Breeding pairs with color aberrations in oriental reed warblers.

In 2017, one pair of Oriental reed warblers (Acrocephalus orientalis) with color aberrations was found in Yongnianwa National Wetland Park, Hebei, China. The female bird exhibited white feathers on the head, neck, and upper back, and the base of the beak was flesh-red in color. The male had a few feathers on the outer edges of the left and right primary wing coverts that were white, which was determined to be leucism after analysis. The breeding pairs laid their first egg on May 29, with a clutch size of four eggs. After an incubation period of 13 days, two chicks hatched on June 13, 2017. The nest was found empty on June 20 when the chicks were 7 days old and before fledging age; therefore, it was presumed that the chicks had been predated. A white parrot egg was added to the nest during the incubation period to test the egg recognition ability of breeding pairs and was successfully rejected. To the best of our knowledge, this is the first report of color aberrations in the Oriental reed warbler, and we found that this color aberrations did not affect some reproductive and antiparasitic behaviors of the birds, but whether it affects their breeding success needs to be further studied.

Three sympatric host nestlings eavesdrop on cuckoo nestling distress calls.

In predator-prey interactions, the prey faces extreme challenges from predation, which drives the evolution of defense or anti-predator mechanisms. Compared with adult birds, nestlings are more vulnerable but not helpless. However, data on whether nestlings eavesdrop on the danger signals transmitted by other prey nestlings and the mechanisms of eavesdropping remain limited. In brood parasitism, common cuckoo (Cuculus canorus) nestlings, raised by host adults who are not closely related, offer an instructive system for studying the transmission and recognition of danger signals among nestlings of different species that share special relationships. We played back the distress calls of common cuckoo nestlings to nestlings of three sympatric host species (the oriental reed warbler Acrocephalus orientalis, which is a primary host of the common cuckoo, the reed parrotbill Paradoxornis heudei, an occasional host, and the vinous-throated parrotbill Sinosuthora webbiana, which is not parasitized in the study area) to investigate whether the host nestlings reduced their begging behavior. We also quantified the degree of inhibition toward begging behavior for these nestlings. The results revealed that, in response to the distress calls, the three sympatric host species markedly suppressed their begging behavior. This response can likely be attributed to the innate response of host nestlings caused by the general characteristics of distress calls, rather than the acoustic similarity and phylogenetic relationship between host nestlings and cuckoo nestlings. Furthermore, we observed that upon hearing the distress calls of cuckoo nestlings, the oriental reed warbler nestlings exhibited the greatest reduction in the total number of calls compared to the other two host species, potentially owing to stronger predation and parasitic pressures. This study suggests that host nestlings can detect danger signals emitted by parasitic nestlings; however, further investigation is needed to determine whether they can respond to distress calls from unfamiliar nestlings in different regions.

Oriental reed warblers retain strong egg recognition abilities during the nestling stage.

Egg recognition and rejection are the most common and effective anti-parasitic strategies against avian brood parasitism in terms of maintaining stability over time and plasticity in response to environmental cues. Conversely, parasites have evolved multiple counter-adaptations to the anti-parasitic defenses of hosts. Among them, the crypsis hypothesis suggests that eggs that appear more cryptic in color and are closely matched to the environment are helping to counter the egg recognition strategy of the host. In this study, we investigated whether the egg recognition ability of Oriental reed warblers (Acrocephalus orientalis), a common host of common cuckoos (Cuculus canorus), changed during different reproductive stages by using model egg experiments. The effect of the crypsis hypothesis on the egg recognition ability of the hosts was also investigated by controlling the color contrast between the inside of the experimental nests and the model eggs. The results showed that the Oriental reed warbler retained strong egg recognition abilities, which were similar to the incubation stage (GLMMs: F 1,27 = 0.424, p = .521), even after entering the nestling stage and preferentially rejected model eggs with distinct contrasting colors (binomial test: Fisher's exact, p = .016). These results are consistent with the crypsis hypothesis. The present study suggests that the host retains a strong egg recognition ability even during the nestling stage and that cryptic-colored eggs that are closely matched with the breeding nest environment help counter the host's egg recognition abilities and increase the chances of successful parasitism by cuckoos. However, the effectiveness of cryptic egg may be weaker than mimic egg in countering egg recognition and rejection by hosts with open-cup nests.

Report of a magpie preying on a post-fledgling Daurian redstart.

A magpie (Pica pica) preying on a fledgling of Daurian redstart (Phoenicurus auroreus) was incidentally recorded with a video shot by mobile phone on 26 May 2021, providing direct evidence for magpie predation. It also shows that predation is an important factor that affects the survival of fledglings, indicating that survival of fledglings should be considered in evaluating breeding success of birds. The fledgling was about 13-day-old posthatching, and it was on its first day of leaving the nest when the incident occurred. It was preyed upon by a magpie 10 m away from the nest by two attempts under strong defensive behaviour from the female.

