Human disturbance affects distribution but not nesting success of the Great Reed Warbler in a semi-urban reed habitat

On Wicken Fen and nearby watercourses eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26 and 16 of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively...

Nestlings of brood parasites exhibit more intensive begging than offspring of their hosts to gain sufficient amount of food or competitive advantage over host nestlings. This begging behaviour should be costly because exuberant acoustic begging may more likely attract nest predators. However, to date, nobody has explored the survival of nests with and without chicks of brood parasites in the common cuckoo (Cuculus canorus) host system. Here, we analysed an extensive dataset of 817 great reed warbler (Acrocephalus arundinaceus) and 788 reed warbler (Acrocephalus scirpaceus) nests to explore the relationships between nest predation and parasitism status (parasitized vs. non-parasitized), nest contents (cuckoo chick vs. host nestlings) and age of nestlings. We found that although parasitized nests had higher predation rate than non-parasitized nests in the incubation stage, the effect of original parasitism status almost disappeared in the nestling stage. In both host species, nests with younger cuckoo chicks survived similarly to nests with host nestlings of the same age (till the ninth day of age). Later on, however, nest contents influenced nest predation in each species differently. While nests with older cuckoo chicks (from the ninth to the 17th day of age) did not survive worse that host nestlings in the great reed warbler, older cuckoos survived much worse than host nestlings in reed warbler nests. Finally, nest survival decreased with nestling age in all three species. Thus, it seems that common cuckoo chicks can be penalized for more intensive begging only in nests of smaller reed warbler hosts. Parental feeding of young is in birds frequently accompanied by striking nestlings begging behaviour serving as a signal of their need. Brood parasites exhibit even more intense food solicitation than their hosts which may attract predators to the nest. However, this hypothesis has never been tested in a widely studied brood parasite species—the common cuckoo. Here, we analysed survival of more than 1600 nests of its two main host species. We found that nests containing older common cuckoo chicks were depredated more frequently than nests with host own nestlings only in the smaller reed warbler hosts but not in the larger and more aggressive great reed warblers. This shows that the intensity of begging could be costly in terms of nest predation at least in some common cuckoo host species.

Nestling cuckoos, Cuculus canorus , eject host eggs or young from the nest and are then raised alone by the hosts. Using reed warblers, Acrocephalus scirpaceus , as hosts, we investigated how the single cuckoo chick can command the same provisioning rate as a whole brood of host young. Large size alone is not sufficient to stimulate adequate provisioning because single blackbird, Turdus merula , or song thrush, T. philomelos , chicks of the same mass as a cuckoo were fed at a lower rate. Our experiments show that the key stimulus is the cuckoo chick9s rapid begging call (‘si, si, si, si ...’), which sounds remarkably like a whole brood of host chicks, and which it matched in calling rate. When single blackbird or song thrush chicks were accompanied by loudspeakers that broadcast either cuckoo begging calls or calls of a brood of reed warblers, the hosts increased their provisioning rate to that for a cuckoo chick. We suggest that the cuckoo needs vocal trickery to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.

Analyses of the stable isotope composition of feathers can provide significant insight into the spatial structure of bird migration. We collected feathers from Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and a small sample of their hybrids in a sympatric breeding population in Kazakhstan to assess natural variation in stable isotope signatures and delineate wintering sites. The Great Reed Warbler is a long-distance migrant that overwinters in sub-Saharan Africa, whereas the Clamorous Reed Warbler performs a short-distance migration to the Indian sub-continent. Carbon (δ13C), nitrogen (δ15N) and deuterium (δD) isotope signatures were obtained from winter-grown feathers of adult birds. There were highly significant differences in δD and less significant differences in δ13C between Great and Clamorous Reed Warblers. Thus, our results show that the stable isotope technique, and in particular the deuterium (δD) signal, resolves continental variation in winter distribution between these closely related Acrocephalus species with sympatric natal origin. The isotope signatures of hybrid Great × Clamorous Reed Warblers clustered with those of the Great Reed Warblers. Hence, a parsimonious suggestion is that the hybrids undergo moult in Afrotropical wintering grounds, as do the Great Reed Warblers. The observed δD values fell within the range of expected values based on available precipitation data collected at precipitation stations across the wintering continents of each species. However, the power to predict the winter origin of birds in our study system using these data was weak as the expected values ranged widely at this broad continental scale.

