Nozomu J. Sato

The Egg Dilution Effect Hypothesis: A Condition Under which Parasitic Nestling Ejection Behaviour will Evolve

Abstract Recent research into the Large-billed Gerygone, a host species of the Little Bronze-Cuckoo, has revealed that the host physically ejects parasitic cuckoo young from its nest, a behaviour not previously observed in any other host-brood parasite system. Curiously, this host does not reject dissimilar foreign eggs despite egg rejection seeming to be a better strategy for it because if successful, there is no risk of the warblers own eggs being ejected by cuckoo young. In order to explain this puzzle, we present a new hypothesis, termed the egg dilution effect, which argues that parasitic cuckoo eggs serve to ensure host egg survival through the dilution effect and protect against parasitism by multiple females. It is therefore beneficial for hosts to accept cuckoo eggs even if they are capable of discriminating cuckoo eggs from their own. The conditions of this hypothesis, such as small clutch size and high parasitism rate, fit the nature of the host species and thus help explain why this unique anti-parasitic strategy has evolved only in the Large-billed Gerygone.

Breeding seasons of landbirds in New Caledonia

Abstract Based on 805 observations of 63 species, we identified the main breeding seasons of avian trophic guilds in New Caledonia. Most omnivorous and carnivorous birds had undefined breeding periods with moderate peaks during the warm/dry season and the hot/rainy season. In contrast, nectarivorous, frugivorous, granivorous and insectivorous species had marked breeding seasons that started at the beginning of the warm/dry season and ended in the hot/rainy season. Introduced species also reproduced during these periods, regardless of the trophic guild to which they belonged.

Mating system and extra-pair paternity in the Fan-tailed Gerygone Gerygone flavolateralis in relation to parasitism by the Shining Bronze-cuckoo Chalcites lucidus

Extra-pair copulation can increase genetic diversity and offspring fitness. However, it may also increase intra-nest variability in avian hosts of brood parasites, which can decrease the discrimination ability of host parents towards the parasite. In New Caledonia, the Fan-tailed Gerygone (Gerygone flavolateralis), which is parasitized by the Shining Bronze-cuckoo (Chalcites lucidus), has two nestling morphs, dark and bright, that can occur in monomorphic and polymorphic broods. Gerygone parents recognize and eject parasite nestlings from their nest, but the presence of polymorphic broods may increase the chances of recognition errors. Using 17 microsatellite markers, we investigated the mating system of the Fan-tailed Gerygone to understand the mechanisms underlying nestling polymorphism. We hypothesised that extra-pair copulations would lead to a higher proportion of polymorphic broods caused by higher genetic variability, thus creating a trade-off between genetic benefits and host defence reliability. Extra-pair paternity occurred in 6 of 36 broods, which resulted in 6 of 69 offspring sired by extra-pair males. Broods with and without mixed paternity were comparably often parasitized. Extra-pair paternity did not influence the proportions of bright, dark and polymorphic broods. Compared to bright siblings in polymorphic broods, dark nestlings tended to have lower heterozygosity, particularly in loci associated with skin coloration. The results also suggested that there is no obstacle for genetic exchange between individuals from forest and savannah, possibly due to dispersal of offspring. We conclude that the Fan-tailed Gerygone is a socially monogamous species with a low rate of extra-pair paternity compared to closely related species. Extra-pair paternity increased offspring genetic variability without measurable associated costs by brood parasitism. The results highlight the importance of studying host mating systems to assess the trade-offs between host defence and offspring fitness in co-evolutionary arms races.

Establishment of Microsatellite Markers to Assess the Mating System of the Fan-Tailed Gerygone (Gerygone flavolateralis) for Studying Cuckoo-Host Arms Race

We developed a set of microsatellite markers for the fan-tailed gerygone (Gerygone flavolateralis), host of a brood parasite, the shining bronze-cuckoo (Chalcites lucidus). We used 454 pyro-sequencing to establish 17 polymorphic microsatellite markers. The number of alleles per locus ranged from 6 to 18, the expected heterozygosity from 0.328 to 0.931 and the polymorphism information content from 0.306 to 0.916. The developed set of microsatellites will allow us to determine the gerygones mating system, which is crucial to understand inheritance of dichromatism in skin colour of their chicks.

Gourmand New Caledonian crows munch rare escargots by dropping

Numerous broken shells of a rare endemic snail Placostylus fibratus, a species rated as vulnerable, were scattered around rocky beds of dry creeks in rainforest of New Caledonian. We set a video camera near one site to identify the predator. We recorded a New Caledonian crow Corvus moneduloides dropping and consuming the snail. This is the first direct evidence of the crow dropping of this native species. The assumed weight of snails relative to the crow is exceptionally heavy among prey dropped by birds.