Eggshell spots are an important cue for the egg retrieval behavior in two tit species.

It is a common occurrence for incubating female ground- or cavity-nesting birds to accidentally roll their own eggs out of the nest cup. To correctly roll eggs that are not in the nest cup back to the nest, birds must be able to discriminate between their eggs and things around their nests that could be easily confused for their eggs (e.g., egg-shaped stones or other birds' eggs). Moreover, there may be a strong interaction between egg retrieval and egg rejection behaviors in the context of avian brood parasitism where recognition and rejection of alien eggs is an effective means for hosts to defend against cuckoo parasitism. It has been shown that green-backed tits (Parus monticolus) and Japanese tits (P. minor) in China have strong egg recognition ability and are able to reject nonmimetic eggs in the nest. Eggshell spots play an essential role in the egg rejection behavior of these two tit hosts. This study investigated the egg retrieval behavior of green-backed tits and Japanese tits by adding one experimental egg to the nest corner to explore whether eggshell spots also play a role in the process of egg retrieval. The results revealed significant differences in the retrieval rates of white-rumped munia (Lonchura striata) eggs painted with spots, Japanese tits' own true eggs, and unpainted, pure white-white-rumped munia eggs. The retrieval rate of white-white-rumped munia eggs was significantly lower than that of spotted white-rumped munia eggs and Japanese tits' own spotted eggs. For green-backed tits, the retrieval rate of spotted white-rumped munia eggs in the nest corner was significantly higher than that of pure white-white-rumped munia eggs, while the rejection rate of spotted white-rumped munia eggs was significantly lower than that of pure white-white-rumped munia eggs. These findings indicate that eggshell spots play a key role in the egg retrieval of green-backed tits and Japanese tits.

Defense behavior of two closely related but geographically distant host species against cuckoo parasitism: A next test for the parallel coevolution.

Interactions between avian brood parasites, such as common cuckoos (Cuculus canorus), and their hosts are one of the best-studied examples of the coevolutionary arms race. Different stages of this arms race can be seen in different races of common cuckoos and their hosts across their range. However, little is known whether selected populations of two closely related but geographically distant species with probably different coevolutionary histories with the common cuckoo are also at different stages of the arms race. In this study, we tested this prediction experimentally using the same non-mimetic model eggs and three-dimensional (3D) printed models of the gray adult common cuckoo (Cuculus canorus). We examined egg recognition and egg rejection and aggression against the common cuckoo in the great reed warbler (Acrocephalus arundinaceus) and Oriental reed warbler (Acrocephalus orientalis), in Slovakia of Europe and northeast China of Asia. The results showed that the great reed warbler exhibited stronger responses to experimental model eggs and 3D models of the common cuckoo than the Oriental reed warbler. We conclude that both the great reed warbler and Oriental reed warbler have well-developed antiparasitic behaviors against common cuckoos in the studied populations, but with different levels of defense intensity, which may be due to local differences in parasitic pressure and the risk of parasitism. This provides an opportunity to study coevolutionary processes between the brood parasite and its hosts together in both species at large geographical scales.

Functions of Snake Sloughs in Bird Nests Vary with Habitats: A Test of the Anti-Predation Hypothesis.

Snake sloughs in bird nests can reduce nest predation and serve as an anti-predator strategy. However, the anti-predator function of snake sloughs in nests has only been tested twice, and it is difficult to speculate around the origin of the differences, which may well include habitat, as predator species and predation risk vary in different habitat types. Habitat would be a good place to speculate as to how differences in habitats could explain differences in responses by nest predators. Thus, we selected three different habitats, namely, the Diaoluoshan National Nature Reserve in Hainan (DLS, tropical forest), the Hainan Normal University campus (HNU, urban area), and Qingchuifeng National Forest Park in Hebei (QCF, suburban area), to verify the anti-predator function of snake sloughs in bird nests. The experimental results showed that snake sloughs in the nests reduced the predation rate of the experimental nests in HNU, whereas no such effect occurred in DLS and QCF. This suggests that the anti-predatory function of snake sloughs may not be the same over some environmental gradients and could be dependent on the species of nest predators and food resources in the habitat, which does not apply to all types of habitats.

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

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.

Modulation of dear enemy effects by male dusky warblers (Phylloscopus fuscatus) at different reproductive stages.

