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Numerous studies have identified individually distinctive vocal characteristics and call consistency in different bird species. If these vocal characteristics are to be utilised as non-invasive markers for monitoring purposes, then they must remain stable over time. Three recent studies have shown that it is possible to identify individual male Common Cuckoos (Cuculus canorus) based on vocal characteristics, but whether these characteristics are stable over the duration of a breeding season remains unknown. We recorded 1032 syllables from 30 male Common Cuckoos in a Northeast Asian population. We banded six of these males and made repeated recordings of their cu-coo advertisement call across a 19-day period of the breeding season in China. We used three methods to identify individuals: discriminant function analyses (DFA), correlation analysis (CA) and spectrographic cross-correlation (SPCC). We also used repeatability analysis to test whether call consistency (the number of syllables in each calling bout) was repeatable within individuals. Based on the same-day recordings, calls from the same male were more similar in their characteristics than compared to those of different males, and yielded correct rates of classifying individuals of 93.6% (SPCC), 90.8% (DFA), and 71.5% (CA). However, these rates declined to 40.5% (SPCC), 40.7% (DFA) and 27% (CA) when using recordings over the 19-day period. Call consistency was repeatable within individuals across two successive calling bouts, but this individual repeatability disappeared when several (more than two) calling bouts from the same day or bouts from the different days of the study were included in the analyses. Declines in the correct rate of identifying individual male cuckoos and call consistency in this study raise concerns that individual male cuckoo calls may be more variable than previously thought.

Despite the fact that parent-absent begging (PAB) has been studied as a communication tool during competition between nestlings for food or as a signal of need, its role in nest predation has been poorly investigated. In this study we examined whether the natural level of Common Cuckoo (hereafter “Cuckoo”) PAB increases the risk of artificial nest predation (n = 48) by using three types of experimental playback of parent-present begging (PPB) which differ in the presence or absence of additional PPB, PAB or periods of silence. Contrary to our predictions, we did not find a significant difference in daily survival rates between PPB + PAB vs. PPB + PPB nests or between PPB + PAB vs. PPB + silence nests, indicating that the natural level of Cuckoo PAB did not influence nest vulnerability to predation. Moreover, we did not find a significant difference between nests with any of the types of playback and completely silent control nests, suggesting that not only PAB, but also begging behaviour itself, does not present an important predation cost for nestlings, at least not in cases when nestlings beg at the natural level used in our experiment. As we filmed all experimental nests during their exposure to predators, we were able to show that eight of 20 filmed nest-predation events were caused by predators that never or rarely predate upon passerine nests. Excluding predation events by these unlikely predators, however, did not change our original results. Nonetheless, we advocate direct identification of nest predators to strengthen the interpretation of future studies on the predation cost of begging.

The Common Cuckoo (hereafter Cuckoo) shows two adult plumage morphs—adult male plumage is grey and adult females are either grey or, less frequently, rufous. The situation is less clear in juveniles, as both sexes exhibit variable proportions of grey and rufous colour. We thus describe the patterns related to sex-specific plumage colour variation in a central European Cuckoo population. We genetically determined the sex of 91 Cuckoo chicks and using visual classification of photographs we scored juvenile plumage colouration of individual chicks into five classes based upon the increasing proportion of rufous colour on feathers. To verify these scores, we sampled chick feathers and quantified the proportion of rufous colour of individual feathers by digital image analysis. We found that juvenile females had a higher proportion of rufous colour of feathers than juvenile males. However, the difference was marginally non-significant based on visual inspection alone, and some male chicks even showed intensively rufous plumage like those of juvenile females. In contrast, we captured only grey adult males (n = 37), while five out of 20 adult females were rufous. The rufous colour of Cuckoo feathers considerably differed from the grey colour and the difference seemed to be larger in adults than in juveniles. We show that chicks, unlike adult females, cannot be visually assigned to either of the adult morphs. Therefore, we encourage further investigation of Cuckoo plumage colouration across the species’ range to examine the process of plumage maturation. A detailed genetic analysis is necessary to understand the origin of Cuckoo feather colouration.