Visual discrimination of polymorphic nestlings in a cuckoo-host system

Mimicry by avian brood parasites favours uniformity over variation within a breeding attempt as host defence against parasitism. In a cuckoo-host system from New Caledonia, the arms race resulted in both host (Gerygone flavolateralis) and parasite (Chalcites lucidus) having nestlings of two discrete skin colour phenotypes, bright and dark. In our study sites, host nestlings occurred in monomorphic and polymorphic broods, whereas cuckoo nestlings only occurred in the bright morph. Irrespective of their brood colour, host parents recognised and ejected parasite nestlings but never ejected their own. We investigated whether host parents visually recognised their own nestlings by using colour, luminance and pattern of multiple body regions. We found that the parasite mimicked multiple visual features of both host morphs and that the visual difference between host morphs was larger than the difference between the parasite and the mimicked host morph. Visual discrimination alone may result in higher chances of recognition errors in polymorphic than in monomorphic host broods. Host parents may rely on additional sensorial cues, not only visual, to assess nestling identity. Nestling polymorphism may be a trace of evolutionary past and may only have a marginal role in true-recognition of nestlings in the arms race in New Caledonia.

Puddle use by New Caledonian rainforest birds

Abstract The water requirements of birds in humid areas are not well understood. Even rainforests have dry seasons, during which birds might experience water shortages. We surveyed use of puddles as a water source for birds in a New Caledonian rainforest during the dry season using camera traps. We found that birds frequently used puddles, and that especially granivorous and omnivorous birds were frequent water users. Almost all bird species visited puddles for bathing, but granivorous birds drank water significantly more often than other bird species. Our results suggest that even in tropical rainforests, birds depend on surface water.

Response to Comments on “The Egg Dilution Effect Hypothesis Revisited”

(Received 17 February 2017; Accepted 13 April 2017) # Corresponding author, E-mail: sato@bird-research.jp While many hosts of the Common Cuckoo Cuculus canorus accept parasitic cuckoo nestlings, the Largebilled Gerygone Gerygone magnirostris, a host of the Little Bronze-Cuckoo Chalcites minutillus, ejects cuckoo nestlings from its nest (Sato et al. 2010a). In Sato et al. (2010b), illustrating a simple theoretical model, we presented a new hypothesis for explaining the existence of these two different types of host (egg ejector and chick ejector). Our theoretical model showed that the egg dilution effect increases the relative pay-off of chick ejectors compared with egg ejectors, which supports the evolution of chick ejection behavior under certain conditions. Grim (2017) has criticized the five assumptions of our model as unrealistic. We welcome his criticisms because they provide an opportunity for us to explain our model in detail and because his criticism and our counter comments will deepen the discussion of the key factors driving the evolution of chick ejection behavior. In our response to the comments by Grim (2017), we claim that: the five assumptions are to simplify the model and are not essential assumptions that qualitatively alter the conclusion of our model, which is that accepting a parasitic egg, but rejecting parasitism during the chick stage, could be adaptive. Even if the five assumptions are not completely met, we think that our model is still useful and provides a novel insight into why some hosts reject parasitism at the chick stage, not at the egg stage. The main aim of our model has been to assess the influence on hosts of multiple parasitism, frequency of parasitism, and clutch size, in the evolution of anti-parasitism behavior. We therefore did not consider other possible factors such as predation. Grim’s opinion 1): Real parasitism rates of host species of the common cuckoo were lower than presented We are confused as to why Grim (2017) criticizes our model based on the low parasitism rates often observed in Europe (e.g., involving the Great Reed Warbler Acrocephalus arundinaceus and the Common Cuckoo). The egg dilution hypothesis we proposed (Sato et al. 2010b) targets a host-brood parasite system in tropical Australia involving the Largebilled Gerygone and the Little Bronze-Cuckoo, that is very different in many aspects from the Great Reed Warbler and Common Cuckoo interactions. We agree with Grim’s (2017) opinion that we did not present true parasitism rates, as we did not use parasitism rates of populations that are not parasitized. In addition, we cited certain data on parasitism rates with the understanding that some were higher than average. In Sato et al. (2010b), we aimed to show that the parasitism rates of host species by Cuculidae cuckoos were relatively lower than those affecting the Large-billed Gerygone (Fig. 2b in Sato et al. 2010b). In our hypothesis, multiple parasitism, resulting from a high parasitism rate, is a key factor driving the evolution of chick ejection behavior in the Large-billed Gerygone. Therefore, the low parasitism rates observed in the host species of Cuculus, Chalcites and Chrysococcyx cuckoos is convincing evidence in support of our hypothesis (chick ejection is not adaptive in interactions with low parasitism rates). Even if the parasitism rate Grim (2017) has presented is more accurate (i.e. even if the real parasitism rate is lower than we presented), it does not undermine our hypothesis so much as support it.