Great Spotted Cuckoo Nestlings but not Magpie Nestlings Starve in Experimental Age‐Matched Broods

Nestlings of non‐evicting avian brood‐parasites have to compete for food with foster parents' own nestlings. The outcome of these competitive contests is determined mainly by body size differences between parasitic and host nestlings. As part of the coevolutionary arms race between brood parasites and their hosts at the nestling stage, it has been reported that some host foster parents discriminate against parasitic chicks and are reluctant to feed them. Here, by experimentally creating size‐matched broods of different composition (only magpie Pica pica chicks, only great spotted cuckoo Clamator glandarius chicks or mixed broods), we show that great spotted cuckoo chicks starved in 20.2 per cent (17 of 84) of the parasitized magpie nests even in absence of size asymmetries, while in none (0 of 72) of the nests a magpie chick starved. As far as we know, this is the first record of non‐evictor brood parasitic nestlings starving without being smaller than their host nestmates in a frequently used host species. Nest composition had no effect on chick starvation. The cuckoo nestling starved even in two of the nests occupied by only one cuckoo chick. Our results could be explained by (1) magpies being reluctant to feed cuckoo chicks; (2) parasitic chicks receiving lower‐quality food items or cuckoo nestlings being sensitive to some particular component of the diet (e.g. cereal grains); and (3) the existence of cuckoo chick discrimination ability by magpie foster parents.     

Complex feeding behaviour by magpies in nests with great spotted cuckoo nestlings

Parent decisions about food allocation are usually based on simple time‐saving rules that optimize their own fitness; however, they can sometimes vary depending on the prevailing ecological conditions both outside and inside the nest. Parent–offspring interactions also become more complex when parents suffer from brood parasitism, which implies that they care for the parasite's eggs and unrelated young. The great spotted cuckoo Clamator glandarius is a specialist brood parasite that uses the magpie Pica pica as its primary host. Here, by filming food allocation by magpie parents in natural non‐parasitized and experimentally parasitized and non‐parasitized magpie nests, we have found that magpie provisioning behaviour is highly complex including two types of feedings apart from normal ones. First, false feedings, when the parent touched the chick's beak but did not leave any food, occurred more frequently when feeding a cuckoo than when feeding magpie nestlings. Second, two types of what we have called coax feedings: 2a) when magpie parents induce a nestling to beg by waking it up by touching it softly with the beak, and 2b) when parents disregard begging signals (always from brood parasitic great spotted cuckoos) while coaxing one non‐begging nestling (always one of their own) to feed it. We suggest that brood parasitism, involving selfish excessively begging nestlings, could have acted as a selective pressure for both false and coax feedings to evolve, as both imply ignoring nestlings that beg too much. We also discuss that these parental responses could have evolved either by a discrimination without recognition mechanism, or, more probably, by a recognition‐based discrimination mechanism.     

Corticosterone levels in host and parasite nestlings: Is brood parasitism a hormonal stressor?

Parasite chicks from non-evictor species usually try to monopolize host parental care, thereby increasing considerably the level of food competition in the nest. Here, we propose that brood parasitism is an important stressor for host and parasite nestlings and explore this hypothesis in the non-evictor great spotted cuckoo (Clamator glandarius) and its main hosts, the same-sized black-billed magpie (Pica pica) and the larger carrion crow (Corvus corone). We experimentally created 3-nestling broods of different brood compositions (only cuckoo chicks, only host chicks, or cuckoo and host chicks together) and measured baseline corticosterone levels of nestlings along their developmental period (early, middle and late). We found that brood parasitism increased corticosterone levels in magpie nestlings in the mid and late nestling period compared to those raised in unparasitized nests. Interestingly, carrion crow nestlings from parasitized nests only increased their corticosterone levels in the mid nestling period, when the competition for food with the cuckoo nestling was highest. Our results suggest that brood parasitism could be a potential physiological stressor for host nestlings, especially during the developmental stages where food requirements are highest. Conversely, cuckoo nestlings could be physiologically adapted to high competition levels since they did not show significant differences in corticosterone levels in relation to brood composition.     

