Innate versus learned recognition of conspecifics in great spotted cuckoos Clamator glandarius

Imprinting in an interspecific brood parasitic bird

Many animals learn to recognize conspecifics after an early experience with them through sexual imprinting. For brood parasitic birds, it is not possible to develop conspecific recognition using cues provided by their foster parents. One solution is that a unique species-specific signal triggers the learning of additional aspects of the conspecific's phenotype. It has been proposed that for brood parasitic cowbirds, this signal is an innate vocalization, the chatter. This vocalization might act in a cross-modal learning process through which juveniles that listen to the song learn to recognize the visual characteristics of the song's producer. We trained two groups of juvenile shiny cowbirds (Molothrus bonariensis). In one group, individuals listened to the chatter or a heterospecific call while they observed a stuffed model of the corresponding species. In the other group, individuals listened to the call of one species (cowbird or heterospecific) while they observed the stuffed model of the other species. In the preference test, juveniles chose the model associated with the chatter, regardless of whether the model was a cowbird or a heterospecific. These results show how the auditory system through a species-specific signal can lead to cross-modal learning of visual cues allowing conspecific recognition in brood parasitic cowbirds.

Preferential Begging Responses of Shiny Cowbirds to the Conspecific Chatter Call

Avian brood parasites lay their eggs into the nests of other species, which incubate them and raise the chicks until their independence. Despite living their early weeks of life surrounded by heterospecifics, young brood parasites have the ability to recognize and associate to conspecifics after independence. It has been proposed that the initial conspecific recognition develops when a young parasite encounters a unique species-specific signal that triggers the learning of other aspects of the producer of the signal. For cowbirds (Molothrus spp.), this species-specific signal is hypothesized to be the chatter call. Young birds also could express auditory biases, which in some cases lead to discrimination in favor of conspecific songs. Therefore, the perceptual selectivity for chatters might be also present in nestlings. Our aim was to assess if nestlings of the shiny cowbird (M. bonariensis) present a preferential begging response to conspecific chatter calls. We evaluated if they respond more to the parasitic vocalization than host chicks and if they respond more to the chatter than to heterospecific nonhost calls. We tested shiny cowbird chicks reared by chalk-browed mockingbirds (Mimus saturninus) or house wrens (Troglodytes aedon) and host chicks, as control species. We randomly presented to 6-day-old chicks the following playback treatments: (1) conspecific chatter calls, (2) host calls, used as positive controls, and (3) nonhost calls, used as negative control. We measured if chicks begged during the playback treatments and the begging intensity. When responding to the playback of chatter calls, shiny cowbird chicks begged at a higher frequency and more intensively than host chicks. Shiny cowbird chicks reared by mockingbirds begged more intensively to playbacks of conspecific chatter calls than to host calls, while those reared by wrens begged with a similar intensity to playbacks of conspecific chatter and host calls. On the contrary, wren nestlings begged more intensively to playbacks of the wren call than to chatter calls. Mockingbird nestlings did not beg during any treatment. None of the three species begged during the playback of nonhost calls. Our results show that the chatter call produced a preferential begging response in cowbird nestlings, which may be the beginning of a process of conspecific recognition.

Quintuple parasitism of a great reed warbler nest by common cuckoos

Multiple parasitism in obligate avian brood parasites occurs when several brood parasitic females lay their eggs in the nest of the same host. While multiple parasitism is common in the highly social, nonevicting cowbird species (Molothrus sp.), in which multiple parasitic nestlings can be raised simultaneously by the same hosts, it is less common in the case of cuckoo species (Cuculus sp.). The first cuckoo nestling to hatch from the egg evicts all nestmates; therefore, it is costly for cuckoo females to lay eggs in already parasitized nests. However, this can occur in sites with very high parasitism rates, and it can even increase the breeding success of the brood parasites, as the presence of multiple parasitic eggs in the nest of the host decreases rejection rates. Here, we present a case of a quintuple brood parasitism of a great reed warbler (Acrocephalus arundinaceus) nest, an extreme form of multiple brood parasitism.

