Multiple mechanisms of egg recognition in a cuckoo host

Nest sanitation as an effective defence against brood parasitism

Egg rejection is a crucial defence strategy against brood parasitism that requires the host to correctly recognise the foreign egg. Rejection behaviour has, thus, evolved in many hosts, facilitated by the visual differences between the parasitic and host eggs, and driving hosts to rely on colour and pattern cues. On the other hand, the need to recognise non-egg-shaped objects to carry out nest sanitation led birds to evolve the ability to discriminate and eject objects using mainly shape cues. However, little is known regarding the evolutionary significance of rejection behaviour in general and the cognitive processes underlying it. Here, we investigated the response of the barn swallow (Hirundo rustica) during pre-laying and laying stages to four objects types that differed in shape (eggs vs stars) and colour/pattern (mimetic vs non-mimetic) to investigate (1) what cognitive mechanisms are involved in object discrimination and (2) whether egg rejection is a direct defence against brood parasitism, or simply a product of nest sanitation. We found that swallows ejected stars more often than eggs in both stages, indicating that swallows possess a template for the shape of their eggs. Since the effect of colour/pattern on ejection decisions was minor, we suggest that barn swallows have not evolved a direct defence against brood parasitism but instead, egg ejection might be a product of their well-developed nest sanitation behaviour. Nonetheless, the fact that mimetic eggs were ejected especially in the pre-laying stage shows that nest sanitation could be an effective defence against poorly timed brood parasitism.

Egg mimicry and host selection by common cuckoos among four sympatric host species breeding within a reed habitat

In the case of cuckoo parasitism, the maintenance of host specificity may be closely related to active host selection. In the reed wetlands of the Yongnian National Wetland Park, China, four host species of common cuckoos (Cuculus canorus) with similar nest structures breed sympatrically, namely, the Oriental reed warbler (Acrocephalus orientalis) (ORW), vinous-throated parrotbill (Sinosuthora webbiana) (VP), reed parrotbill (Paradoxornis heudei) (RP) and blunt-winged warbler (Acrocephalus concinens) (BW). Cuckoo parasitism rates on ORW (14.5%) and BW (18.2%) were found to be significantly higher than those on two species of parrotbills (RP 1.4% and VP 0%), and cuckoo eggs mimicked the size and coloration of ORW eggs significantly more closely than those of the eggs of the other three hosts. Egg recognition experiments revealed that the ability of ORW and VP to recognize nonmimetic eggs was significantly greater than that of BW and RP. Different life history strategies of the four hosts, including antiparasitic strategy, breeding time period and population density may partly explain the difference of parasitism in this multiple cuckoo-host system. Our study suggests that determining host life history traits and antiparasitic strategies are important for a better understanding of the specificity of host selection in common cuckoos.

Impact of nest sanitation behavior on hosts' egg rejection: an empirical study and meta-analyses

Egg rejection in birds is a specific adaptation toward avian brood parasitism, whereas nest sanitation is a general behavior for cleaning the nest and avoiding predation. However, both behaviors refer to the action of ejecting objects out of the nest, and nest sanitation has been proposed as a pre-adaptation for egg rejection. Here, we tested the eliciting effect of nest sanitation on egg rejection in the red-whiskered bulbul Pycnonotus jocosus, a potential host species that are sympatric with parasitic cuckoos. We conducted meta-analyses of previous studies on both nest sanitation and egg rejection, in order to evaluate the consistency of our conclusions. Our results showed that nest sanitation did not elicit egg rejection in P. jocosus. The conclusions concerning such an eliciting effect from previous studies were mixed, whereas the methodologies were inconsistent, making the studies unsuitable for comparisons. However, the ejection frequency of nest sanitation was consistently higher than the frequency of egg rejection across different host species or populations. These results suggest that nest sanitation, which is an ancient behavior, is more fundamental than egg rejection, but the effect of the former on the latter is complex and needs further study. Standardized methodologies and the integration of behavior, physiology, and modeling may provide better opportunities to explore the relationship between nest sanitation and egg rejection.

