Accurate recognition of salient cues is critical for adaptive responses, but the underlying sensory and cognitive processes are often poorly understood. For example, hosts of avian brood parasites have long been assumed to reject foreign eggs from their nests based on the total degree of dissimilarity in colour to their own eggs, regardless of the foreign eggs' colours. We tested hosts' responses to gradients of natural (blue-green to brown) and artificial (green to purple) egg colours, and demonstrate that hosts base rejection decisions on both the direction and degree of colour dissimilarity along the natural, but not artificial, gradient of egg colours. Hosts rejected brown eggs and accepted blue-green eggs along the natural egg colour gradient, irrespective of the total perceived dissimilarity from their own egg's colour. By contrast, their responses did not vary along the artificial colour gradient. Our results demonstrate that egg recognition is specifically tuned to the natural gradient of avian eggshell colour and suggest a novel decision rule. These results highlight the importance of considering sensory reception and decision rules when studying perception, and illustrate that our understanding of recognition processes benefits from examining natural variation in phenotypes.

• MeSH
• barva * MeSH
• hnízdění * MeSH
• ovum MeSH
• ptáci * MeSH
• vaječná skořápka * MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

Brood parasites lay their eggs in other females' nests, leaving the host parents to hatch and rear their young. Studying how brood parasites manipulate hosts into raising their young and how hosts detect parasitism provide important insights in the field of coevolutionary biology. Brood parasites, such as cuckoos and cowbirds, gain an evolutionary advantage because they do not have to pay the costs of rearing their own young. However, these costs select for host defenses against all developmental stages of parasites, including eggs, their young, and adults. Egg rejection experiments are the most common method used to study host defenses. During these experiments, a researcher places an experimental egg in a host nest and monitors how hosts respond. Color is often manipulated, and the expectation is that the likelihood of egg discrimination and the degree of dissimilarity between the host and experimental egg are positively related. This paper serves as a guide for conducting egg rejection experiments from describing methods for creating consistent egg colors to analyzing the findings of such experiments. Special attention is given to a new method involving uniquely colored eggs along color gradients that has the potential to explore color biases in host recognition. Without standardization, it is not possible to compare findings between studies in a meaningful way; a standard protocol within this field will allow for increasingly accurate and comparable results for further experiments.

• MeSH
• biologická evoluce MeSH
• hnízdění * MeSH
• ovum růst a vývoj   MeSH
• ptáci MeSH
• zvířata MeSH
• Check Tag
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• audiovizuální média MeSH
• časopisecké články MeSH
• práce podpořená grantem MeSH

Avian brood parasites lay their eggs in other birds' nests, and hosts can mitigate the fitness cost of raising unrelated offspring by rejecting parasitic eggs. A visually-based cognitive mechanism often thought to be used by hosts to discriminate the foreign egg is to compare it against the hosts' own eggshell by size, shape, maculation, and/or ground coloration (i.e., absolute chromatic contrast). However, hosts may instead discriminate eggs based on their colors along a scale of natural avian eggshell coloration (i.e., directional chromatic contrast). In support of this latter visual process, recent research has found that directional chromatic contrasts can explain some host species' rejection behavior better than absolute chromatic or achromatic contrasts. Here, for the first time, we conducted an experiment in a cavity-nesting host species to test the predictions of these different visual mechanisms. We experimentally parasitized nests of the Common Redstart Phoenicurus phoenicurus, a regular host of a mimetic-egg laying Common Cuckoo Cuculus canorus host-race, using painted, immaculate 3D-printed model eggs in two geographically distant areas (Finland and Czech Republic). We found that directional chromatic contrasts better explained rejection behaviors in both parasitized (Finland) and non-parasitized (Czech Republic) host populations, as hosts rejected eggs that were noticeably browner, but not eggs that were noticeably bluer, than redstart eggs. These results support the paradigm of a single rejection threshold predicted by the directional chromatic contrast model and contribute to a growing generality of these patterns across diverse avian host-brood parasite systems.

• MeSH
• barva MeSH
• biologické modely * MeSH
• hnízdění fyziologie   MeSH
• interakce hostitele a parazita MeSH
• ovum fyziologie   MeSH
• Passeriformes fyziologie   MeSH
• podněty MeSH
• ptáci fyziologie   MeSH
• rozpoznávání (psychologie) MeSH
• vaječná skořápka * MeSH
• zraková percepce fyziologie   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Recognition is considered a critical basis for discriminatory behaviours in animals. Theoretically, recognition and discrimination of parasitic chicks are not predicted to evolve in hosts of brood parasitic birds that evict nest-mates. Yet, an earlier study showed that host reed warblers (Acrocephalus scirpaceus) of an evicting parasite, the common cuckoo (Cuculus canorus), can avoid the costs of prolonged care for unrelated young by deserting the cuckoo chick before it fledges. Desertion was not based on specific recognition of the parasite because hosts accept any chick cross-fostered into their nests. Thus, the mechanism of this adaptive host response remains enigmatic. Here, I show experimentally that the cue triggering this 'discrimination without recognition' behaviour is the duration of parental care. Neither the intensity of brood care nor the presence of a single-chick in the nest could explain desertions. Hosts responded similarly to foreign chicks, whether heterospecific or experimental conspecifics. The proposed mechanism of discrimination strikingly differs from those found in other parasite-host systems because hosts do not need an internal recognition template of the parasite's appearance to effectively discriminate. Thus, host defences against parasitic chicks may be based upon mechanisms qualitatively different from those operating against parasitic eggs. I also demonstrate that this discriminatory mechanism is non-costly in terms of recognition errors. Comparative data strongly suggest that parasites cannot counter-evolve any adaptation to mitigate effects of this host defence. These findings have crucial implications for the process and end-result of host-parasite arms races and our understanding of the cognitive basis of discriminatory mechanisms in general.

