Abstract In the coevolutionary interactions between avian brood parasites and their hosts, egg recognition based on color and/or pattern is a common and effective defense to counter parasitism. However, for egg recognition based on size, only a few studies have found affirmative results, and they do not provide unambiguous evidence that egg size recognition in hosts has evolved as an important and specific anti-parasite adaptation against parasite eggs. We studied the brood parasite system between the Asian emerald cuckoo Chrysococcyx maculatus and its host, the chestnut-crowned warbler Phylloscopus castaniceps. The cuckoo parasitizes the warbler using non-mimetic and larger eggs at a parasitism rate of 12.9%. The warbler nests used in this experiment were built in a dark environment with the nest illuminance near 0 lux. Experiments with 2 types of model eggs with colors and patterns resembling cuckoo eggs of different sizes (cuckoo egg size or host egg size) showed that the warblers were able to reject 63.6% of cuckoo model eggs under these dim light conditions. However, model eggs with the same color and pattern similar to the warbler egg size were always accepted. This study provides strong evidence supporting the theory that egg size recognition can be evolved in hosts as a specific anti-parasite adaptation against cuckoos. We suggest that the egg size recognition of the warbler is an outcome of the tradeoff between the costs of violating the parental investment rule and suffering cuckoo parasitism.

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Egg rejection based on egg size recognition as a specific strategy against parasitic cuckoos

Nest parasitism by cuckoos (Cuculus spp.) results in enormous reproductive failure and forces hosts to evolve antiparasitic strategies, i.e., recognition of own eggs and rejection of cuckoo eggs. There are often sexual conflicts between male and female individuals in the expression of antiparasitic behavior due to the differences in reproductive inputs and division of labor. By adding a foreign egg made of blue soft clay to the host nest during early incubation period in the field, and by removing several host eggs and adding experimental eggs to control the proportion of two egg types in the nest, we examined egg rejection ability, egg recognition mechanism and sexual difference in egg rejection of the Oriental Reed Warbler (Acrocephalus orientalis), one of the major hosts of Common Cuckoos (Cuculus canorus). Our results indicated that Oriental Reed Warblers can recognize and reject nearly 100% (73/75) of the non-mimetic eggs made of blue soft clay, and they could reject foreign eggs with 100% accuracy, regardless of the ratio of experimental eggs and its own eggs in the nest. Furthermore, all cases of egg rejections recorded by videos were only carried out by females. Oriental Reed Warblers have a high egg recognition ability and show a true recognition mechanism. Only female warblers perform egg rejection, suggesting that the sex for host egg incubation seems to play an important role in the evolution of egg recognition mechanisms.

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Egg rejection and egg recognition mechanisms in Oriental Reed Warblers

Thrush species are rarely parasitized by cuckoos, but many have a strong egg recognition ability. To date, there is a limited understanding of the relationship between host egg rejection and cuckoo parasitism rate. By using egg experiments in the field, we compared egg rejection between two non-parasitized potential host species and two parasitized hosts of cuckoos in the same region. The White-bellied Redstart (Luscinia phoenicuroides), a host of the Common Cuckoo (Cuculus canorus), rejected 66.6% of blue model eggs; the Elliot’s Laughingthrush (Trochalopteron elliotii), a host of the Large Hawk Cuckoo (Hierococcyx sparverioides), rejected 25% of blue model eggs and 46.1% of white model eggs; and the Chestnut Thrush (Turdus rubrocanus) and the Chinese Thrush (T. mupinensis), in which cuckoo parasitism has not been recorded, rejected 41.1 and 83.3% of blue model eggs, respectively. There were no significant differences in the egg rejection among them, although the Chinese Thrush showed the highest rate of egg rejection. This study indicates that the egg recognition ability of cuckoo hosts has no correlation with the actual parasitism rate of cuckoos. We suggest that the egg recognition ability of the two potential host species may have been retained from a parasitic history with the cuckoo, while the two common host species have developed their egg rejection abilities due to current parasitism pressure. In addition, our study highlights the importance of the multi-cuckoo parasite system for better understanding the selection pressure of parasitism on the evolution of host egg recognition abilities.