The co-evolutionary arms race between brood parasites and their hosts involves stepwise adaptive changes on the side of the parasites as well as hosts. In response to avian brood parasitism, host females may eject a parasitic egg, bury the parasitized clutch or desert it. After nest desertion, females commonly re-nest and may move further to avoid being parasitized again. Here we tested whether and under which conditions the within-season re-nesting prevents brood parasitism in the great reed warbler (Acrocephalus arundinaceus). We analysed 78 re-nesting events of 58 naturally parasitized host females that deserted their nests in response to the common cuckoo (Cuculus canorus) parasitism. The parasitism rate in the replacement nests of these females was 60%. Most of these females built their replacement nests less than 143 m from the previous nests. The probability for replacement nests to be parasitized increased with increasing instantaneous parasitism rate but not with the re-nesting distance or timing of the replacement clutch. We explain this by the high level of cuckoo parasitism across the whole study site during the major part of the breeding season. To better understand the patterns and consequences of host re-nesting behaviour, further studies in other host populations with different levels of cuckoo parasitism would be desirable. Although various factors affecting avian breeding dispersal have been studied, little is known about the relationship between the within-season re-nesting distances and fate of replacement nests. Moreover, there is a lack of studies focusing on the consequences of re-nesting dispersal in response to brood parasitism and, to our best knowledge, this is the first study investigating this topic in a host of an evictor parasite.

Hosts of the common cuckoo (Cuculus canorus), an avian brood parasite, develop antiparasite defense mechanisms to increase their reproductive success. Ejection of the parasite egg and desertion of the parasitized nest are the most typical adaptations in response to brood parasitism, but nest desertion may also occur in response to partial clutch reduction, independently from parasitism. Some great reed warblers (Acrocephalus arundinaceus) showed both mechanisms in the same incidence of cuckoo parasitism: in 18% of successful ejections of the parasite eggs, they deserted their nests. We studied if such cases of post-ejection nest-desertion are caused by brood parasitism or reduced clutch value. We experimentally parasitized clutches consisting of five or three host eggs with two painted conspecific eggs to mimic parasitic eggs, as multiple parasitism is frequent in the area. Although hosts ejected these parasitic eggs in both clutch categories (100% and 67% for the larger and smaller inital clutch sizes, respectively), we found that after manipulation, post-ejection nest-desertion frequently occurred at small (3-egg) clutches (40%), but rarely at large (5-egg) clutches (17%). The same phenomenon also occurred when unparasitized 3-egg clutches were reduced by two eggs, but not when 5-egg clutches were reduced in the same way. A logistic regression model revealed that only initial clutch size affected nest desertion of parasitized nests in our experiments. Therefore, we conclude that post-ejection nest-desertion is not a second antiparasite mechanism, which might serve as a redundant antiparasite defense, but a reaction to typically small and further decreased clutch size.

1) Interspecific relationships between sympatric Great and Schrenck's reed warblers Acrocephalus arundinaceus and A. bistrigiceps were studied at Sugadaira (36°20′N, 138°20′E), central Honshu, during the breeding seasons in 1982-87. An intensive observation on the breeding schedule, home range and interspecific behavior was made in 1987.2) Great Reed Warblers usually arrived at the breeding ground earlier than Schrenck's Reed Warblers. The earliest arrivals were male Great Reed Warblers, followed by female Great, male Schrenck's, and finally female Schrenck's reed warblers.3) The mean staying period of mated males in the study area was about 60 days for both species. The home ranges of Schrenck's Reed Warblers were unstable as compared with those of Great Reed Warblers; Schrenck's often changed their home ranges.4) Breeding success (fledglings/egg laid) in the study area was 40.4% for Great and 61.1% Schrenck's reed warblers in 1987.5) In establishing home ranges the two species were segregated by time and space. Great Reed Warblers usually establish their home ranges earlier than Schrenck's. Schrenck's Reed Warblers had their home ranges either in the neutral area among the treat's home ranges or in the area distant from them.6) The habitat preferred by the two species differed horizontally, although both species were overlapped in their preference for reed beds. Mean nest height was 1.02 m for Great and 0.63 m for Schrenck's reed warblers.7) Three types of territorial behavior were observed: counter singing, chasing, and fighting. In Great Reed Warblers counter singing was noticeable. Great Reed Warblers often preferred tall trees for singing post, while Schrenck's selected reed stem for it.8) Sizes of home ranges and singing areas of Great Reed Warblers were considerably larger than those of Schrenck's. Some males of Schrenck's Reed Warblers travelled from their home ranges to further places and sometimes established a second home range.9) Territorial Great Reed Warblers were always dominant against intruding Schrenck's, whereas the latter could not successfully chased out the intruded Great Reed Warblers.10) The two sympatric reed warbler species enabled to avoid competition not by interspecific territory, but by different time schedule for breeding and establishing home ranges. Schrenck's Reed Warblers had their home ranges in the neutral area between neighboring territorial males of Great Reed Warblers and made their breeding successful by non-territorial sneaking behavior