Co-operative breeders provide parental care to non-filial offspring—a behaviour known as ‘alloparental care’. While inclusive fitness benefits are a widely accepted driver of alloparental care in kin-based social groups, such indirect benefits are lost in non-kin societies. Among such societies, theory predicts that the degree of parental and alloparental effort should therefore be proportional to an individual’s genetic contribution to mixed broods—depending upon reproductive options. Using genotyping data across five to 12 microsatellite loci for individuals from 20 social groups (67 adults and 153 nestlings), we assessed whether kinship or proportional reproductive success explained trends in parental and alloparental effort in the Smooth-billed Ani (Crotophaga ani), a joint-nesting cuckoo species. Nocturnal incubation in this species appears to be performed almost exclusively by a single male. We first report significantly higher degrees of relatedness between adults within social groups ( $$\bar{r}$$ = 0.208, n = 114 dyads), than between social groups ( $$\bar{r}$$ = 0.120, n = 893 dyads), suggesting that inclusive fitness benefits may in part explain uneven allocation of parental effort. Second, we show that nocturnal incubation status is a significant predictor of reproductive success in males, as nocturnal incubators sire a greater proportion of nestlings in mixed-parentage broods. While patterns of reproductive skew appear high at 80% paternal confidence ( $$\bar{B}$$ = 0.052, p = 0.061), we report no significant deviation from an egalitarian breeding framework. Our results revealed similar patterns of reproductive allocation to closely related Groove-billed Anis (Crotophaga sulcirostris); however, differences in male reproductive skew and within-group relatedness across crotophagids are highlighted and offer insight into social evolution among anis.

In some systems, brood parasites may be attracted by vocal or visual signals connected with host breeding. We studied a Great Reed Warbler (Acrocephalus arundinaceus) population where annually 30–50% of nests are parasitized by the Common Cuckoo (Cuculus canorus). We observed host males and quantified their song-related behaviors, including time spent singing, distance of movements and time spent singing in particular positions on a reed stem. We predicted that nests of more exposed males (i.e., those spending more time singing, moving a larger total distance, and spending more time on the top of stems) would be more likely to be parasitized than the nests of less exposed males. Additionally, we measured male song characteristics that we assumed to be most audible, and thus potentially the most conspicuous to the Common Cuckoo. We counted the number of “kara” syllables per song and measured their peak frequencies. Since these song structures are of low frequency and thus might be audible at longer distances, we predicted that males producing more kara syllables or uttering kara syllables of lower peak frequencies would also be more parasitized. However, we found that neither male singing behavior nor conspicuous song characteristics were significant predictors of parasitism. Only the visibility of host nests to the parasite, which we treated as a covariate, proved to be significant. Visible nests were more often parasitized than hidden nests. Our findings indicate that the Cuckoo females use nest visibility, or host behavior other than male singing, as the cue to locate host nests.

Distinctive individual vocalizations are advantageous in several social contexts. Both genetic and environmental effects are responsible for this phenomenon resulting in different frequencies and time domains of sounds in birds. This individuality can be utilized in breeding bird censuses and abundance estimates. In this study we explored the individuality of the advertisement calls of male Common Cuckoos (Cuculus canorus) with the aims of describing the ways in which individuals differ from each other acoustically, and characterizing the practical requirements for using statistical learning methods for individual recognition. We collected calls from a Hungarian Cuckoo population and conducted discriminant function analysis on acoustic parameters to distinguish individuals. We show that individuals differ in both the frequency and time of their calls, most importantly in maximum frequency of the first syllable. Our discrimination of the calls of 26 male individuals was almost 100 % accurate, and even when the number of samples was reduced to five calls per individual, and the number of acoustic parameters was decreased to five variables, accuracy still exceeded 90 %. Our acoustic individual discrimination technique is applicable to a wide range of ecological and behavioural studies because it is easy to perform and can be readily automated.

