Reactions of wintering passerines to male calls of the European cuckoo Cuculus canorus

The reaction of birds to the nest parasite, the European cuckoo Cuculus canorus, has been the subject of extensive testing in various aspects. However, while the cuckoo is a long-distance migrant, some of its hosts are sedentary species. In this study, we aimed to investigate whether species, primarily hosts, react to the presence of the cuckoo also in the winter season. This behaviour may involve an attempt to drive the parasite away from locations that will subsequently become their breeding sites. During playback experiments conducted in the winter of 2021/2022 in Poland, we demonstrated that numerous bird species react to the male cuckoo calls in winter. These calls may be perceived as a source of danger, particularly by cuckoo hosts, who responded to this call more frequently than non-hosts and the control species (pigeon). Nonetheless, the birds' reactions were not strong, as they did not approach the source of the call. However, our results are constrained by the limited number of cuckoo host species wintering in Poland. To better evaluate the intensity of bird responses to the male cuckoo's call during the non-breeding season, further studies should be conducted in regions where a greater variety of species, especially those most susceptible to parasitism, overwinter.

Soft calls do not modulate aggressiveness in male common cuckoos toward conspecific territorial intruders: A sequential playback study

In many bird species loud broadcast calls serve as aggressive signals with a large effective radius, whereas soft calls may indicate aggression at a closer distance, and are often directed at a nearby conspecific individual. Male common cuckoos (Cuculus canorus) are famous for their simple cu-coo calls, which are long-range broadcast calls, uttered in long sequences during the breeding season. Cuckoos also produce soft calls, including a single gowk (wah), a series of gowks (gowk series), or a harsher single variant, the guo call. To our knowledge, no previous study has attempted to understand the function of cuckoos' soft calls. We conducted a sequential playback experiment by (i) first attracting male cuckoos with "cu-coo" calls, and then (ii) playing heterospecific control calls or one of several types of cuckoo soft calls. While the "cu-coo" call attracted focal males, neither the soft conspecific calls nor the heterospecific control calls elicited an additional response. Finally, the gowk series calls did not alter cuckoos' approach to the speaker when played together with "cu-coo" calls. As cuckoos' soft calls did not increase or decrease aggression, they instead may advertise the cuckoo's presence and position for a nearby conspecific.

Quantitative analysis of vocalisation types in male common cuckoos' "gowk" call complex

Acoustic communication of animals often contains two types of vocalizations: loud sounds for long-range and soft sounds for short-range signalling. Brood parasitic common cuckoos (Cuculus canorus) are suitable study objects for research on acoustic signalling as they have a simple acoustic repertoire, of which the loud territorial advertisement calls of males, the "cu-coo", is the most famous type. Although a distinct group of soft calls has also been reported in early naturalists' works, no systematic studies compared them acoustically. Even the classification of these soft calls is lacking. Here we surveyed these neglected types of calls, and evaluated them through bioacustic analyses. Specifically, we compared the following soft calls: single gowk, guo, and the series of gowk calls. The advertisement call ("cu-coo") was used as the referent for these comparisons. As the male's gowk series call most typically contains 3-5 elements with decreasing volume (disappearing at the end), we compared the first two elements of this call. Our analyses revealed great acoustic similarities between three call types (gowk series call 1st note, gowk series call 2nd note, and single gowk call), and the distinctiveness of the other call type (guo) from this group. Structurally the gowk cannot be regarded as a separate call from a male's gowk series call, and the only difference is that the series call contains a set of the gowk calls. Our classification of cuckoo calls offers the possibility for further research into their functional importance and communicative role in male-male or female-male social contexts.

Female Cuckoo Calls Deceive Their Hosts by Evoking Nest-Leaving Behavior: Variation under Different Levels of Parasitism

Simple Summary

Obligate brood parasite birds such as the common cuckoo (Cuculus canorus) can trick their hosts via a variety of means. Cuckoos are threats to the host nests but not to the host adults themselves. To successfully parasitize the host nests, female cuckoos have been hypothesized to distract the hosts’ attention from their nests by mimicking the calls of sparrowhawks (Accipiter nisus), a predator of the hosts. We performed playback experiments in two populations of the host oriental reed warbler (Acrocephalus orientalis) that have experienced different levels of parasitic pressure. We found that female cuckoo calls evoked both populations of the hosts to leave their nests more frequently than did the calls of male cuckoos or doves that do not pose threats to the hosts. This indicated that the call of the female cuckoo functions to deceive the host and thus favors host brood parasitism. However, we propose that such a deceiving effect of the female cuckoo call is due to the rapid cadence of the call rather than sparrowhawk mimicry.