How does human disturbance affect brood parasitism and nest predation in hosts inhabiting a highly fragmented landscape?

Mimicry cannot explain rejection type in a host–brood parasite system

It is well known that avian brood parasites lay their eggs in the nests of other bird species, called hosts. It remains less clear, however, just how parasites are able to recognize their hosts and identify the exact location of the appropriate nests to lay their eggs in. While previous studies attributed high importance to visual signals in finding the hosts’ nests (e.g. nest building activity or the distance and direct sight of the nest from vantage points used by the brood parasites), the role of host acoustic signals during the nest searching stage has been largely neglected. We present experimental evidence that both female and male common cuckoos Cuculus canorus pay attention to their host’s, the great reed warbler’s Acrocephalus arundinaceus alarm calls, relative to the calls of an unparasitized species used as controls. Parallel to this, we found no difference between the visibility of parasitized and unparasitized nests during drone flights, but great reed warblers that alarmed more frequently experienced higher rates of parasitism. We conclude that alarm calls might be advantageous for the hosts when used against enemies or for alerting conspecifics, but can act in a detrimental manner by providing important nest location cues for eavesdropping brood parasites. Our results suggest that host alarm calls may constitute a suitable trait on which cuckoo nestlings can imprint on to recognize their primary host species later in life. Our study contributes to the growing body of knowledge regarding the context-dependency of animal signals, by providing a novel example of a beneficial acoustic trait intercepted by a heterospecific and used against the emitter.

Brood parasites reduce the reproductive success of many bird species by laying eggs in their nests. Hosts that reject parasitic eggs (“rejecters”) avoid most costs of brood parasitism altogether by physically ejecting eggs from nests or abandoning parasitized nesting attempts. Species that accept parasitic eggs once these are laid (“accepters”) may reduce or eliminate costs by aggressively responding to brood parasites at their nests to prevent parasitism from taking place. Accordingly, accepters should recognize brood parasites and nest predators as different nest threats with different levels of aggression, whereas rejecters may not. We exposed active Eastern Phoebe (Sayornis phoebe, an accepter host) and American Robin (Turdus migratorius, a rejecter host) nests to models of a female brood parasitic Brown-headed Cowbird (Molothrus ater), an eastern chipmunk (Tamias striatus, nest predator), and a European Starling (Sturnus vulgaris, nonthreatening control) during the incubation stage. Phoebes alarm-called equally toward the nest predator and brood parasite models, but attacked the nest predator model more than the brood parasite model. Robins, in contrast, alarm-called toward and attacked all 3 models equally. Interpreting these results is challenging due to experimental design elements, specifically small sample sizes and restricting the experiment to the incubation stage. Nonetheless, our experiment contributes to the paucity of comparative studies on accepter versus rejecter nest defense behavior in response to both nest parasites versus predators, and adds a new tested accepter species to the literature.