Adaptive radiation is a suitable system for the study of the (co)evolution of traits because phylogeographic constraints should be of minor importance. Adaptive radiation of beak morphology in birds is a classic example, but the evolution of other traits remains unclear. Here, using a comparative phylogenetic analysis, we studied plumage coloration in relation to terrestrial:aerial locomotion in Malagasy Couas. We hypothesized that rufous coloration would be beneficial for crypsis on the ground, and found that rufous coloration increased with the relative investment in terrestrial:aerial locomotion, measured as tarsus:wing length ratio. Terrestrial:aerial locomotion may explain the diversity of plumage coloration, at least in part.

One of the most obvious adaptations to the brood parasitic mode of reproduction is the formation of eggs with unusually strong shells, which apparently reduce chances of egg breakage during laying and puncture ejection attempts of parasitic eggs by the hosts. We tested a hypothesis that strong eggshells of the Common Cuckoo, Cuculus canorus, may have also led to a stronger hatching muscle, musculus complexus. First, the Cuckoo hatching muscle had a higher density of fibers than that of the similarly sized Great Reed Warbler Acrocephalus arundinaceus chicks; and second, the cross-sectional area of fibers of the hatching muscle was smaller in the Cuckoo than in the Great Reed Warbler. We propose that the increased density of muscle fibers in the Cuckoo facilitates hatching out of structurally strong eggshells because chicks possessing this trait should be able to exert greater pressure on the shell during the hatching process. Our results are consistent with the hypothesis that the higher density of fibers in the musculus complexus represents another adaptation facilitating hatching from unusually strong parasitic eggs that has presumably evolved during coevolution involving the Cuckoo and its hosts.

The three species of New Zealand’s endemic Mohoua genus are sole hosts of the obligate brood parasitic Long-tailed Cuckoo (Eudynamys taitensis), making their intrageneric phylogenetic relationships particularly important for coevolutionary studies. Also, recent molecular phylogenetic analyses have not identified the family-level placement of this genus. To resolve both intrageneric and family relationships, we generated new nuclear and mitochondrial sequence data and conducted phylogenetic analyses using Bayesian inference among representatives of endemic New Zealand passerines and Australasian ‘core Corvoidea’ lineages. The results establish strong intrageneric relationships of all three Mohoua species, confirm the monophyly of the genus, and suggest its placement in a re-erected monotypic family: Mohouidae.

The existence of begging costs has recently been the focus of evolutionary studies on parent–offspring conflict. Indirect findings seem to support the existence of a nest predation cost of begging. However, empirical direct evidence of nestling begging attracting the attention of nest predators has been criticized because of methodological problems, mainly the use of artificial nests and playbacks. In order to avoid this problem, we used natural and active nests and real chicks. We utilized Common Blackbird (Turdus merula) nests to experimentally create high and control begging nests by introducing a Great Spotted Cuckoo (Clamator glandarius) or Blackbird chick respectively. We also explored the effect of brood size comparing between Blackbird broods augmented and reduced by one chick. Experimentally parasitized nests were significantly more predated than simply augmented or reduced broods: nest predation for Cuckoo introduced (high begging) nests was more than double that predation for nests of the other two groups. However, we did not find any effect of the brood size. The presence of a Great Spotted Cuckoo chick in a nest also favors an earlier detection of the nest by predators. Parental activity at the nests seems not to be a factor explaining these results as we did not detect significant differences between Cuckoo and Blackbird introduced nests in visit rates. Therefore, our findings support the hypothesis that nestling begging has a severe cost in terms of predation and, hence, this cost may impose an important restriction to the evolution of begging behavior.

