ON THE ORIGIN AND RARITY OF INTERSPECIFIC NEST PARASITISM IN BIRDS

Interspecific nest parasitism is surprisingly rare in birds given the potential advantages for the parasite of exploiting the parental care of other species. One possibility is that chicks will not thrive with the parental care and food of heterospecifics. I simulated parasitism in nonparasitic congeners by switching eggs between nests of three spe cies of titmice (great tit Parus major, blue tit Parus caeruleus, and coal tit Pants ater). The experiment showed that compatibility of pare ntal care was not a constraint preventing parasitism. I also used the model system to compare fitness consequences of inter- and intraspecif ic nest parasitism, addressing the problem of which form is ancestral. Fledging success (body mass, survival) was higher when an egg was add ed.to the nest of a smaller species than to the nest of a conspecific and also higher when the parasitic chick hatched early rather than lat e relative to host chicks. This suggests that interspecific nest paras itism may not require a stage of intraspecific nest parasitism before evolving but may start from a larger species directly exploiting the p arental care of a smaller species or a species with shorter incubation period directly exploiting a species with longer incubation period.