Common cuckoo females remove more conspicuous eggs during parasitism

Abstract

Avian obligate brood parasites gain an advantage by removing the eggs of the cuckoos who have already visited the nest, which can increase the chances of survival for their offspring. Conversely, to prevent their eggs from being picked up by the next parasitic cuckoo, they need to take some precautions. Egg mimicry and egg crypsis are two alternative strategies to prevent the parasitized egg from being picked up by another parasitic cuckoo. Here, we tested whether the egg crypsis hypothesis has a preventative effect when common cuckoos ( Cuculus canorus ) parasitize their Oriental reed warbler ( Acrocephalus orientalis ) hosts. We designed two experimental groups with different crypsis effects to induce common cuckoos to lay eggs and observed whether the cuckoos selectively picked up the experimental eggs with low crypsis levels in the process of parasitism. Our results supported the egg crypsis hypothesis; the observed cuckoos significantly preferred to select the more obvious white model eggs. This shows that even in an open nest, eggs that are adequately hidden can also be protected from being picked up by cuckoo females during parasitism so as to increase the survival chance of their own parasitic eggs.

Oriental reed warblers retain strong egg recognition abilities during the nestling stage

“…In contrast, we believe that the rejection of model egg in this study is more likely to be a continuation of egg recognition behavior, where the removal of non‐conspecific model eggs from neighboring nests is an expression of egg recognition stimuli. A similar study on the Oriental Reed Warbler in another population during the egg stage also found that they also rapidly reject model eggs in neighboring nests (Wang, Zhang, et al., 2021 ). Previous studies have suggested that hosts can fine‐tune their egg recognition abilities in response to external parasitism risk (Lindholm & Thomas, 2000 ; Zhang et al., 2021 ).…”

“…The former is widely recognized as an effective strategy evolved to counteract the host's egg recognition ability and the interaction process proceeds and constitutes escalating coevolution (Soler, 2014 ). The crypsis hypothesis is used to explain exceptional cases where non‐mimetic parasitic egg would survive in host nests because of dim coloration which is cryptic enough to avoid detection by the host (Antonov et al., 2011 ; Wang, He, et al., 2021 ; Wang, Zhang, et al., 2021 ; Yang et al., 2023 ), secondary parasite (Gloag et al., 2014 ; Thorogood et al., 2017 ; Wang et al., 2020 ) or predator (Mason & Rothstein, 1987 ). Previous studies have found that some parasites lay dark‐colored parasitic eggs and mostly use hosts with darker nesting environments, presumably using darker nesting environments to hide darker parasitic eggs as a response to host egg recognition and rejection, in line with the crypsis hypothesis (Langmore et al., 2009 ).…”

“…In addition, in contrast to egg mimicry, some parasites of hosts with darker parasitic nest environments lay parasitic eggs of darker colors resulting in less visible in the nest, which is a potential alternate evolutionary trajectory possibly in response to egg recognition and rejection by the host, in support of the crypsis hypothesis (Brooker et al., 1990 ; Harrison, 1968 ; Langmore et al., 2009 ). Studies have found that cryptic‐colored eggs could play a role in host recognition and rejection (Antonov et al., 2011 ; Wang, He, et al., 2021 ; Yang et al., 2023 ), avoiding egg‐removing behavior by other parasites in a nest suffering multiple parasitism (Gloag et al., 2014 ; Thorogood et al., 2017 ; Wang, Zhang, et al., 2021 ), and reducing predation risk (Mason & Rothstein, 1987 ). However, many studies have also found that hosts did not alter egg recognition and rejection responses via increasing nest light intensity or shifting in egg‐nest contrasts experimentally (Aidala et al., 2015 ; Hauber et al., 2015 ; Medina & Langmore, 2019 ).…”

Revealing the roles of egg darkness and nest similarity for a cryptic parasite egg versus host's cognition: an alternate coevolutionary trajectory

“…To explain these exceptions, the cryptic egg hypothesis was proposed to deduce that a non-mimetic cuckoo egg would survive in host nests because it had dim coloration as an adaptive response to predation pressure or secondary parasitism (i.e. egg removal) by other cuckoos [14,15,18], or simply because it was cryptic enough to avoid detection by the host [20,21]. These assumptions were not mutually exclusive, and we hypothesized that no single assumption could fully account for the evolutionary patterns observed, especially for the first two mentioned.…”

“…Therefore, the cryptic egg hypothesis was proposed to suggest an alternate coevolutionary trajectory of adaptation in parasite eggs [15,16]. This hypothesis demonstrated that the use of crypticity rather than mimicry of a parasite egg evolved as an adaptive trait, because it would favour the parasite egg (i) escaping from being recognized by the host [17], (ii) avoiding replacement by other parasites during egg-laying [14,18] or (iii) reducing predation risk by a predator [19]. Crypticity consists of two aspects: egg darkness and nest similarity (similarity to host nest appearance).…”