Differences in intestinal microbiota between avian brood parasites and their hosts

The intestinal microbiota determines the effectiveness of digestion in vertebrates, and is influenced by the external environment (mainly the diet), gut characteristics, and phylogeny. Avian brood-parasitic nestlings of the sub-family Cuculinae develop in nests of phylogenetically distant passerines and can be fed with the host diet. If the shaping of bacterial communities is dominated by phylogenetic constraints, and therefore the microbiota of parasitic nestlings differs from that of host nestlings, the energy and micronutrients that parasites and hosts obtain from a similar amount of food would be different. In this case, the bacterial communities of parasitic and host nestlings would have important consequences with respect to brood parasite development. By experimentally creating mixed broods of magpies ( Pica pica ) and great spotted cuckoos ( Clamator glandarius ), we investigated their cloacal microbiota using ribosomal intergenic spacer analysis. We found significant differences in bacterial assemblages of the parasitic and host nestlings, although none of the phylotypes were specific in either great spotted cuckoos or magpies. Cuckoos presented more complex communities, which could help the brood parasitic life style and allow the digestion of food provided by different potential hosts. Moreover, the intestinal morphology is different between the two species due to phylogenetic differences in the two taxa, which would influence the dissimilar bacterial assemblages. The detected differences in microbiota of great spotted cuckoo and magpie nestlings, which might occur in other brood parasite–host systems, may imply a lower digestion efficiency in parasites. Thus, the higher level requirements of cuckoo nestlings may be explained, at least in part, by cuckoos having a suboptimal bacterial community for processing the host diet.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 406–414.     

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.Subject terms: Microbial ecology, Community ecology     

Great spotted cuckoos respond earlier to the arrival of feeding foster parents and perform less erroneous begging when hungry than their magpie host nest‐mates

In many bird species, parents usually feed the first nestling that starts to beg before its nest‐mates. The pressure to avoid missed feeds could trigger nestlings to perform in erroneous begging in absence of parents, which has the same costs as begging in the presence of parents but without any reward. So, nestlings should try to minimize both erroneous begging and missed feeds simultaneously. The threshold to start begging is predicted to be lower for hungry nestlings and for nestlings that are unrelated to their nest‐mates, because they suffer lower inclusive fitness costs when depriving nest‐mates of food. In line with this idea, we found that brood parasitic great spotted cuckoo nestlings responded sooner than their magpie nest‐mates when an adult arrived to the nest. Under laboratory conditions, nestlings of both species rarely incurred in erroneous begging when food was abundant, but under conditions of restricted food, magpie nestlings increased erroneous begging while cuckoo nestlings did not. Highly conspicuous begging in cuckoos results in an increased predation risk, which could have resulted in stronger selection pressures on cuckoos to avoid erroneous begging, probably resulting in better developed perceptual abilities, allowing cuckoos to perform better than their host nest‐mates.     

Bacterial diversity at the cloaca relates to an immune response in magpie Pica pica and to body condition of great spotted cuckoo Clamator glandarius nestlings

Diversity of the gut bacterial community is of prime importance for optimal food digestion and, therefore, for nutritional condition of avian nestlings. Consequently, bacterial community should be considered as a predictor of the future survival and recruitment of young birds. To explore this hypothesis, we studied the cloacal microbiota, by using RISA procedure, in two avian species sharing environmental conditions during growth, the avian brood parasitic great spotted cuckoo Clamator glandarius , and their main host in Europe, the magpie Pica pica . As estimates of phenotypic condition of nestlings we studied two nutrition-dependent traits, the immune response to an innocuous antigen (phytohemagglutinin), and the residuals of body mass on tarsus and wing length of nestlings. According to the hypothesis, we found significant relationships between microbial diversity and nestling phenotypic traits related to probability of recruitment. Briefly, both magpie and cuckoo nestlings having more similar microbial diversity were also those with similar immune response and body condition index respectively. Our results show a possible association between bacterial communities and variables related to the probability of post-fledging survival and recruitment of birds, as well as possible reasons explaining magpie-cuckoo differences in the nutritionally conditioned variables better associated with their bacterial diversity.     