Territorial animals show less aggression during intrusion by neighbours than by strangers, a phenomenon known as the "dear enemy effect". Recent studies have shown that the dear enemy effect is variable in some birds throughout the breeding season. However, there is limited research on changes in dear enemy effects in songbirds during the breeding season. In this study, we monitored dusky warblers (Phylloscopus fuscatus) throughout their breeding stages, conducting playback experiments simulating incursions by neighbours and strangers during their egg-laying and incubation/nestling periods, and recording physical responses and song responses from territory owners. The results showed that male dusky warblers responded similarly to neighbours and strangers during the female egg-laying period, but there was a significantly stronger response to strangers than to neighbours during the incubation/nestling period. This suggests that male dusky warblers adjust the intensity of their defences against neighbours according to the reproductive status of their own mate. This may be because neighbouring males threaten the paternity of territorial males during the egg-laying period, so territorial males demote the 'dear enemy' friendly relationships with their neighbours to defend their mate and paternity. In contrast, during the non-reproductive period of females, when neighbours are less of a threat to the paternity of territorial males, lowering defences against neighbouring males allow them to devote more time and energy to activities that improve their fitness, such as foraging and seeking extra-pair mating.

Responses of cuckoo hosts to alarm signals of different nest intruders in non-nesting areas.

The "call for help" hypothesis proposes that alarm calls produced by a bird can transmit warning information to both conspecific and interspecific neighbors. Neighbors who are attracted by social transmission might benefit from knowing about the presence of danger or by gaining information about the presence of predators or brood parasites nearby. Brood parasite hosts can distinguish threats from different intruders and exhibit varied responses correspondingly. However, most previous studies have conducted sound playback at host nest sites and focused on conspecific individuals attracted by the alarm calls. In this study, we used random location playback to investigate the responses of different host species to alarm signals of oriental reed warblers (Acrocephalus orientalis) toward different intruders (brood parasite, predator, and harmless control) in order to reveal how hosts evaluate different threats from different intruders using vocal information in non-nesting areas during the breeding season. We found that the alarm calls given in response to different intruders incurred similar numbers of approaching species for both conspecific and interspecific birds. However, the number of attracted individuals differed significantly among the various species, with conspecifics and vinous-throated parrotbills (Paradoxornis webbianus) dominating, both of which are major hosts of common cuckoos (Cuculus canorus). Nevertheless, interspecific birds did not present any aggressive behavior according to the alarm calls, which implied that visual information may be needed for further confirmation of threats. In addition, determining whether alarm call structure promoted an evolutionary convergence phenomenon still needs further verification.

Comparative urbanization of birds in China and Europe based on birds associated with trees.

Urbanization effects on living organisms are spatially heterogeneous. Here we quantified the abundance of birds per tree in forested urban and rural habitats for 85,829 trees mainly in China and Europe. A population model was based on the assumption that: 1) birds have a normally distributed habitat preference; 2) an increase in population size linked to the habitat preference; 3) a population size dependent on the habitat preference; and 4) the removal of a certain fraction of individuals giving rise to extinction. We tested for large-scale differences in the impact of urbanization on the frequency distribution of the difference in abundance between urban and rural habitats in China and parts of Europe. The difference in the frequency distribution of urban population density of birds in trees minus rural population density of birds in trees in China and Europe was statistically significant, suggesting that the abundance of birds differed between trees in urban and rural habitats, but more so in China than in Europe. We hypothesize that more pronounced differences in China than in Europe may have arisen due to the Four Pests Campaign in 1958-1962 that resulted in death of hundreds of millions of birds (mainly tree sparrows Passer montanus, but also numerous other less common species that were starting to become urbanized around 1960). Species that were less common in 1960 could not sustain reductions in population size in urban areas and hence these species are still rare or absent in urban areas today 60 years later.

Costs of breeding far away from neighbors: Isolated host nests are more vulnerable to cuckoo parasitism.

A series of parasitic and anti-parasitic strategies has evolved during the long-term coevolution between cuckoos and their hosts. The first stage of the arms race is host nest choice by cuckoos, followed by nest defense by hosts. This study examined nest defense strategies of the Oriental reed warbler (Acrocephalus orientalis) in relation to parasitism by the common cuckoo (Cuculus canorus). Attack rate of Oriental reed warblers against common cuckoo dummies was 100% and neighboring individuals participated in 87.1% of such attacks. Furthermore, the number of hosts attacking cuckoo dummies was significantly positively correlated with the number of neighbors at a distance from 40 to 70 m, indicating social anti-parasitic behavior. Analysis of nest-site parameters indicated that the distance to the nearest neighboring unparasitized nest was significantly shorter than that of parasitized nests. Our study demonstrated that isolated nests of Oriental reed warblers with distant neighbors were more vulnerable to common cuckoo parasitism suggesting a cost of breeding far away from neighbors. We hypothesize that cooperative behavior within local populations of Oriental reed warblers can probably be considered as an anti-parasitic strategy developed through long-term coevolution. Choice and parasitism of isolated host nests far away from neighbors may be an adaptive parasitic strategy by common cuckoos to increase the probability of successful parasitism.