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An evolved resemblance of host eggs by the eggs of avian brood parasites, resulting from Darwinian natural selection.
The interactions between avian brood parasites, such as cuckoos or cowbirds, and their hosts have emerged as model systems to study coevolutionary processes under natural conditions. Instead of building a nest and tending their offspring, brood parasites lay their eggs in the nests of other birds and abandon the care of their young to the host. Adult parasites tend to remove or damage host eggs when depositing their own and parasite chicks generally eliminate the rest of the host’s brood after hatching. Consequently, hosts evolve defences against brood parasites, which select counteradaptations in parasites, further counteradaptations in hosts, and so on. While evidence of reciprocal adaptations and counteradaptations in hosts and brood parasites are evident at all stages of the host’s nesting cycle (Feeney et al. 2014b), the exquisite mimicry of host eggs by the eggs of some brood parasites comprise among the most charismatic and iconic examples of deception in the animal kingdom.
Egg Mimicry in Avian Brood Parasites
Mimicry as an adaptation to increase the likelihood of successful parasitism has evolved numerous times in the avian brood parasites (Feeney et al. 2014b). Best studied in the Eurasian common cuckoo (Cuculus canorus), it is most notably recognized as a counteradaptation to host discrimination (Brooke and Davies 1988). Preference for and coevolution with different hosts species can result maternally inherited host-specific egg phenotypes (Fossøy et al. 2016) that mimic those of their respective host species (i.e., host-specific egg “gentes” or “races”; Gibbs et al. 2000), allowing for host specialization without speciation. Correspondingly, studies have empirically demonstrated the link between the degree of color (Stoddard and Stevens 2011) and pattern (Stoddard and Stevens 2010) matching and host discrimination across common cuckoo lineages and host species, when accounting for the host’s visual perspective.
Outside of the common cuckoo and its hosts, studies have shed insights into other agents of selection for egg mimicry by brood parasites, other kinds of mimicry resulting from brood parasite–host coevolution, and the amount of time it can take for these phenotypes to evolve under natural conditions. For example, in the dark nests of their crevice-nesting hosts, African greater honeyguides (Indicator indicator) produce eggs that mimic the size and shape of those of their hosts. However, rather than mimicry being a product of coevolution with their hosts, egg mimicry is this system is a product of selection imposed by competing honeyguides, which destroy nonmimetic eggs in order to maximize the likelihood of their own young surviving (Spottiswoode 2013).
In Australia, numerous small songbirds build dimly lit dome-shaped nests and produce white eggs with varying degrees of speckling. Rather than producing an egg that closely resembles that produced by any particular host, the Horsfield’s bronze-cuckoo (Chalcites basalis) appears to have circumvented the defences of multiple host species, as well as the need to diverge into host-specific races, by producing an egg that generally resembles those of its multiple hosts (also known as “compromised,” “generalist,” or “jack-of-all-trades” mimicry; Feeney et al. 2014a).
Finally, the interactions between the African cuckoo finch (Anomalospiza imberbis) and its tawny-flanked prinia (Prinia subflava) host provides an elegant example of coevolution between the eggs of a brood parasite and its host, as well as unique insights into the timespan over which these kinds of evolutionary changes can occur. Unlike the common cuckoo, in which female-lineages are host-specific, selection imposed on the prinia eggs by cuckoo finches has selected numerous egg-races in prinias, which allows to more easily identify and reject cuckoo finch eggs that do not closely match their own. However, cuckoo finches have responded by evolving eggs that resemble those of the different prinia lineages (Spottiswoode and Stevens 2010). Further, Spottiswoode and Stevens (2012) used an egg collection that spans over 40 years to track the coevolution of egg phenotypes in prinias and cuckoo finches. They demonstrated that the pressure imposed on hosts by brood parasites can select new phenotypes (i.e., egg colors) within this time period and that parasite egg phenotypes closely tract those of their hosts.
Brood parasites commonly evolve egg mimicry in response to pressures imposed on them by either their hosts, or by other parasites competing for access to hosts. Selection imposed on brood parasites can select for host or host-race specific egg phenotypes, providing a clear and elegant example of naturally occurring coevolution.
- Feeney, W. E., Welbergen, J. A., & Langmore, N. E. (2014b). Advances in the study of coevolution between avian brood parasites and their hosts. Annual Review of Ecology, Evolution, and Systematics, 45(1), 227–246. https://doi.org/10.1146/annurev-ecolsys-120213-091603.CrossRefGoogle Scholar