Egg recognition in Yellow-billed and Black-billed Magpies in the absence of interspecific parasitism: Implications for parasite-host coevolution

Yellow-billed Magpies (Pica nuttalli) ejected 100% of nonmimetic eggs place d in their nests despite such behavior having no detectable present day ben efits. They are not currently parasitized, nor is there any evidence of a r ecently extinct brood parasite. Furthermore, there was no molecular evidenc e of conspecific parasitism, and Yellow-billed Magpies accepted eight of ni ne conspecific eggs transferred between nests, so recognition would rarely be of benefit if conspecific parasitism occurred. Thus, we suggest that egg recognition in Yellow-billed Magpies is a plesiomorphic trait, a primitive character inherited from a remote ancestor, its nearest relative, the Blac k-billed Magpie (Pica pica). The latter suffers from parasitism by Great Sp otted Cuckoos (Clamator glandarius) in Eurasia and displays rejection behav ior throughout Europe in populations that are allopatric and sympatric with this cuckoo. As would be expected if Yellow-billed Magpies inherited rejec tion behavior from the Black-billed Magpie, or the common ancestor of both species, we found that North American Black-billed Magpies also have well d eveloped egg recognition despite being unaffected by interspecific parasiti sm. The long-term retention of host defenses as is indicated by magpies has major implications for the coevolutionary interactions between brood paras ites and their hosts.