The thick-billed warbler (Phragamaticola aedon, passeriformes, Acrocephalidae) as a host species for the common cuckoo (Cuculus canorus, Cuculiformes, Cuculidae) in the middle Amur River basin

On Wicken Fen and nearby watercourses eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26 and 16 of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively...

Nestling cuckoos, Cuculus canorus , eject host eggs or young from the nest and are then raised alone by the hosts. Using reed warblers, Acrocephalus scirpaceus , as hosts, we investigated how the single cuckoo chick can command the same provisioning rate as a whole brood of host young. Large size alone is not sufficient to stimulate adequate provisioning because single blackbird, Turdus merula , or song thrush, T. philomelos , chicks of the same mass as a cuckoo were fed at a lower rate. Our experiments show that the key stimulus is the cuckoo chick9s rapid begging call (‘si, si, si, si ...’), which sounds remarkably like a whole brood of host chicks, and which it matched in calling rate. When single blackbird or song thrush chicks were accompanied by loudspeakers that broadcast either cuckoo begging calls or calls of a brood of reed warblers, the hosts increased their provisioning rate to that for a cuckoo chick. We suggest that the cuckoo needs vocal trickery to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.

Cuckoo nestlings thrive as avian brood parasites. To acquire sufficient food from the host parents, cuckoo nestlings generally make louder begging calls than host nestlings, but this may cause them to be more likely to attract the attention of predators. Studies have shown that nestlings would respond to the alarm calls of their parents by begging less, or crouching and remaining silent as an adaptation to reduce the risk of being heard by predators. Nevertheless, research is lacking on how parasite nestlings respond to alarm calls of their host parents. We studied the common cuckoo (Cuculus canorus) and one of the most common cuckoo host species, the oriental reed warbler (Acrocephalus orientalis), using a playback experiment in Yongnianwa National Wetland Park during the breeding seasons from June to July, 2020–2021. The begging behaviors of either cuckoo or host nestlings were quantified by playing back the alarm calls of host adults toward common cuckoo, sparrowhawk (Accipiter nisus), or oriental turtle doves (Streptopelia orientalis). Meanwhile, normal begging without playback, playback of the natural singing (NS) of host adults, and background noise (BN) were included as behavioral reference, non-threatening comparison, and playback control, respectively. The results showed that the cuckoo and host nestlings produced similar levels of begging with or without playback of NS and BN; however, both types of nestlings inhibited their begging intensity after hearing the playback of alarm calls, although they did not respond differently to the various alarm call playbacks. This study therefore elucidated that coevolution has selected the common cuckoo nestlings that adapt their begging behavior to the parent–offspring communication of alarm signaling in their host, oriental reed warblers.

Strategic Variation in Mobbing as a Front Line of Defense against Brood Parasitism

Brood parasitism selects for defensive mechanisms that enhance host fitness. Therefore, host populations under different parasitism pressures may express different levels of defense against brood parasites. We tested the rejection responses of currently parasitized and unparasitized Common Redstart (Phoenicurus phoenicurus) populations in Finland to artificial Common Cuckoo (Cuculus canorus) eggs. We predicted a higher level of defense in the parasitized population, but in fact the rejection rate was higher in the nonparasitized population. Nonmimetic artificial eggs were rejected more often than mimetic ones. Desertion probability was higher in the nonparasitized population and was independent of artificial egg type. Common Redstarts in the parasitized population rejected the artificial eggs mostly through ejection, whereas desertion was a more frequent rejection method in the nonparasitized population. Our results suggest that current selection pressures from brood parasites do not always explain the current levels of defense. Respuestas de Poblaciones Parasitadas y No Parasitadas de Phoenicurus phoenicurus Contra el Parasitismo Artificial de Cuculus canorus

Brood parasitism selects for defensive mechanisms that enhance host fitness. Therefore, host populations under different parasitism pressures may express different levels of defense against brood parasites. We tested the rejection responses of currently parasitized and unparasitized Common Redstart (Phoenicurus phoenicurus) populations in Finland to artificial Common Cuckoo (Cuculus canorus) eggs. We predicted a higher level of defense in the parasitized population, but in fact the rejection rate was higher in the nonparasitized population. Nonmimetic artificial eggs were rejected more often than mimetic ones. Desertion probability was higher in the nonparasitized population and was independent of artificial egg type. Common Redstarts in the parasitized population rejected the artificial eggs mostly through ejection, whereas desertion was a more frequent rejection method in the nonparasitized population. Our results suggest that current selection pressures from brood parasites do not always explain the current levels of defense.Respuestas de Poblaciones Parasitadas y No Parasitadas de Phoenicurus phoenicurus Contra el Parasitismo Artificial de Cuculus canorus

Growth is a key life history trait that is closely related to individual fitness. In altricial birds, growth is restricted to a relatively short period, and depends primarily on the amount or quality of food and hence on parental care. Obligate brood parasites do not care for their own offspring but impose this burden on other species (hosts). As many brood parasites exploit various host species, their progeny are expected to receive different levels of parental care. Parasite growth has thus often been explored in the context of host parenting abilities and only rarely with respect to its sex specificity. Here, we fill this gap in knowledge and explore sex differences in the growth of Common Cuckoo (Cuculus canorus) nestlings reared by 2 warbler hosts in the genus Acrocephalus. As adult Common Cuckoo males are 5–16% heavier than females, we assumed that nestlings would also differ in size and thus in growth performance. To test this assumption, we used a nonlinear mixed effects modeling approach...

