Egg characteristics are unreliable in determining maternity in communal clutches of guira cuckoos Guira guira

Egg characteristics have been used to determine egg maternity for birds in situations where two or more females lay eggs in a single nest (as in communal breeders and intraspecific parasites). We assessed the applicability of egg morphometry and eggshell appearance in ascribing egg ownership in communal clutches of guira cuckoos Guira guira , a species where up to seven females may lay eggs in a joint nest. We used both combined variables (including egg mass, length, width, shape and two eggshell variables), and a shape index to test whether eggs laid by each female were similar but different from eggs laid by other females. Also, we conducted discriminant function analyses to verify if eggs could be correctly classified to their mothers based on their characteristics. The correct maternity was determined by yolk protein electrophoresis of freshly-laid eggs. Individual female chutches were separated through egg characteristics or shape alone in 29% and 41% of the groups tested, respectively. Differences were mostly due to a single female that differed from her nest-mates in a unique egg variable. On average, 55% of the eggs analyzed were not assigned to the correct mother using egg dimensions and eggshell speckling pattern. In conclusion, the egg characteristics used do not reliably indicate maternity in guira cuckoo communal clutches. We therefore recommend a protein-based verification of egg appearance characteristics for assigning maternity in other species in which multiple female laying occurs.     

Maternal androgens in eggs of communally breeding guira cuckoos (Guira guira)

Variation of maternal androgens in avian eggs may be a mechanism of maternal influence on offspring development, growth, and/or behavior. We studied yolk androgen concentrations in eggs of guira cuckoos (Guira guira) to understand how females might enhance the success of offspring in a complex communal breeding system. We measured concentrations of androstenedione, 5alpha-dihydrotestosterone, and testosterone in yolks and identified eggs and clutches of individuals in joint nests by yolk protein electrophoresis. Androstenedione had the highest yolk concentration, at least 10 times higher than that of testosterone and 5alpha-dihydrotestosterone. The first eggs of individual females that laid two or three eggs in a joint nest had lower androstenedione concentrations than their second and third eggs, the latter having a lower probability of being ejected from the nest. This implies that guira cuckoo females may influence offspring survival and competitiveness in communal nests by means of differential allocation of androstenedione and laying tactics. There was significant variation in yolk androstenedione among females, but the order in which females entered laying in the communal clutch had no effect on the concentrations. Androstenedione yolk concentrations increased with communal clutch size, which may indicate that higher levels of competition in larger groups lead to higher yolk androgen concentrations. Finally, androstenedione concentrations were higher in clutches in the later wetter periods of the rainy season than during the earlier drier period. This may be explained by the high frequency of large clutches in the later periods, with more females contributing to a joint clutch.     

Individual female clutch identification through yolk protein electrophoresis in the communally breeding guira cuckoo (Guira guira)

Avian communal breeding systems generate alternative behavioural strategies for females, resulting in differences in reproductive success. Identifying eggs of different females in such systems is problematic, however, due to egg destruction before incubation, difficulty of capturing adults, and/or inaccuracy of egg identification based on egg morphometry. Here, we describe a technique that uses electrophoresis of yolk proteins to determine egg ownership, which we applied to communally breeding guira cuckoos (Guira guira). Validation of the method included identical yolk protein banding patterns in all eggs of the same female, but different patterns in eggs of different females in budgerigars (Melopsittacus undulatus), and identical patterns in yolk follicles of the same females in guira cuckoos. We applied the protocol to 195 guira cuckoo eggs from 34 joint nests in 2 years. All multiple guira cuckoo eggs laid on the same day in single nests had distinct banding patterns of yolk proteins, practically eliminating the possibility of more than one female being represented by the same pattern. Some identical banding patterns were repeated in different days within a nesting bout, indicating that some females laid several eggs in shared nests. Identical patterns occasionally occurred in renestings of groups, indicating that some females lay eggs in consecutive nestings. Yolk protein electrophoresis is a useful tool to identify egg maternity in other circumstances, such as polygynous mating systems with joint nests and intraspecific parasitism. Additionally, it is an alternative method for species where electrophoresis of egg white proteins does not show sufficient polymorphism.     