Nest predation and brood parasitism are costly for nest owners, and natural selection should therefore favour the evolution of parental counterdefences. We addressed the question of whether Blackcaps Sylvia atricapilla change their incubation behaviour in response to various nest intruders and whether this adjustment matches the intensity of mobbing exhibited towards these intruders. Near focal nests, we successively exposed a dummy of a brood parasite, nest predator and an innocuous species. After the parents had responded, we removed the dummy and filmed their incubation. The most aggressive response towards the Common Cuckoo Cuculus canorus and high nest attendance after its disappearance indicated recognition of the brood parasite. Low-intensity response to the Jay Garrulus glandarius, together with reduced subsequent parental care, suggested that Blackcaps perceived it either as less deleterious at the egg stage than the Cuckoo or as a danger to themselves. Almost no aggression towards the Turtle Dove Streptopelia turtur, along with the resumption of incubation after its removal, implied that Blackcaps recognised it as harmless. In addition, we found that the level of aggression positively correlated with nest attendance, suggesting a link between the intensity of mobbing and subsequent parental care. Altogether, our results demonstrate that the issue of enemy recognition may be viewed as a complex of both aggressive and post-presentation behaviours.

The brood parasitic habits of the European Cuckoo Cuculus canorus have excited wonder, disbelief and speculation since the fourth century BC. Accurate knowledge of cuckoo biology, however, accumulated only slowly and mostly since 1700. The aim of this study is to review six main topics: (1) the placement of cuckoo eggs in host nests; (2) cuckoo ‘clutch’ size; (3) cuckoo egg characteristics, mimicry and rejection; (4) choice of hosts; (5) eviction of eggs and chicks; and (6) the reasons why cuckoos are brood parasites and are incapable of rearing their own young. Early errors in reporting cuckoo biology were often a consequence of poor or incomplete observations leading to erroneous interpretations. Many of the early observers were egg collectors who focussed almost exclusively on the egg-laying period, thus ignoring cuckoo chick biology. Major landmarks in cuckoo studies included the facts that: (1) cuckoo eggs often resembled those of their hosts (1760s) and that this mimicry was adaptive (1850s); (2) hosts sometimes evicted cuckoo eggs (1770s); (3) female cuckoos laid individually distinctive eggs and that specific cuckoo gentes may exist (1850s); and (4) although well recognised that cuckoo chicks were reared alone, prior to Jenner’s work in the 1780s female cuckoo parents were thought to either eat or evict the host eggs or young. Jenner’s results was more readily accepted in Britain than in Germany. Between 1700 and 1859, cuckoo brood parasitism was difficult to reconcile with the prevalent conceptual framework of physico-theology (later known as the argument from design). Thereafter, Darwin’s idea of natural selection provided a superior conceptual framework, which in conjunction with experimental testing of specific hypotheses has continued to advance our understanding of brood parasitism. Our knowledge of cuckoo biology is far from complete, however, and we predict that continuing research often incorporating new technologies will refine and extend our understanding of the cuckoo’s extraordinary biology.

Remote sensing data have been used in previous studies to assess the effects of winter ecological conditions in Africa on biological parameters recorded in bird populations during the following breeding season in Europe. Based on the results of these studies, we hypothesized that a high productivity of vegetation during the winter and, thus, high resource availability, should advance the arrival of long-distance migrants to the European breeding areas due to enhanced ecological conditions. To test this hypothesis, between 1982 and 2000 we examined the first arrival date to the Iberian Peninsula of five species (White Stork, Cuckoo, Common Swift, Barn Swallow and Nightingale) in relation to several explanatory variables: ecological conditions in their African wintering grounds and passage areas, as reflected by the normalized difference vegetation index (NDVI), temperature and precipitation in their passage areas and the winter North Atlantic Oscillation (NAO). Ecological conditions in the wintering areas were important for White Stork, Cuckoo and Barn Swallow phenology, while both NDVI in passage areas and NAO did not have an effect on any species. Migratory birds arrived earlier after winters with high vegetation productivity in Africa. Temperature in passage areas was important for the later species (i.e. Cuckoo, Common Swift and Nightingale), although in all cases the true relevance of this factor was scarce due to the poor explanatory capacity of the models. These species were recorded in the Iberian Peninsula earlier in the spring of those years with warmer temperatures in passage areas. The nexus between African NDVI and arrival phenology is hypothesized through increases in wintering survival rates and/or the faster acquisition of pre-migratory body condition and progression through sub-Saharan areas.