Blackcaps Sylvia atricapilla reject artificial cuckoo eggs, and their eggs vary little in appearance within clutches, whereas among clutches eggs vary considerably. Low variation within clutches facilitates discrimination of parasitic eggs, whereas high variation among clutches makes it harder for the cuckoo to mimic the eggs of a certain host species. These traits have most probably evolved as counteradaptations against brood parasitism by the common cuckoo Cuculus canorus, even though blackcaps are not regularly parasitised today. In this study, we investigated how fine-tuned the rejection of parasitic eggs is in this species by introducing three types of eggs into their nests: a real non-mimetic egg the approximate size of a cuckoo egg, an artificial mimetic egg the size of a cuckoo egg and a real conspecific egg. As the rejection frequency of both mimetic and non-mimetic artificial cuckoo eggs has been shown to be high in previous studies, the variation in rejection behaviour between individuals is low, indicating that most individuals within the population are able to reject parasitic eggs. Thus, we predict that (1) the intraclutch variation in egg appearance should be generally low in all individuals, and that (2) regarding conspecific eggs, rejection decisions should be highly dependent on the degree of mimicry between parasitic and host eggs. We found support for these predictions, which indicates that due to their highly sophisticated countermeasures against brood parasitism, blackcaps can probably be regarded as current winners of the arms race with the common cuckoo. Furthermore, the high and consistent rejection frequency of cuckoo eggs found throughout Europe for this species supports the spatial habitat structure hypothesis, which claims that woodland-nesting species breeding near trees, like blackcaps, presumably experienced a high level of parasitism throughout their range in the past and, therefore, their rejection behaviour, once evolved, spread rapidly to all populations.

BackgroundObligate brood parasites exert strong selective pressure on target hosts. In response, hosts typically evolve anti-parasitism strategies, of which egg recognition is one of the most efficient. Generally, host egg-recognition capacity is determined using model eggs. Previous studies have shown that some host species, which are capable of detecting parasite eggs, do not reject model eggs. However, it is unknown that whether the reaction to model eggs varies among distinct populations of the same host in relation to the degree of parasitism pressure.ResultsHere, we compared the rejection frequencies of model eggs and real eggs between mainland and island populations of the plain prinia (Prinia inornata), which are respectively sympatric and allopatric with their brood parasite, the common cuckoo (Cuculus canorus). Our results indicated that the mainland and island populations rejected real eggs at similar rates, but rejected model eggs, which were similar in size to real eggs but heavier, at significantly different rates: the island population rejected fewer model eggs, possibly because the rejection motivation of this population was lower due to absence of parasitism.ConclusionsOur results indicated that some factors affecting the decision to reject, such as rejection motivation, varied according to the degree of parasitism pressure, and thus influenced the frequency of egg rejection. Furthermore, our results suggested that model eggs should be used with caution in comparative studies of egg recognition abilities among species or populations subjected to different intensities of brood parasitism. That is, model eggs may fail to accurately detect egg recognition in host populations with little to no risk of parasitism.

Egg rejection belongs to a widely used host tactic to prevent the costs incurred by avian brood parasitism. However, the genetic basis of this behaviour and the effect of host age on the probability of rejecting the parasitic egg remain largely unknown. Here, we used a set of 15 polymorphic microsatellite loci, including a previously detected candidate locus (Ase64), to link genotypes of female great reed warblers (Acrocephalus arundinaceus), a known rejecter, with their egg rejection responses in two host populations. We also tested whether host female age, as a measure of the experience with own eggs, plays a role in rejection of common cuckoo (Cuculus canorus) eggs. We failed to find any consistent association of egg rejection responses with host female genotypes or age. It seems that host decisions on egg rejection show high levels of phenotypic plasticity and are likely to depend on the spatiotemporal variation in the parasitism pressure. Future studies exploring the repeatability of host responses towards parasitic eggs and the role of host individual experience with parasitic eggs would greatly improve our understanding of the variations in host behaviours considering the persistence of brood parasitism in host populations with rejecter phenotypes.

Although Old World buntings (Emberizinae) may be considered suitable Common Cuckoo (Cuculus canorus) hosts, there is at present no evidence that any of the European species are regularly parasitized. Most historical parasitism records refer to the Yellowhammer (Emberiza citrinella) and Reed Bunting (E. schoeniclus). Both of these species reject almost 100% of experimentally added nonmimetic eggs, and also a considerable proportion of experimentally added conspecific eggs, showing exquisite egg discrimination abilities. In this paper, we report Common Cuckoo parasitism and egg rejection behavior in a Bulgarian population of another Old World Emberizinae, the Corn Bunting (Miliaria calandra). We found this species was regularly parasitized (9%, 8 of 90 nests) and that the parasitism rate was consistent among the three years of our study. Naturally laid Common Cuckoo eggs were fairly good mimics of host eggs and most were accepted (5 of 7). The Corn Bunting proved to be a suitable Common Cuckoo host...