The gut microbiota of brood parasite and host nestlings reared within the same environment: disentangling genetic and environmental effects

Gut microbiota are essential for host health and survival, but we are still far from understanding the processes involved in shaping their composition and evolution. Controlled experimental work under lab conditions as well as human studies pointed at environmental factors (i.e., diet) as the main determinant of the microbiota with little evidence of genetic effects, while comparative interspecific studies detected significant phylogenetic effects. Different species, however, also differ in diet, feeding behavior, and environmental characteristics of habitats, all of which also vary interspecifically, and, therefore, can potentially explain most of the detected phylogenetic patterns. Here, we take advantage of the reproductive strategy of avian brood parasites and investigate gut microbiotas (esophageal (food and saliva) and intestinal) of great spotted cuckoo (Clamator glandarius) and magpie (Pica pica) nestlings that grow in the same nests. We also estimated diet received by each nestling and explored its association with gut microbiota characteristics. Although esophageal microbiota of magpies and great spotted cuckoos raised within the same environment (nest) did not vary, the microbiota of cloacal samples showed clear interspecific differences. Moreover, diet of great spotted cuckoo and magpie nestlings explained the microbiota composition of esophageal samples, but not of cloaca samples. These results strongly suggest a genetic component determining the intestinal microbiota of host and parasitic bird species, indicating that interspecific differences in gut morphology and physiology are responsible for such interspecific differences.

Female-female aggression and male responses to the two colour morphs of female common cuckoos

Female-only colour polymorphism is rare in birds, but occurs in brood parasitic cuckoos (Cuculidae). Obligate brood parasites leave incubation and parental care to other species (hosts), so female-female interactions can play a role in how parasites guard critical resources (host nests) within their laying areas. The plumage of adult female common cuckoos (Cuculus canorus) is either rufous (typically rare) or grey (common), whereas adult male conspecifics are monochromatic (grey). In previous studies, hosts and conspecific males responded with less intensity toward the rare female morph in support of a negative frequency-dependent benefit of female plumage polychromatism. Here, we assessed responses of both conspecific females and males to vocal playbacks of female calls, coupled with one of two 3D models of the different morphs of female cuckoos. At our study population, the rufous female morph was as common as the grey morph; therefore, we predicted similarly high rates of conspecific responses in both treatments. Both female and male cuckoos responded to playbacks acoustically, which demonstrated the primary role of acoustic communication in social interactions amongst cuckoos. Following this, some cuckoos flew closer to the models to inspect them visually. As predicted, no significant differences were detected between the live cuckoos’ responses toward the two colour morphs in this population. We conclude that dichromatism in female cuckoos evolved to serve one or more functions other than conspecific signalling.

Migration behavior and performance of the great spotted cuckoo (Clamator glandarius)

The study of brood parasitism has traditionally been focused on the breeding period, but recent evidence suggests that it urgently needs a new spatio-temporal perspective to explore novel avenues on brood parasite-host co-evolutionary interactions. Many brood parasites are migrants, but their ecology outside their short breeding season is poorly known. The great spotted cuckoo (Clamator glandarius) is one of the classical models in the study of brood parasitism, however, there is very little information on its migratory strategy, route and wintering grounds. Furthermore, there is no previous information on the geographical distribution of mortality and its causes in this species; information that is critical to understand the fluctuations in cuckoo populations and detect potential conservation risks. Using satellite tracking technology, we provide novel insight into the migratory behavior and performance of the great spotted cuckoo. We found individuals from southern Spain to be long-distance nocturnal migrants that use the East Atlantic Flyway for both post and pre-breeding migration, and that winter in the western Sahel. We found evidence of individual variation in their migration route, particularly regarding their post-breeding behavior in Spain. Our study also suggests that the south of Morocco is the most dangerous area due to a large number of deaths during the post-breeding migratory period. Furthermore, we found that natural predation seems to be the main cause of death, probably due to raptors, although human activities (i.e. hunting) could also played a role in the southern Mediterranean shore. Our study offers novel findings and challenges traditional ideas on the ecology of this species providing a good example of how the new spatio-temporal perspective can expand our knowledge on brood parasites.

