasitic eggs. The results of their study provide the strongest evidence to date for mafia behavior in a brood parasite.
PNAS Article # 06-09710: http://www.pnas.org/papbyrecent.shtml
The research was conducted in the Cache River watershed in southern Illinois. For questions about this research contact Dr. Jeff Hoover, Division of Ecology and Conservation Science, Illinois Natural History Survey, Champaign, Illinois. Daytime telephone (352)392-1721 x511 or evening phone: (352)378-2519 or by e-mail at jhoover@flmnh.ufl.edu. For media contacts at the Illinois Natural History Survey, contact Marsha Hatchel, Information Services Manager (217)244-1620 or hatchel@uiuc.edu.
For media contacts at PNAS contact Jenni Smith, PNAS Communications Coordinator, (202) 334-1310 or PNASnews@nas.edu.
The Illinois Natural History Survey is a division of the Illinois Department of Natural Resources and an allied agency of the University of Illinois.
Dr. Hoover is an alumnus of the Animal Biology Department at the University of Illinois. Dr. Hoover's reseach is also featured in the Chicago Tribune.
