Birds exhibit mafia-like behavior as nest enforcers

Birds exhibit thuggish mafia-like behavior as nest enforcers
mongabay.com
March 5, 2007

Parasitic birds engage in mafia-like reprisals to encourage host acceptance of their eggs according to researchers writing in the online early edition of PNAS.

While a number of bird species, notably cuckoos, are brood parasites that lay their eggs in the nests of unsuspecting hosts, the reason as to why host birds raise chicks that are obviously not their own has not been scientifically documented. Researchers have observed the destruction of nests and eggs of hosts that reject their parasitic eggs, but until no controlled studies on the behavior had been performed.

To test this theory, Jeffrey P. Hoover and Scott K. Robinson of the University of Florida devised an experiment that controlled a parasitic bird's (brown-headed cowbird - Molothrus ater) access to a host's (prothonotary warbler - Protonotaria citrea) nest and then "manipulated cowbird egg rejection to observe the consequences."

Warbler next parasitized with cowbird eggs. Courtesy of Jeffrey Hoover.

Male warbler at nest box. Courtesy of Jeffrey Hoover.

The researchers found that 56 percent of rejecter nests were subsequently destroyed by cowbirds, compared with only 6 percent of accepter nests. If cowbird access was denied, then no nests were damaged. The researchers noted that cowbird-accessed but non-parasitized nests were damaged 20 percent of the time, "suggesting that cowbirds may occasionally 'farm' hosts to create additional opportunities for parasitism" (i.e. so cowbirds can lay their eggs after the hosts rebuild). Sure enough the researchers found that 85 percent of rebuilt warbler nests were parasitized by cowbird eggs.

Hoover and Robinson found that rejecter warblers produced fewer offspring than accepters, suggesting that by accepting cowbird eggs, warblers improve their odds of reproduction and evolutionary fitness.

Jeffrey P. Hoover and Scott K. Robinson (2007). Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. PNAS Early Edition, March 6, 2007. www.pnas.orgcgidoi10.1073pnas.0700618104

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