Western Yellow-Billed Cuckoo Nest-Site Selection and Success in Restored and Natural Riparian Forests

The western distinct population segment of yellow-billed cuckoo (Coccyzus americanus; western cuckoo) has been extirpated from most of its former breeding range in the United States because of widespread loss and degradation of riparian cottonwood (Populus spp.)-willow (Salix spp.) forests. Restoration and management of breeding habitat is important to the recovery of this federally threatened species, and identification of high-quality breeding habitat can help improve the success of recovery. In 2005, the Lower Colorado River Multi-Species Conservation Program, a long-term, multi-agency effort, was initiated to maintain and create wildlife habitat within the historical floodplain of the lower Colorado River (LCR) for federally endangered and threatened species, including western cuckoos. We conducted an empirical, multi-scale field investigation from 2008–2012 to identify habitat characteristics selected by nesting western cuckoos along the LCR. Multiple logistic regression models revealed that western cuckoos selected nest sites characterized by increased densities of small, native, early successional trees measuring 8–23 cm diameter at breast height, and lower diurnal temperature compared to available habitat in restoration and natural forests. Nesting cuckoos selected sites with increased percent canopy closure, which was also important for nest success in restoration sites along the LCR. Our results show habitat components selected by nesting western cuckoos in restoration and natural riparian forests and can help guide the creation, enhancement, and management of riparian forests with habitat conditions necessary to promote nesting of western cuckoos. © 2021 The Wildlife Society.     

Functional morphology of the cranio‐mandibular complex of the Guira cuckoo (Aves)

The cranio‐mandibular complex is an important structure involved in food capture and processing. Its morphology is related to the nature of the food item. Jaw muscles enable the motion of this complex and their study is essential for functional and evolutionary analysis. The present study compares available behavioral and dietary data obtained from the literature with novel results from functional morphological analyses of the cranio‐mandibular complex of the Guira cuckoo (Guira guira) to understand its relationship with the zoophagous trophic habit of this species. The bite force was estimated based on muscle dissections, measurements of the physiological cross‐sectional area, and biomechanical modeling of the skull. The results were compared with the available functional morphological data for other birds. The standardized bite force of G. guira is higher than predicted for exclusively zoophagous birds, but lower than for granivorous and/or omnivorous birds. Guira guira possesses the generalized jaw muscular system of neognathous birds, but some features can be related to its trophic habit. The external adductor muscles act mainly during food item processing and multiple aspects of this muscle group are interpreted to increase bite force, that is, their high values of muscle mass, their mechanical advantage (MA), and their perpendicular orientation when the beak is closed. The m. depressor mandibulae and the m. pterygoideus dorsalis et ventralis are interpreted to prioritize speed of action (low MA values), being most important during prey capture. The supposed ecological significance of these traits is the potential to widen the range of prey size that can be processed and the possibility of rapidly capturing agile prey through changes in the leverage of the muscles involved in opening and closing of the bill. This contributes to the trophic versatility of the species and its ability to thrive in different habitats, including urban areas.     

Reproductive skew and individual strategies: infanticide or cooperation?

Infanticide in species with shared reproduction may indicate attempts to control the degree of skew within groups. Alternatively, individuals that have done poorly in reproduction could use infanticide for hastening the next breeding attempt. Many factors influence the individual’s decision over whether it is best to cooperate or engage in destruction. We present a qualitative model incorporating kinship among adults and progeny, social status, group size and seasonality to predict behavioural options for individuals of diverse backgrounds. The social system used for the model was that of the guira cuckoo (Guira guira), a Neotropical bird that breeds communally. We suggest communal breeders utilise the best available predictors for enhancing seasonal reproductive success, with favourable consequences for lifetime inclusive fitness. Predictors rely upon parameters that change over time, including the individual’s assessment of present and future possibilities. The model produces predictions that may be tested through field observations and molecular analyses for species with similar social dynamics.     

Phylogeny of the cuckoo genus Coccyzus (Aves: Cuculidae): A test of monophyly

Coccyzus comprises nine species of New World cuckoos (Aves: Cuculidae) that breed from southern Canada to central South America. The phylogeny of this genus was reconstructed using 2490 base pairs of the mitochondrial genes cytochrome oxidase II and III, and cytochrome b. Maximum likelihood, maximum parsimony, and Bayesian inference approaches produced similar topologies in which Coccyzus, as currently classified, is polyphyletic. Topological‐constraint analyses demonstrated that trees resulting from this study were significantly better than those derived from conventional classifications. Furthermore, results support paraphyly of Piaya, another genus of New World cuckoos. These conclusions reflect some early classifications of these genera and support the resurrection of Micrococcyx to house the ash‐coloured (Coccyzus cinereus) and dwarf (C. pumilus) cuckoos, and Coccycua for the little cuckoo (Piaya minuta).     

