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

Hosts eject conspecific parasitic eggs according to the egg size in a passerine

Conspecific brood parasitism (CBP) is an alternative reproductive tactic in egg-laying species, where a female lays her eggs in the nest of a conspecific. In a population of spotless starlings (Sturnus unicolor), we observed that some eggs are ejected from the nest during CBP events. It is unclear whether this ejection is a parasitic strategy (a host egg is ejected) or an anti-parasitism strategy (the parasitic egg is ejected). To clarify this, we genotyped the eggs ejected on the ground and found that 100% of them were parasitic. Egg discrimination might be based on tactile or visual cues, and we hypothesized that egg size could be used by hosts to eject parasitic eggs. We conducted experiments in the field using dummy eggs of varying sizes. The results showed that starlings were more likely to eject eggs if they were smaller than their own eggs. In contrast, no significant pattern of egg ejection was observed for larger eggs. Our results suggest that starlings use egg volume recognition as an anti-parasitism strategy to avoid the costs of parasitism. Whether this is a frequency-dependent strategy is worth further studies.

Egg shape and color mediate acceptance thresholds in diverse avian host species with different rates of antiparasitic egg rejection

Eggshell recognition in parental birds is vital for nest management, defense against brood parasitism, optimal embryonic development and minimizing disease and predation risks. This process relies on acceptance thresholds balancing the risk of rejecting own eggs against the benefit of excluding foreign ones, following signal detection theory. We investigated the role of object shape in egg rejection decisions among three host species of the obligate brood parasitic brown-headed cowbird (Molothrus ater), each with a varying known response to parasitic eggs. Following previous studies on the American robin (Turdus migratorius; a robust cowbird-egg rejecter), we presented Eastern bluebirds (Sialia sialis; moderate rejecter) and red-winged blackbirds (Agelaius phoeniceus; an accepter) with 3D-printed blue model eggs varying in width or angularity. To examine the shape-color interaction, we also presented Eastern bluebirds with these series in two colors and maculation: light blue (bluebird mimetic) and white with speckles (more cowbird-like). Both American robins and Eastern bluebirds were less likely to accept blue models as their width decreased. For the blue angularity series, acceptance decreased significantly with increased angularity for the red-winged blackbird, as has been previously seen in the American robin, but not for the Eastern bluebird. For bluebirds with the white-maculated models, these patterns remained but statistical significance reversed: acceptance did not decrease significantly with width, but did decrease significantly with angularity. These results suggest that egg shape variation influences antiparasitic egg rejection behaviors, is modulated by shell color and maculation patterns, and varies among different host species, highlighting the complexity of behavioral defense cues against brood parasitism.

Common cuckoo eggs are more resistant to puncture by the host

The puncture resistance hypothesis suggests that thick-shelled eggs of parasitic birds can resist puncture-ejection by the host. However, few experiments have yet been conducted to test this hypothesis in terms of natural host behavior (e.g., pecking at foreign eggs). To explore whether the eggshells of common cuckoos (Cuculus canorus) are resistant to puncture-ejection by their common hosts, Oriental reed warblers (Acrocephalus orientalis), we designed experiments to investigate if and how breeding Oriental reed warblers peck at foreign eggs that includes common cuckoo, Oriental reed warbler and budgerigar (Melopsittacus undulatus) eggs. The results showed that, given the same frequency of egg pecking, the probability of eggshell breakage was 87.5% for eggs of the Oriental reed warbler and 0% for eggs of the common cuckoo, with a significant difference (P = 0.001, Fisher's exact test). Our study shows clearly that common cuckoos' eggshells are less susceptible to puncture-ejection than those of Oriental reed warblers and budgerigars. This indicates that the eggshells of common cuckoos can resist host Oriental reed warblers' puncture-ejection, supporting the puncture resistance hypothesis.

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.

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

In the co-evolutionary 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 with 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 two types of model eggs with colors and patterns resembling cuckoo eggs 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 evolve 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.