Brood parasitism is associated with increased bacterial contamination of host eggs: bacterial loads of host and parasitic eggs

Factors related to bacterial environment of nests are of primary interest for understanding the causes of embryo infection and the evolution of antimicrobial defensive traits in birds. Nest visitors such as parasites could act as vectors for bacteria and/or affect the hygienic conditions of nests and hence influence the nest bacterial environment. In the present study, we explored some predictions of this hypothetical scenario in the great spotted cuckoo (Clamator glandarius)–magpie (Pica pica) system of brood parasitism. Great spotted cuckoos visit the nests of their magpie hosts and frequently damage some of the host eggs when laying eggs or on subsequent visits. Therefore, it represents a good system for testing the effect of nest visitors on the bacterial environment of nests. In accordance with this hypothesis, we found that the bacterial load of magpie eggshells was greater in parasitized nests, which may suggest that brood parasitism increases the probability of bacterial infection of magpie eggs. Moreover, comparisons of bacterial loads of cuckoo and magpie eggs revealed that: (1) cuckoo eggshells harboured lower bacterial densities than those of their magpie hosts in the same nests and (2) the prevalence of bacteria inside unhatched eggs was higher for magpies than for great spotted cuckoos. These interspecific differences were predicted because brood parasitic eggs (but not host eggs) always experience the bacterial environments of parasitized nests. Therefore, the results obtained in the present study suggest that parasitic eggs are better adapted to environments with a high risk of bacterial contamination than those of their magpie hosts. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 836–848.     

Manipulation of hunger levels affects great spotted cuckoo and magpie host nestlings differently

Brood parasitic nestlings usually exhibit an exaggerated begging behaviour, which is mainly attributed to reduced inclusive fitness costs since they typically share the nest with unrelated individuals. However, energetic costs also constrain begging expression and accordingly a relation between food requirements and intensity of begging behaviour could also exist in brood parasites, just as in nesting bird species. Here, we tested this hypothesis in the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by studying the effect of an appetite enhancer, cyproheptadine hydrochloride, on nestling provisioning and development (size, body mass and cell‐mediated immune response). To study nestling provisioning, neck‐collars were meticulously placed around nestling necks allowing normal respiration but avoiding the ingestion of food delivered by adult magpies during ca 2.5 h. Loss in body mass during neck‐collar trials was used as a proxy for energetic begging costs, while the amount of food received during these trials and growth during the whole nestling period were used as variables reflecting short‐ and long‐term effects of the experimental treatment. During neck‐collar trials, we found that experimental nestlings of both species received more food than control nestlings. However, experimental magpies, but not cuckoos, lost more body mass than control nestlings. These results suggest a short‐term beneficial effect of an escalated begging behaviour in both species that would be energetically cheaper for cuckoos than for magpies. We found positive long‐term effects of the appetite enhancer only in magpies (in terms of tarsus and wing length at fledging, but not in terms of immune response and body mass); suggesting that exaggerated begging would be beneficial for hosts only. We discuss the possible effect of begging behaviour on the risk of predation and on inclusive fitness, but also the possibility that our results may be explained by some kind of limitation in the capability of food assimilation by parasitic species.     

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.     

MAGPIE HOST MANIPULATION BY GREAT SPOTTED CUCKOOS: EVIDENCE FOR AN AVIAN MAFIA?