Molecular phylogeny of cuckoos supports a polyphyletic origin of brood parasitism

We constructed a molecular phylogeny of 15 species of cuckoos using mitochondrial DNA sequences spanning 553 nucleotide bases of the cytochrome b gene and 298 nucleotide bases of the ND2 gene. A parallel analysis for the cytochrome b gene including published sequences in the Genbank database was performed. Phylogenetic analyses of the sequences were done using parsimony, a sequence distance method (Fitch-Margoliash), and a character-state method which uses probabilities (maximum likelihood). Phenograms support the monophyly of three major clades: Cuculinae, Phaenicophaeinae and Neomorphinae-Crotophaginae. Clamator, a strictly parasitic genus traditionally included within the Cuculinae, groups together with Coccyzus (a nonobligate parasite) and some nesting cuckoos. Tapera and Dromococcyx, the parasitic cuckoos from the New World, appear as sister genera, close to New World cuckoos: Neomorphinae and Crotophaginae. Based on the results, and being conscious that a more strict resolution of the relationships among the three major clades is required, we postulate that brood parasitism has a polyphyletic origin in the Cuculiformes, with parasite species being found within the three defined clades. Evidence suggests that species within each clade share a common parasitic ancestor, but some show partial or total loss of brood parasitic behaviour.     

The comparative breeding behaviour of two sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, in Western Australia: a new model for the evolution of egg morphology and host specificity in avian brood parasites

The breeding behaviour of two similarly sized sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, was studied over four breeding seasons at Gooseberry Hill, Western Australia. Both cuckoos usually began laying in late August; Shining Bronze-Cuckoos laid for up to 13 weeks and Horsfield's Bronze-Cuckoos for up to 15 weeks. Four host species were parasitized and major hosts were parasitized throughout most of their laying periods. The frequency of parasitism varied between hosts and between years, but Splendid Fairy-wrens Malurus splendens and Yellow-rumped Thornbills Acanthiza chrysorrhoa (major hosts) were always parasitized more heavily than Western Thornbills A. inornata and Scarlet Robins Petroica multicolor. Western Thornbills were parasitized by both cuckoos. Horsfield's and Shining Bronze-Cuckoos laid monomorphic eggs; those of Horsfield's Bronze-Cuckoos were highly mimetic whereas those of Shining Bronze-Cuckoos were non-mimetic and dark in colour. Both cuckoos laid one egg per host nest, deposited eggs directly into the nest, laid very quickly in the early morning, removed at least one host egg at laying, laid eggs small for the size of the birds, hatched after 12 days and evicted nest companions shortly after hatching. Laying was well synchronized with the start of incubation by hosts. Field observations and experiments with egg models indicated that neither of the major hosts, nor the secondary host in common, discriminate against foreign eggs. The nestling period for Horsfield's Bronze-Cuckoo was 17 days, and for the Shining Bronze-Cuckoo 20 days. There was a corresponding difference in nestling growth rate between the cuckoo species. About 50% of cuckoo eggs produced fledglings. Reproductive success for both cuckoos was highest in nests of the secondary host in common, the Western Thornbill. Young cuckoos reached independence 5–6 weeks after hatching. The adaptive significance of competition between cuckoos as a selective agent for cuckoo egg morphology and host specificity is discussed.     

Egg shape mimicry in parasitic cuckoos

Parasitic cuckoos lay their eggs in nests of host species. Rejection of cuckoo eggs by hosts has led to the evolution of egg mimicry by cuckoos, whereby their eggs mimic the colour and pattern of their host eggs to avoid egg recognition and rejection. There is also evidence of mimicry in egg size in some cuckoo–host systems, but currently it is unknown whether cuckoos can also mimic the egg shape of their hosts. In this study, we test whether there is evidence of mimicry in egg form (shape and size) in three species of Australian cuckoos: the fan‐tailed cuckoo Cacomantis flabelliformis, which exploits dome nesting hosts, the brush cuckoo Cacomantis variolosus, which exploits both dome and cup nesting hosts, and the pallid cuckoo Cuculus pallidus, which exploits cup nesting hosts. We found evidence of size mimicry and, for the first time, evidence of egg shape mimicry in two Australian cuckoo species (pallid cuckoo and brush cuckoo). Moreover, cuckoo–host egg similarity was higher for hosts with open nests than for hosts with closed nests. This finding fits well with theory, as it has been suggested that hosts with closed nests have more difficulty recognizing parasitic eggs than open nests, have lower rejection rates and thus exert lower selection for mimicry in cuckoos. This is the first evidence of mimicry in egg shape in a cuckoo–host system, suggesting that mimicry at different levels (size, shape, colour pattern) is evolving in concert. We also confirm the existence of egg size mimicry in cuckoo–host systems.     

Do common cuckoos (Cuculus canorus) possess an optimal laying behaviour to match their own egg phenotype to that of their Oriental reed warbler (Acrocephalus orientalis) hosts?