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Potential cuckoo hosts have similar egg rejection rates to parasitized host species

Parasites often manipulate host behaviors to achieve their own selfish fitness goals. However, the efficiency with which parasitic begging calls solicit foster parental care has not yet been compared across different avian obligate host-brood parasite systems. For example, the begging calls of nestmate-evictor parasites are predicted to solicit sufficient levels of foster parental provisioning for the sole parasitic chick in the nest, whereas the calls of nest-sharing parasites combine together with the begging by host nestmates to solicit provisions for the whole brood. We studied the red-winged blackbird (Agelaius phoeniceus), a host of the brood parasitic brown-headed cowbird (Molothrus ater), to test the effects of begging call playbacks of nestmate-evictor common cuckoos (Cuculus canorus) or nestmate-sharing cowbirds upon parental feeding patterns. As predicted, cuckoo calls elicited more parental feeding trips and the amount of food delivered than cowbird calls. Food provisioning to nestlings while hearing cuckoo begging calls was similar to that while hearing redwing calls, and both were higher relative to a control silence treatment, which suggested that the begging call acoustics of the cuckoo served as an efficient stimulus for parental care in this cuckoo-naive host. The study also confirms that cowbird calls are not supernormal stimuli for parental provisions in this host species, as redwing begging calls themselves were similarly effective stimulants for conspecific parental provisioning behaviors. Future research should examine the acoustical bases and perceptual biases present in the cuckoo’s begging calls to efficiently solicit parental provisioning in both naive and coevolved hosts. Nestlings of different avian brood parasites use distinct strategies to survive and solicit parental provisions by heterospecific foster parents: some of them evict or otherwise kill host nestmate to grow up alone in the nest (e.g., common cuckoos), whereas others typically share the nest with host chicks (e.g., brown-headed cowbirds). However, the efficiency to solicit parental care through begging calls by brood parasitic young with respect to their nestmate-evictor versus nest-sharing strategies has not yet been evaluated. We focused on a cuckoo-naive and cowbird-exposed host species, the red-winged blackbird, to compare the effects of begging call playbacks of parasites and conspecifics, relative to silence controls. We found that the begging calls of cuckoos, rather than the cowbirds, elicited more parental feeding in this cuckoo-naive host. This implied that it was the nestmate-evictor strategy, rather than host familiarity, that improved the efficiency of the parasitic begging calls. Our results provide the first experimental data on the efficiency of begging calls between cuckoos and cowbirds within the same host species, and it contributes to our understanding of the divergent ontogenetic strategies of evolutionarily distinct avian brood parasites.

Parasitic begging calls of nestmate-evictor common cuckoos stimulate more parental provisions by red-winged blackbirds than calls of nest-sharing brown-headed cowbirds

The arms race between brood parasites and their hosts provides a classic model to study coevolution. Hosts often reject the parasitic egg, and brood parasites should therefore select host nests in which the colour of the eggs best matches that of their own. Although this hypothesis has received some support, direct experimental evidence is still lacking. Here, we report on a study of Daurian redstarts, which show a distinct egg-colour dimorphism, with females laying either blue or pink eggs. Redstarts are often parasitized by common cuckoos, which lay light blue eggs. First, we showed that cuckoo eggs were more similar in spectral reflectance to the blue than to the pink redstart egg morph. Second, we report that the natural parasitism rate was higher in blue than in pink host clutches. Third, we performed a field experiment in which we presented a dummy clutch of each colour morph adjacent to active redstart nests. In this set-up, cuckoos almost always chose to parasitize a blue clutch. Our results demonstrate that cuckoos actively choose redstart nests in which the egg colour matches the colour of their own eggs. Our study thus provides direct experimental evidence in support of the egg matching hypothesis.

