Nest defenses and egg recognition of yellow-bellied prinia against cuckoo parasitism

Importance of cooperation: How host nest defenses effectively prevent brood parasitism from the cuckoos

As the frontline defense against avian brood parasitism, nest defense is important in reducing nest parasitism and increasing host fitness. However, systematic studies on its effectiveness (i.e., on whether it successfully prevents cuckoo parasitism) are scarce. Based on 214 video recordings from 10 years of field observations, we evaluated the effectiveness of nest defenses of the Oriental reed warbler (Acrocephalus orientalis) in deterring common cuckoo (Cuculus canorus) parasitism. Under a breeding pair situation (≤ 2 hosts individuals), warblers were hardly effective in resisting the egg-laying of the cuckoo and many of the cuckoos under attack were able to complete the parasitic process without being harmed. However, when hosts ≥ 3 individuals, the warblers could effectively thwart the cuckoos and cause lethal damage to the cuckoo in partial cases. This indicated that the host group effectively resisted the cuckoo, demonstrating the importance of cooperation among host neighbors in resisting cuckoo parasitism.

Coevolution between Himalayan cuckoos and 2 sympatric Pycnonotidae hosts

Selection due to cuckoo parasitism is responsible for the evolution of anti-parasitism defenses in hosts. Different host species breeding sympatrically with a single parasitic cuckoo may evolve different strategies to reduce the risk of counter cuckoo parasitism, resulting in different interactions between cuckoos and hosts in areas of sympatry. Here, we studied the coevolutionary interactions between Himalayan cuckoos Cuculus saturatus and 2 sympatric and closely related potential hosts belonging to the family Pycnonotidae, the brown-breasted bulbul Pycnonotus xanthorrhous and the collared finchbill Spizixos semitorques. We investigated parasitism rates and nest-site selection (nest height, nest cover, human disturbance, perch height, forest distance, and degree of concealment) related to parasitism risk, nest defense against a cuckoo dummy, and egg rejection against cuckoo model eggs. Bulbuls used specific nest sites that were further away from forests than those of finchbills, and they behaved more aggressively toward cuckoos than finchbills. In contrast, bulbuls possessed moderate egg rejection ability, whereas the finchbill rejected 100% of cuckoo model eggs. We suggest that selection of a nest site away from forests by the bulbul explains the absence of parasitism by Himalayan cuckoos. We suggest that these interspecific differences in nest-site selection and nest defense indicate alternative responses to selection due to cuckoos.

Behavioral and Acoustic Responses of the Oriental Reed Warbler (Acrocephalus orientalis), at Egg and Nestling Stages, to the Common Cuckoo (Cuculus canorus)

Cuckoo nest parasites lay eggs in host nests and thereby transfer all reproduction costs to the hosts. This greatly reduces host fitness. Parasitism has selected for the evolution of anti-parasitic strategies in hosts, including nest defense. The dynamic risk assessment hypothesis holds that nest parasitism only threatens the nests during the egg stage, so hosts should reduce the level of defense against nest parasites after the egg stage. We studied the behavioral and acoustic responses of oriental reed warblers (Acrocephalus orientalis), during both the egg and nestling stages, toward the common cuckoo (Cuculus canorus), sparrowhawks (Accipiter nisus) and oriental turtle doves (Streptopelia orientalis). A. orientalis can visually distinguish cuckoos from sparrowhawks and doves, indicating that hawk mimicry did not work for the cuckoos. The behavioral response of hosts in the nestling stage was stronger than in the egg stage, which supports the offspring value hypothesis and suggests that cuckoos may also act as nest predators. However, there was no difference in the alarm calls A. orientalis produce in response to different invaders, indicating that different types of alarm calls may not contain specific information.