Why should the hosts of brood parasites accept and raise parasitic offspring that differ dramatically in appearance from their own? There are two solutions to this evolutionary enigma. (1) Hosts may not yet have evolved the capability to discriminate against the parasite, or (2) parasite-host systems have reached an evolutionary equilibrium. Avian brood parasites may either gain renesting opportunities or force their hosts to raise parasitic offspring by destroying or preying upon host eggs or nestlings following host ejection of parasite offspring. These hypotheses may explain why hosts do not remove parasite offspring because only then will hosts avoid clutch destruction by the cuckoo. Here we show experimentally that if the egg of the parasitic great spotted cuckoo Clamator glandarius is removed from nests of its magpie Pica pica host, nests suffer significantly higher predation rates than control nests in which parasite eggs have not been removed. Using plasticine model eggs resembling those of magpies and observations of parasites, we also confirm that great spotted cuckoos that have laid an ejected egg are indeed responsible for destruction of magpie nests with experimentally ejected parasite eggs. Cuckoos benefit from destroying host offspring because they thereby induce some magpies to renest and subsequently accept a cuckoo egg.     

Rejection of parasitic eggs in relation to egg appearance in magpies

The coevolutionary process between avian brood parasites and their hosts predicts that low intraclutch variation in egg colour appearance favours egg discrimination of parasite eggs by hosts. Low intraclutch variation would also result in high interclutch variation, which would increase the difficulty of evolution of mimicry by the cuckoo, because many host colour patterns might coexist in the same host population. We explored this possibility using an experimental approach in the common magpie, Pica pica, and great spotted cuckoo, Clamator glandarius, system. We artificially parasitized magpie nests with great spotted cuckoo model eggs to assess host response in two populations in Spain (Guadix and Doñana) in relation to intraclutch variation in egg appearance, measured by ultraviolet-visible reflectance spectrophotometry. Individuals that rejected model cuckoo eggs had higher intraclutch variation than accepters, suggesting that an increase, rather than a decrease, in intraclutch variation in magpie egg appearance was advantageous for cuckoo egg discrimination.     

Eavesdropping cuckoos: further insights on great spotted cuckoo preference by magpie nests and egg colour

Reproductive success of brood parasites largely depends on appropriate host selection and, although the use of inadvertent social information emitted by hosts may be of selective advantage for cuckoos, this possibility has rarely been experimentally tested. Here, we manipulated nest size and clutch colouration of magpies (Pica pica), the main host of great spotted cuckoos (Clamator glandarius). These phenotypic traits may potentially reveal information about magpie territory and/or parental quality and could hence influence the cuckoo’s choice of host nests. Experimentally reduced magpie nests suffered higher predation rate, and prevalence of cuckoo parasitism was higher in magpie nests with the densest roofs, which suggests a direct advantage for great spotted cuckoos choosing this type of magpie nest. Colouration of magpie clutches was manipulated by adding one artificial egg (blue or cream colouration) at the beginning of the egg-laying period. We found that host nests holding an experimental cream egg experienced a higher prevalence of cuckoo parasitism than those holding a blue-coloured egg. Results from these two experiments suggest that great spotted cuckoos cue on magpie nest characteristics and the appearance of eggs to decide parasitism, and confirm, for the first time, the ability of cuckoos to distinguish between eggs of different colours within the nest of their hosts. Several hypothetical scenarios explaining these results are discussed.     

COMPARATIVE POPULATION STRUCTURE AND GENE FLOW OF A BROOD PARASITE,THE GREAT SPOTTED CUCKOO (CLAMATOR GLANDARIUS), AND ITS PRIMARY HOST,THE MAGPIE (PICA PICA)

The amount of gene flow is an important determinant of population structure and therefore of central importance for understanding coevolutionary processes. We used microsatellite markers to estimate population structure and gene flow rates of the great spotted cuckoo (Clamator glandarius) and its main host in Europe, the magpie (Pica pica), in a number of populations (seven and 15, respectively) across their distribution range in Europe. The genetic analysis shows that there exists a pattern of isolation by distance in both species, although the cuckoo data are only indicative due to a small sample size. Gene flow seems to be extensive between nearby populations, higher for magpies than cuckoos, and especially high for magpie populations within the area of distribution of the great spotted cuckoo. There is no correlation between genetic distances between magpie populations and genetic distances between cuckoo populations. We discuss the implications of extensive gene flow between magpie populations in sympatry with cuckoos for the population dynamics of hosts, in particular for the occurrence of egg rejection behavior in host populations and how the different rates of migration for both species can affect the dynamics of coevolutionary processes.     