Experimental evidence that cuckoos choose host nests following an egg matching strategy

Background Among potential hosts, the rejection of foreign eggs, which is a common and effective strategy to counter brood parasitism, depends on egg recognition. Multimodal and multicomponent recognition cues of brood parasitic eggs, which include both tactile (size, shape, and texture) and visual (size, shape, color, and maculation) cues, are potentially involved in the perception and discrimination of foreign eggs by hosts. An egg rejection experiment on the host with different types of model eggs can help to accurately assess the relative contribution of different components on egg recognition and constraints to rejection, in which videos can help identify the method of host rejection. Methods Here, we assessed egg recognition and rejection responses by Oriental reed warblers (Acrocephalus orientalis), one of the most common hosts of common cuckoos (Cuculus canorus) which breed in eastern China. We designed six groups of model eggs for rejection experiments in which sensory cues included three grades of size and two categories of visual mimicry. Results Our experiments confirmed that the multimodal traits, which included variation in size, were significant predictors of egg rejection: We detected significantly higher rejection rates of mimetic spotted model eggs than of nonmimetic blue eggs. However, large model eggs did not yield higher rejection rates and, instead, these were less likely to be rejected and more likely to be deserted compared with smaller eggs. Further video-recording data showed that there was no significant effect of egg size on the egg recognition rate (percentage of nests with evidence of egg pecking). No evidence that the egg appearance had an effect on the method of egg rejection (ejection or nest desertion) was found. Conclusions Only visual signals, such as color and maculation, contributed to the recognition of foreign eggs by Oriental reed warblers as recognizable clues, but not the egg size. The egg size had an impact on the type of egg rejection. It was less feasible for the warblers to eject large eggs and that is why they opted more often for desertion as the mean of model egg rejection. The significantly lower egg rejection rate of large eggs suggested that although some of them were recognized as foreign eggs, hosts failed to reject these eggs and finally the eggs were assumed to being accepted by the commonly used nest-checking methods.

Contrasting effects of egg size and appearance on egg recognition and rejection response by Oriental reed warblers.

Lack of fine-tuned egg rejection adjustment in barn swallows with variable local abundance of common cuckoos.

Background Infective endocarditis (IE), though uncommon, is a potentially lethal disease. Blood culture-negative endocarditis (BCNIE) accounts for 2.5%–31% of all cases of IE and can lead to life-threatening complications, including aortic root pseudoaneurysm. It is associated with considerable diagnostic and therapeutic dilemmas. TrueVue and TrueVue Glass include the latest two technologies applied in advanced three-dimensional echocardiography, which allow for novel photorealistic images of cardiac structures, and provide abundant previously unavailable diagnostic information. Herein, based on a series of novel three-dimensional echocardiographic methods, we report a case of BCNIE with aortic valve involvement, leading to aortic valve perforation and prolapse, and developing into a giant aortic root pseudoaneurysm. Case summary In this study, we presented a case of a 64-year-old man exhibiting symptoms of intermittent fever, asthenia, and dyspnea following light exertion. Physical examination, laboratory tests, and electrocardiograms were suspected of IE, though the results of blood cultures were exactly negative. Three-dimensional transthoracic echocardiography, as well as a series of novel advanced techniques, was adopted to clearly visualize the lesions of the aortic valve and aortic root. However, despite active medical treatment modalities, the patient eventually suffered from a sudden, unexpected death 5 days later. Conclusion BCNIE with aortic valve involvement and development into a giant aortic root pseudoaneurysm is a rare and serious clinical event. In addition, TrueVue and TrueVue Glass offer unprecedented photographic stereoscopic images, enhancing the diagnostic performance of such structural heart diseases.

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Case report: Novel three-dimensional echocardiographic methods mapping aortic root pseudoaneurysm secondary to blood culture-negative endocarditis with bicuspid aortic valve involvement

Shuang Guan

Papers

Contrasting effects of egg size and appearance on egg recognition and rejection response by Oriental reed warblers.

Lack of fine-tuned egg rejection adjustment in barn swallows with variable local abundance of common cuckoos.

Case report: Novel three-dimensional echocardiographic methods mapping aortic root pseudoaneurysm secondary to blood culture-negative endocarditis with bicuspid aortic valve involvement