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“Mafia” behavior noted in birds

March 5, 2007
Courtesy PNAS
and World Science staff

It’s well known that some spe­cies of birds lay their eggs in oth­er spe­cies’ nests, to force oth­ers to raise their off­spring. 

Now, re­search­ers have iden­ti­fied a new low in the be­hav­ior of some of these “par­a­site” birds: they re­tal­i­ate mafia-style against those that re­ject their im­po­si­tion, by ran­sack­ing their nests.

A warbler next parasitized with cowbird eggs. (Courtesy PNAS)


Many spe­cies, no­ta­bly cuck­oos, are brood par­a­sites that lay their eggs among un­wit­ting hosts. 

Some of the free­load­ers lay eggs that look like the hosts’ eggs, ex­plain­ing why the hosts ac­cept them. But in oth­er cases, the in­t­rud­ers’ eggs look dra­ma­t­i­cal­ly dif­fer­ent from those of the hosts; this is the case with the par­a­sit­ic brown-headed cow­bird. 

That raises the ques­tion of why the vic­tim pa­rents ac­cept the eggs. Al­though some of them toss the al­ien eggs from their nest, it hap­pens sel­dom enough that the par­a­site strat­e­gy works as a whole.

One ex­pla­na­tion could be that the free­loaders en­force ac­ceptance by de­stroy­ing the eggs or nests of hosts that re­ject their eggs. While such be­hav­ior has been re­ported in a cuck­oo spe­cies, con­trolled stud­ies haven’t been per­formed, ac­cord­ing to the in­ves­ti­ga­tors in a new study, which sought to rem­e­dy this. 

They con­trolled cow­birds’ ac­cess to the nest of a host, the war­bler. They then ma­nip­u­lat­ed war­blers’ re­jection of cow­bird eggs to see the con­se­quenc­es. The re­ported re­sults: cow­birds ran­sacked 56 per­cent of re­jecter nests, com­pared to just 6 per­cent of ac­cepter nests. 

Ran­sack­ing was­n’t lim­it­ed to re­tal­i­a­tory sit­u­a­tions, though. Cow­birds al­lowed ac­cess to host nests al­so were found to ran­sack one in five non-par­a­si­tized nests. This sug­gests cow­birds “farm” for hosts, de­stroy­ing war­bler nests so they can lay their eggs af­ter the hosts re­build, the sci­en­tists ar­gued. Sup­port­ing this no­tion, they added, cow­birds par­a­si­tized 85 per­cent of re­built nests. 

Over­all, re­jecter war­blers pro­duced few­er off­spring than ac­cepters, sug­gest­ing hosts may be bet­ter off in ev­o­lu­tion­ary terms ac­cepting cow­bird eggs, the in­ves­ti­ga­tors said.

The re­search, by Jeff Hoo­ver Il­li­nois Nat­u­ral His­to­ry Sur­vey in Cham­paign, Ill., and Scott K. Rob­in­son of the Flor­i­da Mu­se­um of Nat­u­ral His­to­ry in Gaines­ville, Fla., is to ap­pear this week in the ear­ly on­line edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.


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It’s well known that some species of birds lay their eggs in other birds’ nests, to force the others to raise their offspring. Now, researchers have identified a new low in the behavior of some of these “parasite” birds: they retaliate mafia-style against those that reject their imposition, by ransacking their nests. Many species, notably cuckoos, are brood parasites that lay their eggs among unwitting hosts. Some of the parasites lay eggs that mimic the appearance of host eggs, explaining why the hosts often accept them. But in other cases, the intruders’ eggs look dramatically different from those of the hosts; this is the case with the parasitic brown-headed cowbird. This raises the question of why the victims of the scam accept the eggs. Although some of them toss the alien eggs from their nest, it happens seldom enough that the parasite strategy works as a whole. One explanation could be that the parasites enforce acceptance by destroying the eggs or nests of hosts that reject their eggs. While such behavior has been reported in a cuckoo species, controlled studies haven’t been performed, according to the invest igators in a new study that sought to remedy this. They controlled cowbirds’ access to the nest of a host, the warbler. They then manipulated the warbler’s rejection of cowbird eggs to observe the consequences. The researchers found that cowbirds ransacked 56% of rejecter nests, compared with only 6% of accepter nests. The ransacking wasn’t limited to retaliatory situations, though. Cowbirds allowed access to host nests also were found to ransack one in five non-parasitized nests. This suggests cowbirds “farm” for hosts, destroying warbler nests so the cowbirds can lay their eggs after the hosts rebuild, the scientists argued. Supporting this notion, cowbirds parasitized 85% of rebuilt nests. Overall, rejecter warblers produced fewer offspring than accepters, suggesting hosts may be better off in evolution ary terms accepting cowbird eggs, the invest igators said. The research, by Jeff Hoover Illinois Natural History Survey in Champaign, Ill., and Scott K. Robinson of the Florida Museum of Natural History in Gainesville, Fla., is to appear this week in the early online edition of the research journal pnas.