Determining the level of extra-pair paternity in yellow-bellied prinias, a socially monogamous passerine

Previous work based on molecular evidence has shown that most socially monogamous birds follow a genetic polyandrous mating system. However, our knowledge about avian mating systems is heavily biased toward the north temperate zone, with data on tropical birds remaining relatively scarce. This uneven distribution of both phylogenetic and spatial sampling has hampered our understanding and interpretation of results. In this study, we investigated the frequency of extra-pair paternity (EPP) in a tropical population of yellow-bellied prinias (Prinia flaviventris) in Guangxi, southern China. A total of 129 individuals belonging to 24 nests were sampled, among which 12 out of 83 chicks (14.46%) in seven nests were found to be EPP offspring. In nests in which all nestlings were sampled, only five out of 56 chicks were EPP offspring, accounting for an unbiased EPP rate of 8.93%. This rate is below the average rate of EPP in the family Sylviidae. The possible causes of EPP in prinias and the occurrence of EPP in birds with high resource investment and intensive parental care are discussed. This study highlights the value of genome-wide markers in determining relatedness in a wild bird species without a reference genome.

Specific responses of cuckoo hosts to different alarm signals according to breeding stage: a test of the offspring value hypothesis

The selective pressure exerted by avian brood parasites forces their hosts to evolve specific defense strategies. When subject to brood parasite attack, avian hosts will often emit alarm calls. To date, few studies have examined whether and how host responses to different alarm calls indicative of different enemies vary with the host's breeding stage. We carried out alarm call playback experiments during both the egg and nestling stages of the oriental reed warbler Acrocephalus orientalis, a host of the common cuckoo Cuculus canorus. The playback exemplars were selected from recorded alarm calls of the warbler to the presence of common cuckoos, sparrowhawks Accipiter nisus, and oriental turtle doves Streptopelia orientalis, which represented brood parasite, predator, and harmless control, respectively. The results showed that the oriental reed warblers did not discriminate alarm calls issued to different intruder types, but the intensity of the response was significantly higher in the nestling stage than in the egg stage. Attack behavior related to sparrowhawk alarm calls was absent in the egg stage, but aggressive behavior increased dramatically and exceeded the attack frequency in response to the cuckoo alarm call in the nestling stage, implying a shift in the tradeoff between the parents' own survival and the loss of offspring. Alarm calls attracted a larger number of conspecifics than members of other species. In general, the oriental reed warbler had consistently stronger responses to different alarm calls in the nestling stage than in the egg stage, supporting the offspring value hypothesis.

Egg recognition abilities of tit species in the Paridae family: do Indomalayan tits exhibit higher recognition than Palearctic tits?

Recent studies have shown that the closely related cinereous tit ( Parus cinereus) and green-backed tit ( P. monticolus) in China display strong egg recognition ability in contrast to tit species in Europe, which lack such ability. However, egg recognition in other populations of cinereous and green-backed tits and additional Paridae species still requires further research. Here, we compared the egg recognition abilities of cinereous tits across China, green-backed tits ( P. m. insperatus) in Taiwan, China, and five other species from the Paridae family, including the marsh tit ( Poecile palustris), varied tit ( Sittiparus varius), willow tit ( Poecile montanus), coal tit ( Periparus ater), and ground tit ( Pseudopodoces humilis). Results showed that the Hebei (58.8% egg rejection, n=17) and Liaoning populations (53.3%, n=15) of cinereous tits, and the Guizhou (100%, n=12) and Taiwan populations (75%, n=12) of green-backed tits all exhibited high egg recognition ability. The egg recognition ability of these tits was significantly greater than that of the other five species in the Paridae family. The varied tit (5.4%, n=37), marsh tit (8.3%, n=12), willow tit (Hebei: 25%, n=20; Beijing: 9.5%, n=21), coal tit (16.7%, n=18), and ground tit (0, n=5) species all showed low egg recognition abilities, with no significant differences found among them. Egg recognition was not associated with a single phylogenetic group but occurred in several groups of tits. In particular, those species widely distributed in the Indomalayan realm, thus overlapping with small cuckoo species, displayed strong egg recognition ability, whereas tit species in the Palearctic realm exhibited low or no egg recognition ability.

