Common cuckoos (Cuculus canorus) in Europe, and brown-headed cowbirds (Molothrus ater) in North America lay their eggs in the nests of other species and leave them to the care of their respective host. A Cuckoo female searches for an unguarded nest of another species and then replaces an egg in it with one of her own. The mother then abandons her egg leaving it to be raised by the nest's natural owners. Clearly, lacking a distinct ability to identify and remove foreign eggs from among its clutch appears maladaptive.
As an example of an evolutionary arms race, cukoos have adapted their kleptoparasitic strategies to the host behavior. Upon return, hosts may reject the foreign egg if it is a poor match, added at the wrong time of day, or the cukoo's presence has been witnessed. Different cuckoo strains exhibit distinct preferences for a specific host species which provide a close visual match to their own eggs. Likewise, hosts have adapted to selection from cuckoos where species suffering high amounts of parasitism are much better at identifying and removing cuckoo eggs than those who suffer little. When parents remove an egg from their nest they do risk removing one of their own or breaking some of the others in the process. Whether taking such risks will be worth it depends on the overall rate of parasitism and the ability to discriminate foreign from own eggs.
In contrast to cuckoos, brownheaded cowbirds exhibit little preference for nests of a specific song bird species. Moreover, their eggs fail to show close mimicry to those of their hosts. Removal of parasitic eggs is much more variable, high in some species and rare in others. Although cowbird fledglings do not evict their nest mates, up to 50% of the nests may be parasitized in some areas.
The lack of co-evolution in brood parasitism of cowbirds compared to that of cuckoos is puzzling. Do these two cases represent different stages of the arms race? Are cowbirds in the early parts of the game while cuckoos have had a chance to adapt for a much longer time? Has the recent change in North America landscape from forest to agricultural lands brought cowbirds into contact with a much greater number of new hosts? Alternatively, both species may be at an evolutionary equilibrium with stabilizing selection. Differing degrees of rejection may well depend on their respective costs of rejecting vs. accepting any given egg.