Optimality theory suggests that parasitic cuckoos should evolve an optimal laying behaviour aiming to positively select host nests in which the eggs match the phenotype of their own eggs, thus minimizing the rejection risk from hosts and, in turn, maximizing the cuckoos' fitness. We tested this hypothesis by investigating cuckoo‐egg matching between parasitized and nonparasitized nests in a common cuckoo (Cuculus canorus) host, the Oriental reed warbler (Acrocephalus orientalis), by use of Vorobyev–Osorio and Nature‐Pattern‐Match models to quantify the matching of egg colour and pattern from avian vision, respectively. The results of our study indicated that cuckoo‐egg matching in parasitized nests was no better than that in nonparasitized nests, and thus we found no support for the optimal laying hypothesis in cuckoos. The mixed conclusions from all previous studies, including the present study, may be explained by (1) the parallel coevolution in different cuckoo–host systems; (2) the inappropriate methodology; and (3) the deficiency of the assumption itself. We suggest that a better methodology should be developed to solve the puzzle of whether cuckoos choose to lay eggs matching those of the host.     

Bluffing in the forest: Neotropical Neomorphus ground‐cuckoos and peccaries in a possible case of acoustic mimicry

Acoustic communication is particularly important in environments such as dense tropical forests, where the dim light constrains the efficacy of visual signals. In these environments, complex species interactions could promote the evolution of acoustic signals and result in intriguing patterns of mimicry and convergence. In the Neotropical region, Neomorphus ground‐cuckoos frequently associate with herds of collared peccaries and white‐lipped peccaries. Bill clacking behavior in ground‐cuckoos closely resembles the sound of teeth clacking in peccaries and these acoustic signals are used in agonistic and foraging contexts in both species. Here we demonstrate that the acoustic characteristics of bill clacking in ground‐cuckoos are more similar to teeth clacking of peccaries than to bill clacking of the more closely related Geococcyx roadrunner. We propose that two hypotheses may explain the evolution of the clacking behavior in these taxa. First, because peccaries are known to successfully ward off attacks from large predators to defend their herds, mimicking their clacking can deceive predators, either by triggering clacking from nearby peccaries, or making it appear to the predators that peccaries are present when they are not. Second, ground‐cuckoos and peccaries could mutually benefit from the use of similar signals to alert each other of the presence of predators. In this context, ground‐cuckoos could serve as sentinels while peccaries could confer protection. We also discuss alternative explanations for this striking acoustic resemblance. Ground‐cuckoos and peccaries provide an interesting opportunity to study how an ecological association could foster the evolution of acoustic mimicry.     

Hawk mimicry in cuckoos and anti‐parasitic aggressive behavior of barn swallows in Denmark and China

Hosts of brood parasites defend their nests against parasitism by aggression and subsequently, if parasitized, by rejection of the parasite egg or nestling. Cuckoos have evolved plumage mimicry with convergence towards the phenotype of Accipiter hawks that are common predators of cuckoo hosts. Here we tested two alternative hypotheses 1) whether barn swallows Hirundo rustica have evolved less aggressive behavior towards cuckoos in areas of sympatry with more abundant Accipiter hawks; and 2) whether barn swallows have evolved more aggressive anti‐parasite behavior in areas with a single species of cuckoo. We presented dummies of common cuckoo Cuculus canorus, sparrowhawk Accipiter nisus and Oriental turtledove Streptopelia orientalis (a benign control) at the nests of barn swallows during breeding, while recording intensity of response and proximity of barn swallows to the dummy. We demonstrated that cuckoos moved away when attacked aggressively and approached more closely by barn swallows showing that barn swallow behavior was efficient at driving away cuckoos. Barn swallows were significantly more aggressive and approached cuckoo and sparrowhawk dummies more closely in Denmark than in China, despite sparrowhawks being relatively more common in Denmark. Responses towards cuckoo dummies differed from responses towards sparrowhawk dummies, showing that barn swallows distinguished between the two different causes of danger. These findings are inconsistent with a less aggressive response towards cuckoo dummies in areas of sympatry with more abundant Accipiter hawks, but consistent with the alternative hypothesis that barn swallows have evolved more aggressive behavior towards cuckoos in areas with a single species of brood parasite, but not in areas with multiple species of parasites, where it is harder for hosts to tell the difference.     