We examined primary sex-ratios of two brood parasitic species, Brown-headed Cowbird (Molothrus ater) and the Common Cuckoo (Cuculus canorus), to determine whether there was any evidence of primary sex-ratio manipulation as has been demonstrated in other species of birds. Despite good reasons for why female brood parasites should manipulate the primary sex-ratio of their young, we found a lack of evidence for a bias in the sex-ratio of eggs produced at the population level, with respect to the host species parasitized or time of breeding season, or in terms of the sex ratio of eggs produced by individual females. Thus, this study provides another example in birds of little evidence for sex-ratio variation in relation to environmental factors.

Returning to a breeding site and decision where to breed belong to the key life-history traits, especially in migratory birds. Yet, we still lack knowledge about the drivers of adult return rates and breeding dispersal distances in populations under pressure of brood parasitism. We explored these issues in a trans-Saharan migratory passerine, the great reed warbler (Acrocephalus arundinaceus), in a population parasitized by the common cuckoo (Cuculus canorus)—an evicting brood parasite. In 2008–2012, a total of 563 great reed warblers were colour-marked and 185 of them were re-encountered 303 times in a year following their breeding at a fishpond area in the Czech Republic. We tested how brood parasitism and host breeding parameters in 1 year affect host return rate and dispersal distances in a following year. Return rate was lower in females fledging a cuckoo and in both sexes that failed to produce any offspring than in birds that fledged own chicks in the preceding year. Individual brood parasitism had a negative effect on the probability of female returning, but this relationship disappeared when excluding females fledging cuckoos. Although return rates did not differ between females that rejected and those that accepted cuckoo eggs, rejecter females dispersed less than acceptors. We conclude that brood parasitism and fostering the parasite might be negatively related to host female survival. The other breeding conditions might rather be related to the decision where to breed in the future. Establishing new long-term studies monitoring parasitized populations might open up avenues for future research.

The common cuckoo Cuculus canorus is an obligate brood parasite that lays its eggs in the nests of small passerines. It has long been hypothesized that cuckoo eggs should be structurally stronger than host eggs or those of non-parasitic cuckoos to reduce chances of breakage during laying, to prevent accidental damage during incubation and/or to hinder their rejection through puncture ejection by the host. Therefore, we analysed selected characteristics of a sample of freshly laid eggs of the common cuckoo with two of its major hosts, the reed warbler Acrocephalus scirpaceus and great reed warbler Acrocephalus arundinaceus, and a sample of species with known puncture resistant eggs. We found that in puncture resistance tests cuckoo eggs tolerated on average 231 g. The cuckoo eggs were 3.3 and 2.5 times stronger than those of the reed warbler and great reed warbler, respectively. Greater shell thickness can explain only 17% of the total extra strength of the cuckoo eggs (125.97 g). When we controlled for the confounding effects of egg size (using a sample of eggs of normal strength from bird species of varying size), the common cuckoo eggs were 2.2 times stronger than expected for their size. Our results are consistent with the hypothesis that cuckoo eggs are structurally stronger and this trait probably represents an adaptation for a brood parasitic life style.

BackgroundIn the last decade, enigmatic male-like cuckoo calls have been reported several times in East Asia. These calls exhibited a combination of vocal traits of both Oriental Cuckoo (Cuculus optatus) and Common Cuckoo (Cuculus canorus) advertising calls, and some authors therefore suggested that the enigmatic calls were produced by either Common × Oriental Cuckoo male hybrids or Common Cuckoo males having a gene mutation. However, the exact identity of calling birds are still unknown.MethodsWe recorded previously unknown male-like calls from three captive Oriental Cuckoo females, and compared these calls with enigmatic vocalizations recorded in the wild as well as with advertising vocalizations of Common and Oriental Cuckoo males. To achieve this, we measured calls automatically. Besides, we video-recorded captive female emitting male-like calls, and compared these recordings with the YouTube recordings of calling males of both Common and Oriental Cuckoos to get insight into the mechanism of call production.ResultsThe analysis showed that female male-like calls recorded in captivity were similar to enigmatic calls recorded in the wild. Therefore, Oriental Cuckoo females might produce the latter calls. Two features of these female calls appeared to be unusual among birds. First, females produced male-like calls at the time of spring and autumn migratory activity and on migration in the wild. Because of this, functional significance of this call remained puzzling. Secondly, the male-like female call unexpectedly combined features of both closed-mouth (closed beak and simultaneous inflation of the ‘throat sac’) and open-mouth (prominent harmonic spectrum and the maximum neck extension observed at the beginning of a sound) vocal behaviors.ConclusionsThe Cuculus vocalizations outside the reproductive season remain poorly understood. Here, we found for the first time that Oriental Cuckoo females can produce male-like calls in that time. Because of its rarity, this call might be an atavism. Indeed, female male-like vocalizations are still known in non-parasitic tropical and apparently more basal cuckoos only. Therefore, our findings may shed light on the evolution of vocal communication in avian brood parasites.