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A well-studied defense against avian brood parasitism in which a host “rejects” a brood parasite’s egg – either through physical ejection, burying the egg in the nest lining, or abandoning the nest.
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 defenses 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. 2014), rejection of foreign eggs by hosts is a well-studied defense that acts as a catalyst for subsequent coevolution between these species.
Egg Rejection as a Defence Against Brood Parasitism
Despite hosts employing an array of defense to prevent brood parasites laying eggs in their nests, it often occurs, which select adaptations in hosts that aim to minimize the cost of parasitism. Of these various adaptations (reviewed in: Feeney et al. 2014), rejection of foreign eggs often emerges as a key defense.
Iconic field experiments demonstrated that egg rejection behaviors are widespread, but not ubiquitous, among hosts of brood parasites (Brooke and Davies 1988; Rothstein 1975). Rejection of foreign eggs selects egg mimicry by parasites (Brooke and Davies 1988), which makes discrimination more difficult and increases the likelihood of hosts mistakenly rejecting its own egg. Hosts can decrease the likelihood of parasites laying eggs that match their own with individually identifiable “egg signatures” (i.e., female or race-specific egg phenotypes; Stoddard et al. 2014). Additionally, they can counter-egg mimicry with cognitive mechanisms that increases the likelihood of them accurately identifying and rejecting the correct egg (or eggs) in their nest (Stevens et al. 2013) and weigh the decision to reject an egg against their perceived risk of parasitism (Thorogood and Davies 2016). Unlike other defensive adaptations, such as recognizing and responding to adult parasites near the nest, egg rejection behaviors appear to quickly become conserved after evolving, and can persist for at least hundreds of years after the pressure imposed by parasites has been removed (Lahti 2006).
Counteradaptations by brood parasites, or external ecological pressures, can generally explain why some species do not reject foreign eggs. For example, in the United States, brood parasitic brown-headed cowbirds (Molothrus ater) employ “mafia tactics” to ensure that hosts do not reject their eggs and increase the likelihood of successful parasitism. Unlike many parasitic cuckoos, cowbird nestlings do not always eliminate the host’s entire brood, which allows for a circumstance where cowbirds monitor parasitized nests and repeatedly destroy the nests of egg rejecters until they succumb and accept the compromised, but not entirely diminished, reproductive output associated with hosting a cowbird nestling (Hoover and Robinson 2007). Similarly, despite Jacobin cuckoo (Clamator jacobinus) eggs being large and conspicuous in cape bulbul (Pycnonotus capensis) nests in sub-Saharan Africa, the combined effects of them being difficult to effectively eject with high external predation and the relatively low likelihood of the parasite’s egg hatching in sync with the host’s own eggs makes acceptance of the parasite’s egg optimal in this species (Krüger 2011).
Egg rejection represents the best-studied counteradaptation that hosts employ to minimize the cost of brood parasitism. Research into the interactions across diverse parasite–host pairs highlights how ecological circumstances, available information, and the particular characteristics of the involved species shape this unique behavior.
- Brooke, M. de. L., & Davies, N. B. (1988). Egg mimicry by cuckoos Cuculus canorus in relation to discrimination by hosts. Nature, 335, 630–632. https://doi.org/10.1038/335630a0.
- Feeney, W. E., Welbergen, J. A., & Langmore, N. E. (2014). 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