Egg rejection by Iberian azure-winged magpies Cyanopica cyanus in the absence of brood parasitism

The Iberian azure-winged magpie Cyanopica cyanus shows a remarkable ability to discriminate against great spotted cuckoo Clamator glandarius eggs. Here, I studied whether egg recognition in this species could be a derived feature resulting from intra-specific brood parasitism. Azure-winged magpies showed a very high level of discrimination and rejection of great spotted cuckoo models (73.7%), and of conspecific eggs (42.8%), even when no evidence of great spotted cuckoo or conspecific brood parasitism has been found in the population. Azure-winged magpie discriminated more readily than magpies, the current favourite host of the great spotted cuckoo. The high rejection rate of conspecific eggs by the azure-winged magpie suggests that it is quite possible that egg discrimination in this species evolved in response to conspecific brood parasitism rather than to cuckoo parasitism.     

Great Spotted Cuckoos Frequently Lay Their Eggs While Their Magpie Host is Incubating

Species that suffer from brood parasitism face a considerable reduction in their fitness which selects for the evolution of host defences. To prevent parasitism, hosts can mob or attack brood parasites when they approach the host nest and block the access to the nest by sitting on the clutch. In turn, as a counter‐adaptation, brood parasites evolved secretive behaviours near their host nests. Here, we have studied great spotted cuckoo (Clamator glandarius) egg‐laying behaviour and defence by their magpie (Pica pica) hosts inside the nest using continuous video recordings. We have found several surprising results that contradict some general assumptions. The most important is that most (71%) of the parasitic events by cuckoo females are completed while the magpie females are incubating. By staying in the nest, magpies force cuckoo females to lay their egg facing the high risk of being attacked by the incubating magpie (attack occurred in all but one of the events, n = 15). During these attacks, magpies pecked the cuckoo violently, but could never effectively avoid parasitism. These novel observations expand the sequence of adaptations and counter‐adaptations in the arms race between brood parasites and their hosts during the pre‐laying and laying periods.     

Telomere dynamics in parasitic great spotted cuckoos and their magpie hosts

Although little is known on the impact of environment on telomere length dynamics, it has been suggested to be affected by stress, lifestyle and/or life‐history strategies of animals. We here compared telomere dynamics in erythrocytes of hatchlings and fledglings of the brood parasite great spotted cuckoos (Clamator glandarius) and of magpies (Pica pica), their main host in Europe. In magpie chicks, telomere length decreased from hatching to fledging, whereas no significant change in telomere length of great spotted cuckoo chicks was found. Moreover, we found interspecific differences in the association between laying date and telomere shortening. Interspecific differences in telomere shortening were interpreted as a consequence of differences in lifestyle and life‐history characteristics of magpies and great spotted cuckoos. In comparison with magpies, cuckoos experience reduced sibling competition and higher access to resources and, consequently, lower stressful environmental conditions during the nestling phase. These characteristics also explain the associations between telomere attrition and environmental conditions (i.e. laying date) for magpies and the absence of association for great spotted cuckoos. These results therefore fit expectations on telomere dynamics derived from interspecific differences in lifestyle and life history of brood parasites and their bird hosts.     

Climatic effects and phenological mismatch in cuckoo–host interactions: a role for host phenotypic plasticity in laying date?

