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The threshold at which hosts of avian brood parasites reject a foreign egg, which can be adjusted according to their perceived risk of parasitism.
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), interactions at the “egg-stage” of the nesting cycle, such as egg rejection by hosts, egg mimicry by parasites, and further discrimination abilities in hosts, are the best studied.
Rejection Thresholds in Hosts
The failure to successfully reject a parasite’s egg, either through not rejecting an egg or mistakenly rejecting an own egg, can cost a host its nesting attempt as well as the time and effort it takes to raise the parasite’s chick (see Egg Rejection). Studies across numerous host–parasite systems have demonstrated that the decision to reject an egg can be based on the egg color, pattern, and size of the foreign egg (Spottiswoode and Stevens 2010; Stoddard and Stevens 2010, 2011) and that rejection decisions are specifically tuned to biologically relevant stimuli (i.e., according to differences between naturally produced egg colors), rather than simply total differentness in color (Hanley et al. 2017). However, mistaken rejection of own eggs can occur when own and foreign eggs closely resemble one another (Feeney et al. 2015), so hosts use cognitive mechanisms and additional information to increase their likelihood of correctly rejecting the parasite’s egg(s).
When faced with the perception of being parasitized, several cognitive mechanisms have been identified in hosts that help them increase the likelihood of correctly discriminating the foreign egg(s) in their nest. For example, they can simply reject the odd egg in the clutch (“discordancy hypothesis”) or reject an egg (or eggs) based upon comparison to a learned or innate template of what one’s eggs should look like (“true recognition”). Most studied species appear to use a template-based recognition system; however, some have been reported to conform to simple discordancy mechanisms or use a combination of both (reviewed in: Stevens et al. 2013). While these mechanisms are intended to help identify foreign eggs, parasites can also exploit them: African tawny-flanked prinias (Prinia subflava) reject cuckoo finch (Anomalospiza imberbis) eggs according to an internal template; however, cuckoo finches regularly parasitize prinia nests, and as the number of foreign eggs in the nest increases, prinias require greater differences between foreign eggs compared to their internal template to reject the eggs, resulting in lower rejection rates in these nests (Stevens et al. 2013).
Hosts also weigh their decision to reject a foreign egg according to their perceived risk of parasitism. For example, Eurasian reed warblers (Acrocephalus scirpaceus), a primary host of the common cuckoo (Cuculus canorus), that live in areas with cuckoos more readily reject foreign eggs compared to those that live in areas without cuckoos (Lindholm and Thomas 2000) and the rate of egg rejection within populations varies according to parasitism rates across years (Thorogood and Davies 2013). Within a breeding attempt, individual reed warblers combine both personal (i.e., the sight of a cuckoo near their nest) and social (i.e., the sight of a cuckoo near a neighbors nest, resulting from it being attracted by the neighbor’s alarm vocalizations) information about the presence of adult cuckoos near their nest (Thorogood and Davies 2016) to inform their egg rejection decisions.
Egg rejection is among the commonest and best-studied defenses that hosts use to minimize the cost associated with brood parasitism. Rejection decisions are primarily based upon hosts being able to discriminate a foreign egg (or eggs) in their nest; however, they also employ cognitive mechanisms and weigh these decisions according to their perceived risk of parasitism to help ensure they correctly identify the foreign egg (or eggs).
- Feeney, W. E., Troscianko, J., Langmore, N. E., & Spottiswoode, C. N. (2015). Evidence for aggressive mimicry in an adult brood parasitic bird, and generalized defences in its host. Proceedings of the Royal Society of London B: Biological Sciences, 282(1810), 20150795. https://doi.org/10.1098/rspb.2015.0795.CrossRefGoogle Scholar
- 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
- Hanley, D., Grim, T., Igic, B., Samaš, P., López, A. V., Shawkey, M. D., & Hauber, M. E. (2017). Egg discrimination along a gradient of natural variation in eggshell coloration. Proceedings of the Royal Society B, 284(1848), 20162592. https://doi.org/10.1098/rspb.2016.2592.CrossRefPubMedPubMedCentralGoogle Scholar