Gut Microbiota of Great Spotted Cuckoo Nestlings is a Mixture of Those of Their Foster Magpie Siblings and of Cuckoo Adults

Diet and host genetic or evolutionary history are considered the two main factors determining gut microbiota of animals, although studies are scarce in natural populations. The system of great spotted cuckoos (Clamator glandarius) parasitizing magpies (Pica pica) is ideal to study both effects since magpie adults feed cuckoo and magpie nestlings with the same diet and, consequently, differences in gut microbiota of nestlings of these two species will mainly reflect the importance of genetic components. Moreover, the diet of adults and of nestling cuckoos drastically differ from each other and, thus, differences and similarities in their microbiotas would respectively reflect the effect of environmental and genetic factors. We used next-generation sequencing technologies to analyze the gut microbiota of cuckoo adults and nestlings and of magpie nestlings. The highest α-diversity estimates appeared in nestling cuckoos and the lowest in nestling magpies. Moreover, despite the greatest differences in the microbiome composition of magpies and cuckoos of both ages, cuckoo nestlings harbored a mixture of the Operational Taxonomic Units (OTUs) present in adult cuckoos and nestling magpies. We identified the bacterial taxa responsible for such results. These results suggest important phylogenetic components determining gut microbiome of nestlings, and that diet might be responsible for similarities between gut microbiome of cuckoo and magpie nestlings that allow cuckoos to digest food provided by magpie adults.

Great spotted cuckoo nestlings have no antipredatory effect on magpie or carrion crow host nests in southern Spain

Host defences against cuckoo parasitism and cuckoo trickeries to overcome them are a classic example of antagonistic coevolution. Recently it has been reported that this relationship may turn to be mutualistic in the case of the carrion crow (Corvus corone) and its brood parasite, the great spotted cuckoo (Clamator glandarius), given that experimentally and naturally parasitized nests were depredated at a lower rate than non-parasitized nests. This result was interpreted as a consequence of the antipredatory properties of a fetid cloacal secretion produced by cuckoo nestlings, which presumably deters predators from parasitized host nests. This potential defensive mechanism would therefore explain the detected higher fledgling success of parasitized nests during breeding seasons with high predation risk. Here, in a different study population, we explored the expected benefits in terms of reduced nest predation in naturally and experimentally parasitized nests of two different host species, carrion crows and magpies (Pica pica). During the incubation phase non-parasitized nests were depredated more frequently than parasitized nests. However, during the nestling phase, parasitized nests were not depredated at a lower rate than non-parasitized nests, neither in magpie nor in carrion crow nests, and experimental translocation of great spotted cuckoo hatchlings did not reveal causal effects between parasitism state and predation rate of host nests. Therefore, our results do not fit expectations and, thus, do not support the fascinating possibility that great spotted cuckoo nestlings could have an antipredatory effect for host nestlings, at least in our study area. We also discuss different possibilities that may conciliate these with previous results, but also several alternative explanations, including the lack of generalizability of the previously documented mutualistic association.

Great spotted cuckoo fledglings often receive feedings from other magpie adults than their foster parents: which magpies accept to feed foreign cuckoo fledglings?

Natural selection penalizes individuals that provide costly parental care to non-relatives. However, feedings to brood-parasitic fledglings by individuals other than their foster parents, although anecdotic, have been commonly observed, also in the great spotted cuckoo (Clamator glandarius) – magpie (Pica pica) system, but this behaviour has never been studied in depth. In a first experiment, we here show that great spotted cuckoo fledglings that were translocated to a distant territory managed to survive. This implies that obtaining food from foreign magpies is a frequent and efficient strategy used by great spotted cuckoo fledglings. A second experiment, in which we presented a stuffed-cuckoo fledgling in magpie territories, showed that adult magpies caring for magpie fledglings responded aggressively in most of the trials and never tried to feed the stuffed cuckoo, whereas magpies that were caring for cuckoo fledglings reacted rarely with aggressive behavior and were sometimes disposed to feed the stuffed cuckoo. In a third experiment we observed feedings to post-fledgling cuckoos by marked adult magpies belonging to four different possibilities with respect to breeding status (i.e. composition of the brood: only cuckoos, only magpies, mixed, or failed breeding attempt). All non-parental feeding events to cuckoos were provided by magpies that were caring only for cuckoo fledglings. These results strongly support the conclusion that cuckoo fledglings that abandon their foster parents get fed by other adult magpies that are currently caring for other cuckoo fledglings. These findings are crucial to understand the co-evolutionary arms race between brood parasites and their hosts because they show that the presence of the host's own nestlings for comparison is likely a key clue to favour the evolution of fledgling discrimination and provide new insights on several relevant points such as learning mechanisms and multiparasitism.

Do first-time breeding females imprint on their own eggs?

