Editing Cuckoo (section)

===== Host egg mimicry =====
Female parasitic cuckoos sometimes specialize and lay eggs that closely resemble the eggs of their chosen host. Some birds are able to distinguish cuckoo eggs from their own, leading to those eggs least like the host's being thrown out of the nest.<ref name="Campbell" /> Parasitic cuckoos that show the highest levels of egg mimicry are those whose hosts exhibit high levels of egg rejection behavior.<ref>{{cite journal |vauthors=Stoddard MC, Stevens M |title=Avian vision and the evolution of egg color mimicry in the common cuckoo |journal=Evolution; International Journal of Organic Evolution |volume=65 |issue=7 |pages=2004–13 |date=July 2011  |pmid=21729055 |doi=10.1111/j.1558-5646.2011.01262.x|s2cid=334052 }}</ref> Some hosts do not exhibit egg rejection behavior and the cuckoo eggs look very dissimilar from the host eggs. It has also been shown in a study of the European common cuckoos that females lay their egg in the nest of a host that has eggs that look similar to its own.<ref>{{cite journal |vauthors=Avilés JM, Stokke BG, Moksnes A, Røskaft E, Asmul M, Møller AP |title=Rapid increase in cuckoo egg matching in a recently parasitized reed warbler population |journal=Journal of Evolutionary Biology |volume=19 |issue=6 |pages=1901–10 |date=November 2006  |pmid=17040387 |doi=10.1111/j.1420-9101.2006.01166.x|s2cid=37592779 }}</ref> 

Other species of cuckoo lay "cryptic" eggs, which are dark in color when their hosts' eggs are light.<ref name="Davies" /> This is a trick to hide the egg from the host, and is exhibited in cuckoos that parasitize hosts with dark, domed nests. Some adult parasitic cuckoos completely destroy the host's clutch if they reject the cuckoo egg.<ref name="Davies" /> In this case, raising the cuckoo chick is less of a cost than the alternative, total clutch destruction.

Cuckoo egg physiology can limit the degree of mimetic accuracy. Due to larger chick size on average for parasites compared to hosts, this is a physiological constraint on egg size, a minimum egg size needed to support a healthy cuckoo chick.<ref name=":1">{{Cite journal |last1=Akkaynak |first1=Derya |first2=Mary Caswell |last2=Stoddard |title=Common cuckoos do not mimic the size and shape of host eggs |journal=[[Behavioral Ecology]] |date=September–October 2023 |volume=34 |issue=5 |pages=804–815 |doi=10.1093/beheco/arad044 |doi-access=free |via=Oxford Academic }}</ref> In these cases, there is selective pressure on cuckoos to lessen their egg size to be a more effective mimic, but physiological constraints hinder the species from doing so.<ref name=":1" />

Mimicry may also be imperfect due to a lack of strong selection pressures towards the parasite. Oriental reed warbler hosts do not discriminate between warbler-sized model eggs and slightly larger model cuckoo eggs.<ref>{{Cite journal |first1=Donglai |last1=Li |first2=Xiaoshuang |last2=Li |first3=Yan |last3=Zhang |first4=Shuang |last4=Guan |first5=Yanan |last5=Ruan |title=Contrasting effects of egg size and appearance on egg recognition and rejection response by Oriental reed warblers  |journal=[[Ecology and Evolution]] |date=October 2020 |volume=10 |issue=19 |pages=10508–10519 |doi=10.1002/ece3.6707 |doi-access=free |pmid=33072276 |pmc=7548169 |bibcode=2020EcoEv..1010508L }}</ref> Since cuckoos in this situation can effectively parasitize despite laying eggs slightly larger than those of their hosts, there are little selective pressures to evolve "perfect" mimicry.

To select the most suitable host nests, cuckoos may "egg-match" as well. Daurian redstarts (''Phoenicurus auroreus''), another cuckoo host, lay clutches of either pink or blue eggs. Cuckoo eggs are more similar in reflectance and color to blue redstart eggs than pink ones. In-field observations revealed parasitism occurred more frequently in blue-egg redstart nests (19.3%) than in pink-egg redstart nests (7.9%).<ref name=":2">{{Cite journal |first1=Jinggang |last1=Zhang |first2=Peter |last2=Santema |first3=Zixuan |last3=Lin |first4=Lixing |last4=Yang |first5=Meijun |last5=Liu |first6=Jianqiang |last6=Li |first7=Wenhong |last7=Deng |first8=Bart |last8=Kempenaers |title=Experimental evidence that cuckoos choose host nests following an egg matching strategy |journal=[[Proceedings_of_the_Royal_Society#Proceedings_of_the_Royal_Society_B:_Biological_Sciences|Proceedings of the Royal Society B: Biological Sciences]] |date=22 February 2023 |volume=290 |issue=1993 |doi=10.1098/rspb.2022.2094 |doi-access=free |pmid=36809803 |via=The Royal Society Publishing |pmc=9943643 }}</ref> This suggests cuckoos prefer parasitizing nests containing eggs resembling their own. Experiments in the lab show similar findings: cuckoos parasitized artificial nests containing blue eggs more frequently than pink ones.<ref name=":2" />

Two main hypotheses on the cognitive mechanisms mediate host distinguishing of eggs. One hypothesis is true recognition, which states that a host compares eggs present in its clutch to an internal template (learnt or innate), to identify if parasitic eggs are present.<ref name=":0">{{Cite journal |last=Rothstein |first=Stephen I. |date=May 1975 |title=Mechanisms of avian egg-recognition: Do birds know their own eggs? |journal=Animal Behaviour |volume=23 |issue=Part 2 |pages=268–278 |doi=10.1016/0003-3472(75)90075-5|s2cid=53268458 }}</ref> However, memorizing a template of a parasitic egg is costly and imperfect and likely not identical to each host's egg. The other one is the discordancy hypothesis, which states that a host compares eggs in the clutch and identifies the odd ones.<ref name=":0" /> However, if parasitic eggs made the majority of eggs in the clutch, then hosts ends up rejecting their own eggs. More recent studies have found that both mechanisms more likely contribute to host discrimination of parasitic eggs since one compensates for the limitations of the other.<ref>{{Cite journal |last1=Feeney |first1=William E.|last2=Welbergen |first2=Justin A. |last3=Langmore |first3=Naomi E. |date=2014 |title=Advances in the Study of Coevolution Between Avian Brood Parasites and Their Hosts |journal=Annual Review of Ecology, Evolution, and Systematics |volume=45 |issue=1 |pages=227–246 |doi=10.1146/annurev-ecolsys-120213-091603|hdl=1885/66602 |hdl-access=free }}</ref>