Climatic effects on breeding phenology vary across organisms and therefore might promote a phenological mismatch in ecologically interacting species, including those engaged in coevolutionary interactions such as brood parasites and their hosts. Recent studies suggest that climatic induced changes in migration phenology may have mismatched cuckoos and their hosts in Europe. However, it is currently unknown whether cuckoo–host phenological mismatch results from different degrees of phenotypic plasticity or to different speeds of microevolutionary processes affecting hosts and parasites. Here we performed 1) cross‐sectional correlations between climate conditions and population level of phenological mismatch between the migratory brood parasite great spotted cuckoo Clamator glandarius and its main resident host in Europe, the magpie Pica pica; and 2) a longitudinal analysis to study within‐individual variation in breeding phenology for individual hosts experiencing different climate conditions over a period of nine years (2005–2013). Cross‐sectional analyses revealed independent and contrary effects of winter and spring temperature on magpie phenology: magpie hosts tend to breed earlier those years with lower February temperatures, however, high temperature in the first half of April spur individuals to lay eggs. Breeding phenology of cuckoos was tuned to that of their magpie host in time and duration. However, annual phenological mismatch between cuckoos and magpie hosts increased with NAO index and January temperature. Longitudinal analyses revealed high individual consistency in magpie host phenology, but a low influence of climate, suggesting that the climatic‐driven phenological mismatch between cuckoos and magpies at the population‐level cannot be explained by a host plastic response to climatic conditions.     

A comparative study of host selection in the European cuckoo Cuculus canorus

Certain kinds of hosts are commonly regarded as being more suitable than other for rearing European cuckoos (Cuculus canorus) – insectivores that lay small eggs and have open, shallow nests – although empirical tests of cuckoo host selection are lacking. We analysed host use by the European cuckoo in 72 British passerines that are potential hosts and for which there was information available on life-history variables and variables related to cuckoo-host coevolution, such as rate of parasitism, rejection rate of non-mimetic model eggs and degree of cuckoo-egg mimicry of host eggs. The relative population size of the host species affected parasitism rate most strongly, followed by relatively short duration of the nestling period, and the kind of nest, with cuckoos selecting open-nesting hosts. However, the effect of the nestling period could be related to host body size and the kind of nest used, because hole-nesting species normally have longer nestling periods than open-nesters. We re-analysed the data excluding hole nesters and corvid species (species with larger body mass), but the results remained identical. The European cuckoo may benefit from selecting hosts with short nestling periods because such hosts provide food for their nestlings at a very high rate. When only those species known as cuckoo hosts were analysed, the variable that best accounted for the parasitism rate was duration of the breeding season. Therefore, availability of potential hosts in both time and space is important for cuckoos in selecting hosts. Received: 16 July 1998 / Accepted: 27 October 1998     

Imperfect mimicry of host begging calls by a brood parasitic cuckoo: a cue for nestling rejection by hosts?

Coevolutionary interactions between avian brood parasites and their hosts often lead to the evolution of discrimination and rejection of parasite eggs or chicks by hosts based on visual cues, and the evolution of visual mimicry of host eggs or chicks by brood parasites. Hosts may also base rejection of brood parasite nestlings on vocal cues, which would in turn select for mimicry of host begging calls in brood parasite chicks. In cuckoos that exploit multiple hosts with different begging calls, call structure may be plastic, allowing nestlings to modify their calls to match those of their various hosts, or fixed, in which case we would predict either imperfect mimicry or divergence of the species into host-specific lineages. In our study of the little bronze-cuckoo (LBC) Chalcites minutillus and its primary host, the large-billed gerygone Gerygone magnirostris, we tested whether: (1) hosts use nestling vocalizations as a cue to discriminate cuckoo chicks; (2) cuckoo nestlings mimic the host begging calls throughout the nestling period; and (3) the cuckoo begging calls are plastic, thereby facilitating mimicry of the calls of different hosts. We found that the begging calls of LBCs are most similar to their gerygone hosts shortly after hatching (when rejection by hosts typically occurs) but become less similar as cuckoo chicks get older. Begging call structure may be used as a cue for rejection by hosts, and these results are consistent with gerygone defenses selecting for age-specific vocal mimicry in cuckoo chicks. We found no evidence that LBC begging calls were plastic.