REED WARBLER HOSTS FINE‐TUNE THEIR DEFENSES TO TRACK THREE DECADES OF CUCKOO DECLINE

Interactions between avian hosts and brood parasites can provide a model for how animals adapt to a changing world. Reed warbler (Acrocephalus scirpaceus) hosts employ costly defenses to combat parasitism by common cuckoos (Cuculus canorus). During the past three decades cuckoos have declined markedly across England, reducing parasitism at our study site (Wicken Fen) from 24% of reed warbler nests in 1985 to 1% in 2012. Here we show with experiments that host mobbing and egg rejection defenses have tracked this decline in local parasitism risk: the proportion of reed warbler pairs mobbing adult cuckoos (assessed by responses to cuckoo mounts and models) has declined from 90% to 38%, and the proportion rejecting nonmimetic cuckoo eggs (assessed by responses to model eggs) has declined from 61% to 11%. This is despite no change in response to other nest enemies or mimetic model eggs. Individual variation in both defenses is predicted by parasitism risk during the host's egg‐laying period. Furthermore, the response of our study population to temporal variation in parasitism risk can also explain spatial variation in egg rejection behavior in other populations across Europe. We suggest that spatial and temporal variation in parasitism risk has led to the evolution of plasticity in reed warbler defenses.     

The perceptual and chemical bases of egg discrimination in communally nesting greater anis Crotophaga major

The eggshells of communally breeding greater anis Crotophaga major consist of a blue‐green pigmented calcite matrix overlaid by a chalky white layer of vaterite, both of which are polymorphs of calcium carbonate. The white vaterite layer is intact in freshly laid eggs and may function in protecting the eggs from mechanical damage, but it also abrades during incubation to reveal the blue calcite shell underneath. Previous research has shown that this color change serves a visual signaling function: nesting greater anis can discriminate between eggs that are freshly laid and those that have already been incubated, which allows them to reject asynchronous eggs laid by extra‐group parasites. Here we use avian visual modeling and pigment extraction to assess the perceptual and chemical bases of such egg recognition. We found that there was no overlap between the avian perceptual space occupied by ani eggshells with and without vaterite, and that vaterite lacked both of the pigments found in the eggshell's calcite matrix, bililverdin and protoporphyrin. The visual contrast between the unpigmented vaterite and the blue‐pigmented calcite appears to pre‐date the evolution of the signaling function, since the related guira cuckoo Guira guira, also a communal breeder, lays similarly structured and pigmented eggs but does not use the visual contrast as a signal to detect parasitism.     

HIERARCHICAL PATTERNS OF GENETIC POPULATION STRUCTURE IN BLACK RAT SNAKES (ELAPHE OBSOLETA OBSOLETA) AS REVEALED BY MICROSATELLITE DNA ANALYSIS

We investigated the distribution of variation at six microsatellite loci in the black rat snake (Elaphe obsoleta obsoleta). Sampling occurred at three hierarchical scales ranging from communal hibernacula to regional populations, with most locales situated within the Frontenac Axis region of eastern Ontario. We detected no statistically significant pairwise differentiation (F_ST and R_ST) between hibernacula within the same subpopulations (interhibernaculum distance <6 km). However, isolation-by-distance was evident among locales within the Frontenac Axis (maximum of 50 km) and among regional populations (maximum of 1500 km). Conservative estimates of N_c derived from heterozygosity values ranged from approximately 600 to 2000. These values suggest relatively large genetic neighborhoods encompassing many communal hibernacula. Our results considered together suggest viscosity of gene flow over relatively short distances (tens of kilometers), but substantial genetic exchange among local hibernacula.     

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.     

Why Does the Frequency of Nest Parasitism by the Cuckoo Differ Considerably Between Two Populations of Warblers Living in the Same Habitat?

The rate of nest parasitism is a product of two interacting phenomena: host selection by cuckoos and defence by hosts. In our study area the rate of nest parasitism by cuckoos is significantly lower in the great reed warbler (GRW; Acrocephalus arundinaceus) than in the reed warbler (RW; A. scirpaceus), even though they breed in the same habitat and their reproductive biology is similar. We hypothesized that the difference in the proportion of parasitized nests may reflect a narrow selection of host by cuckoos (they prefer RW nests) or/and the relatively better alien egg discrimination in the GRW. In the egg discrimination experiment the GRW rejected the higher proportion of alien eggs than the RW. However, in both species the discriminative ability considerably varied in time, both within the day and within the breeding cycle. A logistic regression model suggests that the GRW would be a frequent host if only nest parasites could exploit the period of its lowest sensitivity to alien eggs. We conclude that the relatively low rate of nest parasitism in the GRW may reflect both its good discriminative ability and the low number of cuckoos that are specialized in dumping eggs to nests of this warbler. The adaptation of cuckoos to the particular host species may involve not only production of mimetic eggs, but also adjusting activity to temporal changes in sensitivity to alien eggs in the host.