The egg-recognition processes underlying egg rejection are assumed to be based on an imprinting-like process (a female learning the aspect of her own eggs during her first breeding attempt). The imprinting-like process and the misimprinting costs have been the objective of many theoretical models and frequently have a leading role in papers published on brood parasitism; however, an experiment has never been undertaken to test the existence of this imprinting-like process by manipulating egg appearance in first-time breeding females. Here, we present the first such experimental study using the house sparrow (Passer domesticus), which is a conspecific brood parasite and which has a good ability to reject conspecific eggs, as a model species. We found that contrary to what the hypothesis predicts first-time breeding females did not reject their own eggs in their second breeding attempt. This lack of response against unmanipulated eggs could indicate that females have an innate preference for their own eggs. However, in a second experimental group in which first-time breeding females were allowed to learn the aspect of their (unmanipulated) own eggs, none ejected manipulated eggs during their second clutch either--a finding that does not support the idea of recognition templates being inherited, but instead suggests that recognition templates could be acquired again at each new breeding attempt. Our results demonstrate that it is likely that egg discrimination is not influenced by egg appearance in the first breeding attempt.

Sexual imprinting misguides species recognition in a facultative interspecific brood parasite

Sexual reproduction relies on the recognition of conspecifics for breeding. Most experiments in birds have implicated a critical role for early social learning in directing subsequent courtship behaviours and mating decisions. This classical view of avian sexual imprinting is challenged, however, by studies of megapodes and obligate brood parasites, species in which reliable recognition is achieved despite the lack of early experience with conspecifics. By rearing males with either conspecific or heterospecific brood mates, we experimentally tested the effect of early social experience on the association preferences and courtship behaviours of two sympatrically breeding ducks. We predicted that redheads (Aythya americana), which are facultative interspecific brood parasites, would show a diminished effect of early social environment on subsequent courtship preferences when compared with their host and congener, the canvasback (Aythya valisineria). Contrary to expectations, cross-fostered males of both species courted heterospecific females and preferred them in spatial association tests, whereas control males courted and associated with conspecific females. These results imply that ontogenetic constraints on species recognition may be a general impediment to the initial evolution of interspecific brood parasitism in birds. Under more natural conditions, a variety of mechanisms may mitigate or counteract the effects of early imprinting for redheads reared in canvasback broods.

Conflict between egg recognition and egg rejection decisions in common cuckoo (Cuculus canorus) hosts

Common cuckoos (Cuculus canorus) are obligate brood parasites, laying eggs into nests of small songbirds. The cuckoo hatchling evicts all eggs and young from a nest, eliminating hosts' breeding success. Despite the consistently high costs of parasitism by common cuckoos, great reed warbler (Acrocephalus arundinaceus) hosts sometime accept and other times reject parasitic eggs. To explore the cognitive basis of this seemingly maladaptive variation in host responses, we documented differences in egg rejection rates within 1-day periods just before and during the egg-laying cycle across host nests. Hosts rejected cuckoo eggs at 28% of nests during the pre-egg-laying stage, but when cuckoos exchanged the first host egg with the parasite egg, rejections increased to 75%. Even later, when several host eggs remained in a nest after parasitism, rejection rate fell to 37.5%. Experimental parasitism with conspecific eggs on the first and second day of host laying showed a similar directional change in relative rejection rates, dropping from 35 to 0%. Mistakes in egg discrimination (ejection error and ejection cost) were observed mostly in the latter part of the laying cycle, mainly when nests contained 5-6 eggs. These correlational and experimental patterns of egg rejection support a cognitive process of egg discrimination through several shifts in hosts' optimal acceptance thresholds of foreign eggs. The results are also consistent with the evolution of foreign egg rejection in the context of nest-sanitation (i.e. the removal of foreign objects). Our results suggest that common cuckoo hosts may recognize more eggs than they reject. This implies that the experience of the host with one or more of its own eggs in the clutch is a key factor in rejecting parasite eggs by allowing inspection and learning about their own clutch.