The breeding ecology of eastern olivaceous warblers Hippolais pallida is poorly known. In this study, we provide data on nest site selection and breeding parameters of the species in a population in northwestern Bulgaria, the only one known to be regularly and heavily parasitized by the common cuckoo Cuculus canorus (hereafter cuckoo). Eastern olivaceous warblers only bred within human settlements of the study area, avoiding seemingly suitable habitat outside them. Nests were built in a wide range of plant taxa but ailanthuses Ailanthus altissima and mulberries Morus spp. were most frequently used (21%). After taking into account the availability of vegetation, there was an apparent preference for several plant taxa but not for mulberries. Mean nest height was 1.65 ± 0.98 (0.53–7.60) m, n = 217, and it varied significantly among different types of nesting substrate. Laying date and clutch size of first breeding attempts averaged 10 June ± 0.98 days, n = 101 and 3.9 ± 0.07 (2–5) eggs, n = 72, respectively. Hatching success, fledging success, and breeding success were 42.5, 86.4, and 36.7%, respectively. The main sources of nest mortality were predation and cuckoo parasitism, with no significant difference in the proportion of nests lost to each. Cuckoo parasitism seemed responsible for the relatively low hatching success in this population.

A cuckoo Cuculus canorus dummy was exposed at 24 nests of great reed warbler Acrocephalus arundinaceus (GRW) and 34 nests of reed warbler Acrocephalus scirpaceus (RW) during the egg-laying stage. The eight GRW pairs attacked the cuckoo directly, striking the dummy, but such a behaviour was not recorded in RWs. Also, other behavioural measures (closest distance from the model, duration of distress calls and number of excitement calls) indicated a lower level of defence by RWs compared to GRWs. In the study area, the parasitism rate was much lower in GRWs (1.7% of nests) than in RWs (11.3%). We suggest that one of the reasons for the lower level of cuckoo parasitism on GRWs is its stronger nest defence and hence higher risk of injury or even death for the cuckoo during egg dumping.

We tested great reed warbler (Acrocephalus arundinaceus) discrimination against two common cuckoo (Cuculus canorus) female color morphs (gray and rufous) in two areas with different parasitism rates and proportions of the two morphs. Hosts recognized the two cuckoo morphs from a control, the feral pigeon (Columba livia), at Apaj, Hungary (where brood parasitism was heavy), whereas no significant differences among the models were recorded at Lužice, Czech Republic (where the parasitism rate was moderate). At Apaj, the hosts discriminated the rufous morph (which is slightly predominant there) better than the gray morph from the control. Between-site comparison (after controlling for background aggression) revealed that great reed warblers were more aggressive towards the rufous morph at Apaj than at Lužice, whereas their responses to the gray morph did not differ, corresponding with much higher between-site difference in the relative abundance of the rufous morph. Our results suggest that both local parasitism pressure and relative abundance of two female color morphs of a brood parasite may significantly influence host nest defenses.

Many internal and environmental factors influence the growth of birds. Here we ask whether the reversed sexual dimorphism in the classically polyandrous black coucal is reflected in differential growth and fledging parameters of females and males. We also investigate whether androgen concentrations were higher in females than males during the nestling development, thus, providing a potential mechanism for the ‘behavioural masculinization’ of female black coucals. Furthermore, we give a detailed description of the breeding biology of this little studied and unusual bird species. Black coucal eggs were smaller, and the incubation and nestling periods were shorter than expected for a bird of that size. In absolute terms female black coucals grew faster than males and left the nest with a larger body mass, suggesting that females are the more expensive sex to rear in this species. However, in relation to adult body size, females grew slower and reached a smaller proportion of their adult body size than males upon fledging. ‘Testosterone’ concentrations were significantly higher in male than in female nestlings, hence not supporting a role of circulating androgens in the ontogeny of sex-role reversal. To our knowledge, the early sex differences in ‘testosterone’ levels of black coucals represent the first evidence that such differences exist in nestlings of altricial birds. We present anecdotal evidence that the differences may be related to an early maturation of male reproductive organs in black coucals. In females but not in males, ‘testosterone’ concentrations were related to structural growth rates.