Egg recognition and brain size in a cuckoo host

The evolution of animal brain size and cognitive ability is a topic of central significance in evolutionary ecology. Interspecific brood parasitism imposes severe selection pressures on hosts favoring the evolution of cuckoo egg recognition and rejection. However, recognizing and rejecting foreign parasitic eggs are enormous cognitive challenges for cuckoo hosts, which might select for an increase in brain size in birds with this capacity. To explore the association between cuckoo parasitism and the evolution of brain size in cinereous tits (Parus cinereus), we used two types of experimental parasitic eggs, real mimetic white-rumped munia (Lonchura striata) eggs and non-mimetic blue model eggs, to test the egg recognition ability of female cinereous tits, thereby comparing brain size variation among individuals that were able to recognize foreign eggs and those that lacked this ability. Interestingly, our results however did not support the prediction that cuckoo parasitism selects for an increase in brain size of host birds, since brain size of egg rejecters was not significantly larger than that of accepters. Hence, this study suggested that the evolution of cognitive ability did not allow recognition of foreign eggs by female cinereous tits. That was the case despite the evolution of a larger brain may have allowed for a reduction in the cost of brood parasitism by cuckoos.

Egg polymorphism and highly sensitive egg recognition of cross-phenotypes in rufescent prinias Prinia rufescens as effective defenses against brood parasitism

Whether escalating egg polymorphism increases the efficiency to reduce the fitness costs of brood parasitism is not supported by empirical studies. Rufescent prinias (Prinia rufescens) laying tetramorphic eggs are highly sensitive to conspecific foreign eggs with different phenotypes that they reject perfectly at 100%, and suffer only 1.4% of plaintive cuckoo (Cacomantis merulinus) parasitism. Our study provided empirical evidence showing that escalation of egg polymorphism in hosts increases their fitness because it reduces successful cuckoo parasitism rate.

Model eggs fail to detect egg recognition in host populations after brood parasitism is relaxed

Background

Obligate brood parasites exert strong selective pressure on target hosts. In response, hosts typically evolve anti-parasitism strategies, of which egg recognition is one of the most efficient. Generally, host egg-recognition capacity is determined using model eggs. Previous studies have shown that some host species, which are capable of detecting parasite eggs, do not reject model eggs. However, it is unknown that whether the reaction to model eggs varies among distinct populations of the same host in relation to the degree of parasitism pressure.

Results

Here, we compared the rejection frequencies of model eggs and real eggs between mainland and island populations of the plain prinia (Prinia inornata), which are respectively sympatric and allopatric with their brood parasite, the common cuckoo (Cuculus canorus). Our results indicated that the mainland and island populations rejected real eggs at similar rates, but rejected model eggs, which were similar in size to real eggs but heavier, at significantly different rates: the island population rejected fewer model eggs, possibly because the rejection motivation of this population was lower due to absence of parasitism.

Conclusions

Our results indicated that some factors affecting the decision to reject, such as rejection motivation, varied according to the degree of parasitism pressure, and thus influenced the frequency of egg rejection. Furthermore, our results suggested that model eggs should be used with caution in comparative studies of egg recognition abilities among species or populations subjected to different intensities of brood parasitism. That is, model eggs may fail to accurately detect egg recognition in host populations with little to no risk of parasitism.

Do swallows (Hirundo daurica) use the visual cue of hatchling down-feathers to discriminate parasite alien nestlings?

Egg recognition is a variable but common anti-parasitism defense among different species of birds with brood parasites. In contrast, nestling recognition is rare. Very few studies have found nestling recognition in brood parasite hosts and determined the rejection mechanism behind this behavior. Hosts may use the number of hatchling down-feathers to reject parasite nestlings. We tested whether hatchling down-feathers is a visual cue for the red-rumped swallow, a host that can recognize and reject parasite nestlings. Our results indicated that red-rumped swallows do not recognize foreign nestlings based on hatchling down-feathers. The closed nest structure and hatchling morph may explain the absence of such a mechanism. None of the rejection mechanisms found in previous studies could explain the nestling recognition in swallows. Olfactory cues, tactile cues, or other visual cues, except for single nestling or hatchling down-feathers, may provide nestling recognition in red-rumped swallows. More study is needed to evaluate these possibilities.

