Common cuckoo females remove more conspicuous eggs during parasitism

Avian obligate brood parasites gain an advantage by removing the eggs of the cuckoos who have already visited the nest, which can increase the chances of survival for their offspring. Conversely, to prevent their eggs from being picked up by the next parasitic cuckoo, they need to take some precautions. Egg mimicry and egg crypsis are two alternative strategies to prevent the parasitized egg from being picked up by another parasitic cuckoo. Here, we tested whether the egg crypsis hypothesis has a preventative effect when common cuckoos ( Cuculus canorus ) parasitize their Oriental reed warbler ( Acrocephalus orientalis ) hosts. We designed two experimental groups with different crypsis effects to induce common cuckoos to lay eggs and observed whether the cuckoos selectively picked up the experimental eggs with low crypsis levels in the process of parasitism. Our results supported the egg crypsis hypothesis; the observed cuckoos significantly preferred to select the more obvious white model eggs. This shows that even in an open nest, eggs that are adequately hidden can also be protected from being picked up by cuckoo females during parasitism so as to increase the survival chance of their own parasitic eggs.

Higher-level pattern features provide additional information to birds when recognizing and rejecting parasitic eggs

Despite a recent explosion of research on pattern recognition, in both neuroscience and computer vision, we lack a basic understanding of how most animals perceive and respond to patterns in the wild. Avian brood parasites and their hosts provide an ideal study system for investigating the mechanisms of pattern recognition. The cuckoo finch, Anomalospiza imberbis , and its host the tawny-flanked prinia, Prinia subflava , lay highly polymorphic eggs with a great deal of variation in colour and patterning, with the cuckoo finch capable of close egg mimicry. Behavioural experiments in Zambia have previously shown that prinias use colour and multiple ‘low-level’ (occurring in early stages of visual processing) pattern attributes, derived from spatial frequency analysis, when rejecting foreign eggs. Here, we explore the extent to which host birds might also use ‘higher-level’ pattern attributes, derived from a feature detection algorithm, to make rejection decisions. Using a SIFT-based pattern recognition algorithm, N ature P attern M atch , we show that hosts are more likely to reject a foreign egg if its higher-level pattern features—which capture information about the shape and orientation of markings—differ from those of the host eggs. A revised statistical model explains about 37% variance in egg rejection behaviour, and differences in colour, low-level and higher-level pattern features all predict rejection, accounting for 42, 44 and 14% of the explained variance, respectively. Thus, higher-level pattern features provide a small but measurable improvement to the original model and may be especially useful when colour and low-level pattern features provide hosts with little information. Understanding the relative importance of low- and higher-level pattern features is a valuable goal for future work on animal coloration, especially in the contexts of mimicry, camouflage and individual recognition. This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.

Cryptic cuckoo eggs hide from competing cuckoos

Interspecific arms races between cuckoos and their hosts have produced remarkable examples of mimicry, with parasite eggs evolving to match host egg appearance and so evade removal by hosts. Certain bronze-cuckoo species, however, lay eggs that are cryptic rather than mimetic. These eggs are coated in a low luminance pigment that camouflages them within the dark interiors of hosts' nests. We investigated whether cuckoo egg crypsis is likely to have arisen from the same coevolutionary processes known to favour egg mimicry. We added high and low luminance-painted eggs to the nests of large-billed gerygones ( Gerygone magnirostris ), a host of the little bronze-cuckoo ( Chalcites minutillus ). Gerygones rarely rejected either egg type, and did not reject natural cuckoo eggs. Cuckoos, by contrast, regularly removed an egg from clutches before laying their own and were five times more likely to remove a high luminance model than its low luminance counterpart. Given that we found one-third of all parasitized nests were exploited by multiple cuckoos, our results suggest that competition between cuckoos has been the key selective agent for egg crypsis. In such intraspecific arms races, crypsis may be favoured over mimicry because it can reduce the risk of egg removal to levels below chance.

Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host–parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird ( Molothrus rufoaxillaris ) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing ( Agelaioides badius ). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds ( Molothrus bonariensis ) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.