Abstract

The common cuckoo (Cuculus canorus) is an obligate brood parasite that has evolved a series of strategies to trick its hosts. The female cuckoo has been hypothesized to mimic the appearance and sounds of several raptors to deceive the hosts into exhibiting anti-predator behavior. Such behavior would relax the protection of the host nest and thus allow the female cuckoo to approach the host nest unopposed. Many anti-parasite strategies have been found to vary among geographical populations due to different parasitic pressures from cuckoos. However, the effect of female cuckoo calls related to different levels of parasitic pressure has not been examined. Here, we studied the effect of female cuckoo calls on the oriental reed warbler (Acrocephalus orientalis), one of the major hosts of the common cuckoo, in two geographical populations experiencing different levels of parasite pressure. Four kinds of sounds were played back to the hosts: the calls from female common cuckoos, male common cuckoos, sparrowhawks (Accipiter nisus), and oriental turtle doves (Streptopelia orientalis). The results showed that the female cuckoo calls induced the hosts to leave their nests more frequently than the male cuckoo or dove calls in both populations, and two populations of the hosts reacted similarly to the female cuckoo calls, implying that the function of female cuckoo calls would not be affected by the difference in parasitism rate. This study indicates that female cuckoo calls function to distract the hosts’ attention from protecting their nests. However, we propose that such a deception by the female cuckoo call may not be due to the mimicry of sparrowhawk calls, but rather that the rapid cadence of the call that causes a sense of anxiety in the hosts.

Syntax errors do not disrupt acoustic communication in the common cuckoo

When acoustic communication signals are distorted, receivers may misunderstand the signal, rendering it ineffective. Common cuckoos (Cuculus canorus) are popularly known for the males' simple, two-note advertisement calls, the "cu-coo" used for declaring the male's breeding territories. Cuckoos do not learn their calls (vocal non-learners), so they are expected to have a limited ability to produce different acoustic signals. Nevertheless, male cuckoos appear to make syntax errors (e.g., repeated, reversed, or fragmented elements) even in their simple advertisement calls. We conducted a playback experiment with male cuckoos, broadcasting ten call types, including seven modified calls with errors (e.g. "cu-cu", and "coo-cu") and three natural calls used for comparisons ("cu-coo", "cu-cu-coo", and interspecific control). Male cuckoos responded in a manner suggesting that the presence of the first ("cu") note of the natural 2-note "cu-coo" call in any form or combination yield effective signals. However, through the elevated frequency (by about 200 Hz) and greater speed of the "cu" note, the natural 3-note version "cu-cu-coo" call appears to have gained a novel communicative function in signalling with female cuckoos. Thus, syntax errors in calls with the "cu" element are not responsible for changing the function of the male cuckoos' "cu-coo" call.

Decoupled Acoustic and Visual Components in the Multimodal Signals of the Common Cuckoo (Cuculus canorus)

Because of its parasitic habits, reproduction costs of the common cuckoo (Cuculus canorus) are mostly spent in pre-laying activities. Female costs are limited to searching host nests and laying eggs, whereas, males spend time in performing intense vocal displays, possibly with territorial purpose. This last aspect, together with a sexual plumage dimorphism, points to both intra- and inter-sexual selections operating within this species. One element triggering sexual selection is a differential fitness accrued by different phenotypes. Before analyzing possible sexual selection mechanisms operating in cuckoos, it is therefore necessary to verify whether there is a variability among male secondary characters by describing and quantifying them. Here we aimed to quantitatively characterize the main two potential candidates of sexual selection traits, i.e., calls and displays, shown by males at perches. During the 2019 breeding season, in a site within the Po Plain, we both audio and video recorded cuckoo males at five different perches. We analyzed acoustic variables as well as display sequences searching for potential correlations. We found a significant variation among calls that could be clustered into four vocal types. We also found that no visual displays were associated with vocal displays; cuckoo males were either vocal and motionless or soundless and active. We discuss our results under the perspective of the potential value of sexual selection in brood parasites and its role in its parasitic habit.