Spatial variation in egg polymorphism among cuckoo hosts across 4 continents

Although egg color polymorphism has evolved as an effective defensive adaptation to brood parasitism, spatial variations in egg color polymorphism remain poorly characterized. Here, we investigated egg polymorphism in 647 host species (68 families and 231 genera) parasitized by 41 species of Old Word cuckoos (1 family and 11 genera) across Asia, Europe, Africa, and Australia. The diversity of parasitic cuckoos differs among continents, reflecting the continent-specific intensities of parasitic selection pressure on hosts. Therefore, host egg polymorphism is expected to evolve more frequently on continents with higher cuckoo diversity. We identified egg polymorphism in 24.1% of all host species and 47.6% of all host families. The common cuckoo Cuculus canorus utilized 184 hosts (28.4% of all host species). Hosts of the common cuckoo and of Chrysococcyx species were more likely to have polymorphic eggs than hosts parasitized by other cuckoos. Both the number of host species and the host families targeted by the cuckoo species were positively correlated with the frequency of host egg polymorphism. Most host species and most hosts exhibiting egg color polymorphism were located in Asia and Africa. Host egg polymorphism was observed less frequently in Australia and Europe. Our results also suggested that egg polymorphism tends to occur more frequently in hosts that are utilized by several cuckoo species or by generalist cuckoo species. We suggest that selection pressure on hosts from a given continent increases proportionally to the number of cuckoo species, and that this selection pressure may, in turn, favor the evolution of host egg polymorphism.

Differently sized cuckoos pose different threats to hosts

Hole-nesting tits Parus spp. have been classified as “unsuitable” hosts for cuckoo parasitism because cuckoos cannot enter a cavity if the entrance is too small. However, Chinese tits could reject alien eggs and egg ejection rate increased with the local diversity of parasitic cuckoo species. Antiparasitic behavior among Chinese tits may have evolved due to greater size variation among sympatric cuckoo species. This raises the question of whether differently sized parasitic cuckoos pose different threats to Chinese tits. A green-backed tit Parus monticolus population that is sympatric with Asian emerald cuckoo Chrysococcyx maculatus (eme-cuckoo, small-sized parasite) and common cuckoo Cuculus canorus (com-cuckoo, large-sized parasite), and a cinereous tit P. cinereus population that is only sympatric with com-cuckoo were chosen as study organisms. We observed behavioral response and recorded alarm calls of the 2 tit species to eme-cuckoo, com-cuckoo, chipmunk Tamias sibiricus (a nest predator) and dove Streptopelia orientalis (a harmless control), and subsequently played back alarm calls to conspecific incubating females. In dummy experiments, both tit species performed intense response behavior to chipmunk, but rarely responded strongly to the 3 avian species. In playback experiments, both tit species responded strongly to conspecific chipmunk alarm calls, but rarely responded to dove alarm calls. The intensity of response of incubating female green-backed tits to eme-cuckoo and com-cuckoo alarm calls were similar to that of chipmunk alarm calls, while the intensity to eme-cuckoo alarm calls was higher than the intensity to dove alarm calls which was similar to that of com-cuckoo alarm calls. In contrast, few female cinereous tits responded to eme-cuckoo and com-cuckoo alarm calls. These findings indicated that the threat level of eme-cuckoo was slightly greater than that of com-cuckoo for sympatric green-backed tits, but not for allopatric cinereous tits.