Persistent fine-tuning of egg rejection based on parasitic timing in a cuckoo host even after relaxation of parasitism pressure

Egg rejection is one of the most effective defenses to avoid avian brood parasites by hosts, and reflects avian cognition during parasite-host coevolution. However, egg rejection varies with different populations and species, or under different contexts. Here we studied the egg recognition behavior in a non-parasitized population of the red-whiskered bulbul (Pycnonotus jocosus), which has been recorded as a host of the banded bay cuckoo (Cacomantis sonneratii). By comparing its egg rejection frequency and latency between egg-laying and egg-incubation stages with a variety of nest ages, which represent different costs of cuckoo parasitism, the results show that the hosts rejected parasite eggs more frequently in the egg-laying stage than in the egg-incubation stage without a difference in latency, and accepted parasite eggs with an increase of nest age. These findings indicated that the hosts are able to adjust the intensity of their anti-parasitic responses according to the parasitic timing. A relaxation of parasitism risk and the tradeoff between parasitism risk and egg rejection cost may contribute to explaining our results.

Cryptic eggs are rejected less frequently by a cuckoo host

Obligate brood parasitism is associated with huge reproduction costs, forcing hosts to evolve various anti-parasitic strategies against brood parasites, among which egg recognition and rejection is the most effective defense strategy. According to the crypsis hypothesis, non-mimetic yet cryptic eggs in a nest can also deceive their hosts and eventually be accepted. To validate this hypothesis, we conducted field experiments on Oriental reed warblers (Acrocephalus orientalis), a common host for common cuckoos (Cuculus canorus). We firstly tested the egg recognition and rejection abilities of Oriental reed warblers, using black and white model eggs in natural nests. Then we designed a comparison test where the cryptic effects of the two groups of experimental eggs were different. We manipulated the nest lining color and added relatively cryptic and bright model eggs to test warblers' rejection behaviors against cryptic and bright foreign eggs. The results showed that warblers have strong egg recognition and rejection abilities. There is a significant tendency for warblers to prefer to peck and reject relatively distinguishable foreign eggs, which supports the crypsis hypothesis. These findings indicate that even in the host-parasite system of open nests, parasitic eggs that are cryptic enough are prevented from being discovered and rejected by the host, and thus obtain the possibility of successful parasitism.

Cuckoos use host egg number to choose host nests for parasitism

To maximize their offspring success common cuckoo (Cuculus canorus) females should lay their eggs into host nests before incubation has begun. This ensures that the parasite chick hatches before all host chicks and can evict its foster siblings to monopolize host parental care. Many studies have demonstrated that most cuckoo eggs are indeed laid before the onset of host incubation. But cues used by female cuckoos to choose the right nest at the right time remain unclear. Here, we combine field observations with a field experiment to test whether female cuckoos use the number of eggs in the nests of their Oriental reed warbler (Acrocephalus orientalis) hosts to direct their choice. Over 8 years of field observations and 5 years of experiments, cuckoo females placed the majority of their eggs in nests with fewer than three host eggs, i.e. early in the laying sequence. For natural nests, the cuckoos may use information gleaned from the activity and behaviour of the host parents to make their choice. In our sets of experimental nests containing different numbers of model eggs, the vast majority of parasitism events occurred in nests containing a single egg. To our knowledge, this is the first field experiment showing that cuckoos choose host nests for parasitism based on the number of host eggs they contain. It appears that cuckoo females use the egg number to estimate the appropriate host nest stage for timely parasitism.

