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Individual identification is crucial for studying animal ecology and evolution. In birds this is often achieved by capturing and tagging. However, these methods are insufficient for identifying individuals/species that are secretive or difficult to catch. Here, we employ an automatic analytical approach to predict the identity of bird females based on the appearance of their eggs, using the common cuckoo (Cuculus canorus) as a model species. We analysed 192 cuckoo eggs using digital photography and spectrometry. Cuckoo females were identified from genetic sampling of nestlings, allowing us to determine the accuracy of automatic (unsupervised and supervised) and human assignment. Finally, we used a novel analytical approach to identify eggs that were not genetically analysed. Our results show that individual cuckoo females lay eggs with a relatively constant appearance and that eggs laid by more genetically distant females differ more in colour. Unsupervised clustering had similar cluster accuracy to experienced human observers, but supervised methods were able to outperform humans. Our novel method reliably assigned a relatively high number of eggs without genetic data to their mothers. Therefore, this is a cost-effective and minimally invasive method for increasing sample sizes, which may facilitate research on brood parasites and other avian species. [ABSTRACT FROM AUTHOR]

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The use of active chemical defence against predators is relatively rare in birds. Among others, it has been reported for some members of family Cuculidae whose chicks, when threatened, expel dark foul-smelling liquid from their cloaca. Apart from the brood parasitic great spotted cuckoo Clamator glandarius, however, this phenomenon has not yet been systematically studied in any other cuckoo species. Here we investigated the repellent behaviour in the evicting brood parasite, the common cuckoo Cuculus canorus, parasitizing the great reed warbler Acrocephalus arundinaceus. We explored whether production of secretions varies with chick age or size, and tested its presumed repellent function against various types of predators. We found that the production of secretions commenced at the age of approximately eight days, then gradually increased and decreased again shortly before fledging. Furthermore, we experimentally confirmed a more intensive repellent effect of the secretions on mammal predators than on avian predators, such as raptors and owls. The secretions have, however, no effect on corvid predators, probably because these scavengers often consume malodorous food. Further experimental studies together with phylogenetic comparative analyses are needed to elucidate the origin and function of this intriguing phenomenon both in parasitic and non-parasitic cuckoos. [ABSTRACT FROM AUTHOR]

In recent decades, numerous studies have examined factors affecting risk of host nest parasitism in well-known avian host-parasite systems; however, little attention has been paid to the role of host nest availability. In accordance with other studies, we found that nest visibility, reed density and timing of breeding predicted brood parasitism of Great Reed Warblers Acrocephalus arundinaceus by the Common Cuckoo Cuculus canorus. More interestingly, hosts had a greater chance of escaping brood parasitism if nesting was synchronized. Cuckoo nest searching was governed primarily by nest visibility at high host-nest density. However, even well-concealed nests were likely to be parasitized during periods when just a few hosts were laying eggs, suggesting that Cuckoos adjust their nest-searching strategy in relation to the availability of host nests. Our results demonstrate that host vulnerability to brood parasitism varies temporally and that Cuckoo females are able to optimize their nest-searching strategy. Moreover, our study indicated that Cuckoos always manage to find at least some nests to parasitize. Thus, in this case, the co-evolutionary arms race should take place mainly in the form of parasitic egg rejection rather than via frontline pre-parasitism defence. [ABSTRACT FROM AUTHOR]

Batesian mimicry in brood parasites is often viewed as an evolutionary strategy to mitigate host aggression. Female common cuckoos ( Cuculus canorus ) exhibit two morphs: the hawk-like grey and the rufous one, potentially maintained by apostatic selection. It was hypothesized that the grey morph's predator-like appearance deters host defences, while the rufous morph benefits from its rarity by evading host attention. Previous research predominantly utilized static cuckoo dummies, lacking insights into real-world interactions. We investigated the effectiveness of the cuckoo morphs in accessing great reed warbler ( Acrocephalus arundinaceus ) nests under natural conditions. Analysing video-recorded cuckoo attempts, we found no significant difference in nest-access success between the morphs. Both experienced a similar probability of physical attacks when hosts were present, and the rufous morph did not evade host detection more often compared with the grey morph. These results fail to support the assumptions of (a) Batesian mimicry, that hawk-like mimicry enhances nest access or reduces host aggression, and (b) apostatic selection, that the rarity of the rufous morph confers an advantage in successfully accessing the host nest. Future research should aim to identify stages in the cuckoo's life cycle or host interactions where colour polymorphism provides an evolutionary benefit.