Egg recognition and chick discrimination in colonial breeding birds

Theory predicts that parents will not raise unrelated offsprings. For colonial breeding birds, evolving an ability to recognize their own eggs and chicks can prevent misdirecting parental behaviour. To verify this hypothesis, egg recognition experiments were performed on colonial breeding Chinese pond herons (Ardeola bacchus) and cattle egrets (Bubulcus ibis) in Chengmai and Lingao, tropical Hainan Island, China. Furthermore, a chick discrimination experiment was conducted on Chinese pond herons. In our study area, we did not record conspecific or interspecific brood parasitism in the two heron species. The results showed that Chinese pond herons had similar recognition rates for the four types of experimental eggs and lacked recognition ability. Cattle egrets had variable abilities to recognize different types of foreign eggs and have low egg recognition ability. Chinese pond herons cannot discriminate foreign chicks. Therefore, we suggest that the two species of colonial breeding herons may lack or have low egg recognition ability, and Chinese pond herons show no nestling recognition ability.

High egg rejection rate in a Chinese population of grey-backed thrush (Turdus hortulorum)

everal previous studies have indicated that nest sanitation behavior is a general adaptation in altricial birds, with egg recognition capacity evolving as a specific response to interspecific brood parasitism (IBP). However, a recent study suggested an alternative hypothesis, concluding that conspecific brood parasitism (CBP) selects for egg rejection in thrushes, with IBP as a by-product. In the present study, we used a spectrophotometer to quantify egg coloration and egg mimicry and performed artificial parasitism experiments in the grey-backed thrush (Turdus hortulorum). We showed that individuals of this species rejected 100% of 12 foreign eggs, without IBP or CBP detected. In a review of previous studies, we also discuss possible explanations for the high egg rejection rate in the grey-backed thrush and suggest areas for future study.

Evolutionary significance of antiparasite, antipredator and learning phenotypes of avian nest defence

Avian nest defence, which is expected to serve both antiparasite and antipredator functions, may benefit or be detrimental to birds, although selective forces that potentially operate on nest defence have not been quantified as a whole. Together with fitness values, we analysed two traits of nest defence, intensity and plasticity, in two distantly related passerine species, yellow warbler (Setophaga petechia) in North America and reed warbler (Acrocephalus scirpaceus) in Europe, both favourite host species for brood parasites. Breeders that escaped parasitism were the most vocal among reed warblers, whereas there was no specific defence phenotype that predicted prevention of parasitism in yellow warblers. Breeders that escaped nest predation were, in both species, those with the most distractive response at the first exposure to a nest-threatening event, such as the experimental predation or parasitism simulated at the nest. However, increasing defence intensity benefited yellow warblers but was detrimental to reed warblers, because intense defence responses attracted predators. Adaptiveness of nest defence was revealed by nest defence phenotypes when examined in concert with the seasonal fitness (i.e. measures of reproductive success). Results revealed selective forces favoured yellow warblers with strong defence phenotypes. Opposite forces were instead revealed among reed warblers whose favoured phenotypes were strong, yet less flexible, defenders.

Nest sanitation elicits egg discrimination in cuckoo hosts

Nest sanitation is a nearly universal behavior in birds, while egg discrimination is a more specific adaptation that has evolved to counter brood parasitism. These two behaviors are closely related with nest sanitation being the ancestral behavior, and it has been hypothesized to constitute a preadaptation for egg discrimination. However, previous studies found little evidence to support this hypothesis. Here, we conducted an empirical test of the association between nest sanitation and egg discrimination in the barn swallow (Hirundo rustica) by inserting a single non-mimetic model egg or a non-mimetic model egg plus half a peanut shell into host nests. Compared to the rejection rate of single model eggs, barn swallows significantly increased egg rejection frequency if a half peanut shell was simultaneously introduced. Our result for the first time shows the impact of nest sanitation on egg discrimination and demonstrates that nest sanitation can elicit egg discrimination in hosts of brood parasites. This study provided evidence for nest sanitation being a preadaptation to egg discrimination by facilitating egg rejection, thereby significantly advancing our understanding of avian cognition of foreign objects. Furthermore, we suggest that egg discrimination behavior in many accepters and intermediate rejecters may be lost or diluted. Such egg discrimination can be elicited and restored after nest sanitation, implying a sensitive and rapid phenotypic response to increased risk of parasitism. Our study offers a novel perspective for investigating the role of so-called intermediate rejecter individuals or species in the long-term coevolutionary cycle between brood parasites and their hosts.