Egg rejection and egg recognition mechanism of chestnut thrushes (Turdus rubrocanus)

Recognizing and rejecting foreign eggs is one of the most effective anti-parasite strategies for hosts in avian brood parasitism. Studies have shown that most cuckoo (Cuculus spp.) hosts have evolved egg recognition abilities. Although some open-nesting birds, especially thrushes belonging to the family Turdidae, are rarely parasitized by cuckoos, they still have high egg recognition ability. This evolutionary selection pressure on egg recognition is currently controversial. Previous studies on egg recognition of thrushes have mainly been carried out in Europe in a single-cuckoo system. In this study, chestnut thrushes (Turdus rubrocanus), which are distributed in a multiple-cuckoo system in China, were used to test their egg rejection and egg recognition mechanism. Our results showed that chestnut thrushes had a rejection rate of 54 % for non-mimetic blue model eggs, showing moderate egg recognition ability. Their egg recognition was true recognition, which relied on a memory template; chestnut thrushes could accurately reject foreign eggs in their nests. This study added the second case to report the egg recognition mechanism of thrushes in the Turdidae family and showed that the evolution of egg recognition ability of chestnut thrushes was likely a retained anti-parasitic strategy because of being parasitized by cuckoos in the past.

Egg retrieval versus egg rejection in cuckoo hosts

Before complex nests evolved, birds laid eggs on the ground, and egg retrieval evolved as an adaptation against accidental displacement of eggs outside the nest. Therefore, egg retrieval is an ancient, and likely ancestral, widespread behaviour in birds. However, it has received little attention in studies of avian brood parasitism, perhaps because most parasitism occurs in species with complex nests, a context in which egg retrieval seems irrelevant. However, for cavity-nesting hosts of avian brood parasites, egg retrieval may still play an important role in the coevolutionary interactions between obligate brood parasites and hosts, because egg retrieval can be considered to be antagonistic to egg rejection behaviour in hosts, yet both may involve cognition to recognize eggs. We hypothesized that (1) cavity-nesting hosts should retrieve misplaced eggs from outside the nest cup, (2) brood parasitism has modulated egg retrieval behaviour in cavity-nesting hosts and (3) hosts use the same visual cues for decision-making during egg recognition in both egg retrieval and egg rejection actions. To test these hypotheses, we performed a series of experiments in a cavity-nesting host, the green-backed tit ( Parus monticolus). Foreign eggs with different levels of mimicry were placed within or outside nest cups of hosts to test their responses. We found that host decisions about whether to retrieve or reject an egg both depended on the degree of mimicry. However, hosts sometimes first retrieved poorly mimetic foreign eggs and then rejected them. Alternatively, hosts sometimes failed to retrieve highly mimetic conspecific eggs. We suggest that egg retrieval in hosts is likely to be a result of the interaction between ancient retrieval behaviour and subsequent adaptation against brood parasitism. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

True recognition of nestlings by hosts selects for mimetic cuckoo chicks

Brood parasitic cuckoos lay their eggs in other birds' nests, whereafter the young cuckoo hatches, ejects its nest-mates and monopolizes the care of the host parents. Theory predicts that hosts should not evolve to recognize and reject cuckoo chicks via imprinting because of the risk of mistakenly imprinting on a cuckoo chick in their first brood and thereafter always rejecting their own chicks. However, recent studies have revealed that some hosts do reject cuckoo chicks from the nest, indicating that these hosts' recognition systems either do not rely on first brood imprinting, or use cues that are independent of chick phenotype. Here, we investigate the proximate mechanisms of chick rejection behaviour in the large-billed gerygone (Gerygone magnirostris), a host of the little bronze-cuckoo (Chalcites minutillus). We find that gerygones use true template-based recognition based on at least one visual chick trait (the number of hatchling down-feathers), and that this is further mediated by experience of adult cuckoos at the nest during egg-laying. Given the theoretical constraints of acquiring recognition templates via imprinting, gerygones must possess a template of own-chick appearance that is largely innate. This true recognition has facilitated the evolution of very rapid hatchling rejection and, in turn, striking visual mimicry of host young by little bronze-cuckoo chicks.