The coevolutionary process among avian brood parasites and their hosts involves stepwise changes induced by the antagonistic selection pressures of one on the other. As long‐term data on an evolutionary scale is almost impossible to obtain, most studies can only show snapshots of such processes. Information on host behaviour, such as changes in egg rejection rates and the methods of rejection are scarce. In Hungary there is an interesting case between the common cuckoo Cuculus canorus and the great reed warbler Acrocephalus arundinaceus , where the level of parasitism is unusually high (around 50%). We compared host rejection rates and methods of rejection from within our own project to that of an early study carried out and published almost 70 yr ago in the same region. Our comparisons revealed high and stable rates of parasitism (range: 52–64%), and marked fluctuations in the ratio of multiply parasitized nests (range: 24–52%). No difference was revealed in egg rejection rates after 7 decades (34–39%). Linear mixed‐effects modelling revealed no year effect on the type host responses toward the parasitic egg(s) during the years of study (categorized as acceptance, ejection, burial, and nest desertion). Cuckoo egg rejection was primarily affected by the type of parasitism, as more cuckoo eggs were rejected during single parasitism than from multiply parasitized nests. Our comparison did not reveal any directional changes in this cuckoo–host relationship, except a slight decrease in the frequency of multiple parasitism, which is likely to be independent from coevolutionary processes.

We surveyed five reed habitats (mining pond, sand pit, large canal, small canal, and lowland river) in north-western Vojvodina (Serbia) between 2009–2011 to study habitat use and to estimate nesting success in an understudied region of the breeding range of the Great Reed Warbler (Acrocephalus arundinaceus). Data from 174 nests showed that habitat use differed considerably between habitat types but was not related to the area of the study site or the reed bed. Higher than expected numbers of nests along the small canal and the river suggested that Great Reed Warblers preferred these to other habitats for nesting. Habitat use was closely linked to the availability of reed edges and the quality of the reed stand. Overall Mayfield nesting success was 43%, slightly lower than in northern and western Europe. Nesting success was low along the small and large canal, where brood parasitism by Common Cuckoos (Cuculus canorus) and nest predation were high because of the nearby presence of tree lines that p...

Brood parasitism represents a unique mode of avian reproduction that requires a number of adaptations. For example, to reduce chances of puncture ejection of their eggs by small hosts, brood parasites may have been selected for laying eggs of unusually great structural strength. However, great structural strength of eggshells should hinder hatching. The goals of our study were to establish if chicks of the Common Cuckoo Cuculus canorus have more difficulty with hatching out of their strong eggs than chicks of species with eggs of similar size, and whether they possess any mechanisms facilitating hatching. To achieve these goals, we compared hatching pattern and selected body characteristics of chicks of the Common Cuckoo with those of another altricial species with eggs of a similar size, the Great Reed Warbler Acrocephalus arundinaceus. Although the rate of pecking was similar in the two species, the Common Cuckoo chicks started pecking earlier in relation to their emergence and consequently required more time and a greater cumulative number of pecks for breaking open their eggs than did young Great Reed Warblers. The two species also differed with respect to the pattern of opening their shells; in contrast to the warbler chicks, which enlarged the original pip circularly, the cuckoo chicks opened the egg by systematically creating a long narrow slit until they emerged. Finally, our study of hatched young revealed several differences; the Cuckoo hatchlings were significantly heavier, had a longer forearm, and their egg tooth was located significantly farther from the tip of the beak. The edge used for cutting through the shell was also significantly longer than that of hatchling Great Reed Warblers. To conclude, our data suggest that hatching is more difficult for a Cuckoo than for a Great Reed Warbler and that Cuckoos possess several mechanisms to overcome the problems of hatching from a structurally strong egg.

While some previous investigations have been made into the effects of vegetation structure on the probability of brood parasitism in reed passerines, the effects of vegetation management remain unclear. Furthermore, we possess little information on how vegetation density influences the probability of brood parasitism in wide-surfaced reed-beds. The aim of this study was to test how vegetation density and reed management by burning influence the probability of cuckoo (Cuculus canorus) brood parasitism in great reed warbler (Acrocephalus arundinaceus) nests in a wide-surfaced reed-bed with pure reed stands. We found that the probability of parasitism was highest in nests constructed in low reed density, with low annual variation of reed density and low annual variation of nest height. Reed management had no effect on the probability of parasitism. Our results suggest that, in wide-surfaced reed-beds with pure reed stands, nests in sparser reed are more exposed to brood parasitism than those concealed in dense reed. Furthermore, low spatio-temporal variation in reed density and nest height benefit brood parasitism, as it provides a relatively constant number of host nests in a similar spatial distribution in the reed-bed from year to year. Vegetation structure was suggested to have an important role in adaptation of hosts’ defence against brood parasites.