Common cuckoos lay eggs that mimic those of the host birds they trick into raising cuckoo chicks. Photo by Sergey Yeliseev.Brood parasitism, in which one bird species lays its eggs in another bird's nest, has long been considered a likely cause of coevolution [$a] between brood parasites and their hosts, because the interaction exerts strong natural selection on both species. Hosts suffer major fitness consequences if they take on the raising of another bird's chick—and brood parasite chicks are often bigger, and more aggressive, than their adoptive "siblings," sometimes pushing them right out of the nest. On the other hand, brood parasites run the risk of losing their offspring to hosts who can recognize a strange egg and eject it from the nest.
One way to avoid raising a cuckoo chick is to lay eggs that look different from cuckoo eggs. Cuckoos counteract this defense by evolving eggs that match their most common hosts—a selective regime proposed to explain rapid rates of species formation in parasitic cuckoo lineages. In the new study, Yang et al. show that this pattern plays out within a single population of ashy-throated parrotbills and the cuckoos that parasitize them. At a forested nature reserve in southwestern China, the team found that parrotbills lay eggs of three different colors: white, blue, or (rarely) pale blue. Common cuckoos in the same area also laid eggs of those three colors, in about the same proportions as the parrotbills—and cuckoo eggs were usually found in host nests with eggs of the same color. Experimental introduction of eggs into parrotbill nests confirmed that parrotbills were more likely to reject eggs colored differently from their own.
That result captures many of the necessary conditions for coevolution between ashy-throated parrotbills and the local cuckoo population; the frequency with which parrotbills reject eggs unlike their own should exert strong selection on the cuckoos, and (conversely) the frequency with which parrotbills fail to reject cuckoo eggs that look like their own should exert selection on the hosts. This isn't the first case in which brood parasites have apparently forced their hosts to diversify, however—notably, African village weaverbirds evolved less varied egg patterning after being introduced into parasite-free habitats on Mauritius and Hispaniola.
Krüger, O., Sorenson, M., & Davies, N. (2009). Does coevolution promote species richness in parasitic cuckoos? Proc. Royal Soc. B, 276 (1674), 3871-9 DOI: 10.1098/rspb.2009.1142
Lahti, D. (2005). Evolution of bird eggs in the absence of cuckoo parasitism. Proc. Nat. Acad. Sci. USA, 102 (50), 18057-62 DOI: 10.1073/pnas.0508930102
Rothstein, S. (1990). A model system for coevolution: Avian brood parasitism. Ann. Rev. Ecology and Systematics, 21 (1), 481-508 DOI: 10.1146/annurev.es.21.110190.002405
Yang, C., Liang, W., Cai, Y., Shi, S., Takasu, F., Møller, A., Antonov, A., Fossøy, F., Moksnes, A., Røskaft, E., & Stokke, B. (2010). Coevolution in action: Disruptive selection on egg colour in an avian brood parasite and its host. PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010816