'Mafia' behavior noted in birds
Mar 16,2007 00:00 by Bend_Weekly_News_Sources

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.

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

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

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.

Courtesy PNAS and World Science staff