Do avian brood parasites eavesdrop on heterospecific sexual signals revealing host quality? A review of the evidence

Eavesdropping can be defined as the extraction of information from the interactions between other individuals. It provides a relatively cheap way of gathering relevant information for fitness enhancement. Here, we propose that obligate avian brood parasites, which always lay their eggs in foreign nests of individuals of other species, may eavesdrop on their host sexual signals to locate nests of high quality individuals in which to lay their parasitic eggs. Sexual signal variation can honestly signal parental quality. Thus, by eavesdropping on sexual signals, parasites may select high quality foster parents for their own offspring. Such a use of sexual signals within host populations by brood parasites differs from signal exploitation theory that proposes that parasite only use signals to locate potential host independently from signaller quality. Here, we review the avian literature concerning host choice within a host species by obligate avian brood parasites and find evidence for host selection within individuals of a host species on the basis of cues potentially functioning as sexually selected traits, or at least revealing parental abilities. We have also found support for the existence of benefits linked to host selection by avian brood parasites. Finally, one study reported on the attenuation of a sexual ornament in host populations under strong pressure by brood parasites. Most of these findings have been interpreted as evidence for host selection by avian brood parasites based on the conspicuousness of sexual signals. We suggest, however, that these findings may in fact reveal eavesdropping on host signalling performance by brood parasites which would use the information extracted to choose the better individuals among conspecifics of a given host. This provides a new perspective for the study of host selection in obligate brood parasites, and raises interesting questions for the study of animal cognition that would deserve experimental studies.

Social responses without early experience: Australian brush-turkey chicks use specific visual cues to aggregate with conspecifics

Almost all birds depend upon early experience with adults and siblings to learn recognition cues. Megapodes, such as the Australian brush-turkey(Alectura lathami), have evolved a very different life history. Eggs are incubated in mounds of decaying organic material. Chicks hatch asynchronously and receive no parental care, so imprinting cannot occur. Nevertheless, chicks subsequently form groups with similar-aged conspecifics. We explored the perceptual basis of this aggregation response, focussing on likely visual cues, such as pecking movements and body colour. Experiments were conducted under naturalistic conditions in a large aviary, using realistic robot models and colour filters. The robots successfully evoked a range of social responses resembling those of a live companion. Aggregation depended upon both behaviour and morphology. Simultaneous choice tests revealed that brush-turkey chicks preferred a pecking robot over either a static model or a scanning robot, suggesting that responsiveness depends upon particular movement patterns. In addition, chicks were sensitive to changes in appearance but only those that affected radiance at short wavelengths. The mechanism underlying social aggregation after hatching hence involves relatively specific cues. This perceptual bias seems to be largely experience independent and may exploit attributes to which potential predators are insensitive.

Ontogeny of social behavior in the megapode Australian brush-turkey (Alectura lathami)

Megapodes meet conspecifics at an unpredictable age, and it is unknown how their social behavior develops under such conditions. The authors induced encounters between 2-day-old socially naive hatchlings and up to 49-day-old chicks of the Australian brush-turkey (Alectura lathami) in a large outdoor aviary. All social behavior patterns found in older chicks were present in hatchlings and did not change in form or the frequency in which they occurred thereafer; the frequency of calling was the only feature that changed significantly with age. Chicks stayed only 0.3-2.0 m (medians) apart and synchronized their feeding activity from the age of 20 days. These results indicate that megapode chicks show social behavior without social experience and that this requires no postnatal leaning.

A password for species recognition in a brood-parasitic bird

Recognition of conspecifics is an essential precursor of sexual reproduction. Most mammals and birds learn salient features of their parents or siblings early in ontogeny and later recognize individuals whose phenotypes match the mental image (template) of relatives closely enough as conspecifics. However, the young of brood parasites are reared among heterospecifics, so social learning will yield inappropriate species recognition templates. Initially, it was inferred that conspecific recognition in brood parasites depended on genetically determined templates. More recently it was demonstrated that learning plays a critical role in the development of parasites' social preferences. Here we propose a mechanism that accommodates the interaction of learned and genetic components of recognition. We suggest that conspecific recognition is initiated when a young parasite encounters some unique species-specific signal or "password" (e.g. a vocalization, behaviour or other characteristic) that triggers learning of additional aspects of the password-giver's phenotype. We examined the possibility that nestlings of the obligately brood-parasitic brown-headed cowbird (Molothrus ater) could use a species-specific vocalization, the "chatter", as a password. We found that six-day-old nestlings responded (begged) significantly more frequently to playbacks of chatters than to other avian sounds and that two-month-old fledglings approached playbacks of chatters more quickly than vocalizations of heterospecifics. Free-living cowbird fledglings and adults also approached playbacks of chatters more often than control sounds. Passwords may be involved in the ontogeny of species recognition in brood parasites generally.