Male common cuckoos use a three-note variant of their "cu-coo" call for duetting with conspecific females

Duetting is a coordinated form of acoustic communication with participants uttering calls or songs simultaneously and/or sequentially. Duetting is often observed in pair-bonded species, with mated females and males both contributing to the communal vocal output. We observed duetting between the sexes in the common cuckoo (Cuculus canorus), an obligate brood parasitic species without known pair formation. Specifically, female cuckoos use their sex-specific bubbling calls for duetting, while male cuckoos use a 3-note variant ("cu-cu-coo") of their typical and well-known 2-note ("cu-coo") territorial advertisement calls. The maximum frequency of the elements in the male's 3-note variants was higher relative to the 2-note calls, while durations of both the elements and the inter-element intervals were shorter. The vast majority (95 %) of the 3-note calling was detected together with the bubbling call, implying an intersexual duetting function, with the female calls preceding these male calls in 67 % of cases. The two call types in duetting followed each other rapidly (mean response time of females was 1.30 ± 0.71 SD s, and 0.76 ± 0.53 SD s in males), and typically overlapped with each other (95 %). Frequently (90 %), the male call was repeated 2-3 times, whereas the female call was repeated less frequently (9%). Our results are consistent with a main function of duetting in intersexual communication and coordination between female and male cuckoos.

Testing the Interspecific Function of Female Common Cuckoo “Bubbling” Call

Female common cuckoo (Cuculus canorous) predator-like “bubbling” calls distract host parental attention and reduce the egg rejection rate. Such “bubbling” calls are also frequently used to attract males and deter territorial rivals in intraspecies contact, and these calls are an ancestral character in many cuckoo species. Although hosts have had sufficient time to become familiar with this call and evolve anti-parasitic strategies, why are the hosts fooled by this “bubbling” call? We propose two hypotheses. The first hypothesis proposes that call variation reduces the opportunity for host species to correctly assess cuckoo tricks. In contrast, the second hypothesis proposes that the cost of behavior may prevent the antiparasitic strategy from evolving. In the study, we tested the prerequisites of these hypotheses, by investigating whether cuckoo calls vary during the day and testing whether the predator-like calls suppress bird activities. Based on field recordings from three different areas, we found high overlap in the calls generated during different periods. Oriental great reed warblers (Acrocephalus orientalis), a host species, did not show different responses toward the playback of female common cuckoo calls generated before noon or afternoon. Based on bird count data, we found that predator-like call playback is insufficient for suppressing bird activities. Therefore, none of the prerequisites were supported by our field data. We discuss the potential reasons for our findings and hope to inspire more research examining female cuckoo vocalizations.

Effective conspecific communication with aberrant calls in the common cuckoo (Cuculus canorus)

Abstract

The obligate brood parasitic common cuckoo (Cuculus canorus) is best known for its two-note “cu-coo” call, which is uttered repeatedly by adult males during the breeding season. This call advertises the male’s claim for his territory. A rare, aberrant version (“cu-kee”) was discovered in a population of cuckoos in central Hungary. In a playback experiment, we simulated conspecific territorial intrusions using either aberrant call sequences or normal calls (as control). Cuckoos responded to both calls similarly by approaching the speaker, flying around it several times, and perching on nearby trees. To identify the role of each note of these cuckoo calls, we also played sequences of the first (“cu”) or second (“coo” or “kee”) notes of the calls. Territorial males responded to first notes at similarly high frequencies as to each of the full calls, whereas responses toward either second note type were nearly absent. Thus, the first notes of both typical and aberrant cuckoo calls contain sufficient information to recognize conspecific males and the novel calls did not reduce the efficiency of male-male communication in cuckoos because the aberration occurred in the less functional second note.

Significance statement

Birds use songs and calls to communicate with each other, including advertising their territories to keep competitors away. However, when the acoustic signal is atypical and distorted, the receiver individual may not process it correctly. Common cuckoos recognize a territorial intruder by their well-known “cu-coo” calls. We studied a rare, aberrant version of the common cuckoo call (“cu-kee”), which differed from the normal call in the second note of the two-partite call. However, cuckoos responded similarly to both of the normal and aberrant calls in a playback experiment. When the first or second parts of the different calls were played separately, only the first part of the cuckoo calls was effective in eliciting territorial defence. Consequently, the aberrant second note did not reduce cuckoos’ communication efficiency.

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.