• MeSH
• analýza rozptylu MeSH
• diskriminace (psychologie) fyziologie   MeSH
• druhová specificita MeSH
• financování organizované MeSH
• hnízdění fyziologie   MeSH
• lidé MeSH
• mateřské chování fyziologie   MeSH
• otcovské chování MeSH
• regresní analýza MeSH
• rozpoznávání (psychologie) fyziologie   MeSH
• zpěvní ptáci fyziologie   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• mužské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• srovnávací studie MeSH

Insectivorous birds feed upon all developmental stages of herbivorous insects, including insect eggs if larvae and adults are unavailable. Insect egg deposition on plants can induce plant traits that are subsequently exploited by egg parasitoids searching for hosts. However, it is unknown whether avian predators can also use egg-induced plant changes for prey localization. Here, we studied whether great tits (Parus major) and blue tits (Cyanistes caeruleus) are attracted by traits of the Scots pine (Pinus sylvestris) induced by pine sawfly (Diprion pini) egg deposition. We chose this plant - insect system because sawfly egg deposition on pine needles is known to locally and systemically induce a change in pine volatile organic compounds (VOCs), and tits are known to prey upon sawfly eggs. In dual choice laboratory experiments, we simultaneously offered the birds an egg-free control branch and a systemically egg-induced branch. Significantly more birds visited the egg-induced branch first. We confirmed by GC-MS analyses that systemically egg-induced branches released more (E)-β-farnesene compared to control branches. Spectrophotometric analyses showed that control branches reflected more light than egg-induced branches throughout the avian visual range. Although a discrimination threshold model for blue tits suggests that the birds are poor at discriminating this visual difference, the role of visual stimuli in attracting the birds to egg-induced pines cannot be discounted. Our study shows, for the first time, that egg-induced odorous and/or visual plant traits can help birds to locate insect eggs without smelling or seeing those eggs.

• MeSH
• borovice lesní chemie   metabolismus   parazitologie   MeSH
• chování zvířat MeSH
• Hymenoptera růst a vývoj   fyziologie   MeSH
• interakce hostitele a parazita MeSH
• ovum fyziologie   MeSH
• Passeriformes fyziologie   MeSH
• plynová chromatografie s hmotnostně spektrometrickou detekcí MeSH
• seskviterpeny chemie   metabolismus   MeSH
• spektrofotometrie MeSH
• těkavé organické sloučeniny chemie   metabolismus   MeSH
• zraková percepce MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

One of the most effective defenses against avian brood parasitism is the rejection of the foreign egg from the host's nest. Until recently, most studies have tested whether hosts discriminate between own and foreign eggs based on the absolute differences in avian-perceivable eggshell coloration and maculation. However, recent studies suggest that hosts may instead contrast egg appearances across a directional eggshell color gradient. We assessed which discrimination rule best explained egg rejection by great reed warblers Acrocephalus arundinaceus, a frequent host to an egg-mimetic race of common cuckoos Cuculus canorus. We deployed 3D-printed model eggs varying in blue-green to brown coloration and in the presence of maculation. Using visual modeling, we calculated the absolute chromatic and achromatic just-noticeable differences (JNDs), as well as directional JNDs across a blue-green to brown egg color gradient, between host and model eggs. While most model eggs were rejected by great reed warblers, browner eggs were rejected with higher probability than more blue-green eggs, and the rejection probability did not depend on maculation. Directional egg color discrimination shown here and in a suite of recent studies on other host species may shape the cognitive decision rules that hosts use to recognize foreign eggs and affect the course of evolution in parasitic egg mimicry.

• MeSH
• barva * MeSH
• hnízdění fyziologie   MeSH
• Passeriformes MeSH
• vaječná skořápka * MeSH
• vnímání barev fyziologie   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

In birds, the colour, maculation, shape, and size of their eggs play critical roles in discrimination of foreign eggs in the clutch. So far, however, no study has examined the role of egg arrangement within a clutch on host rejection responses. We predicted that individual females which maintain consistent egg arrangements within their clutch would be better able to detect and reject foreign eggs than females without a consistent egg arrangement (i.e. whose eggs change positions more often across incubation). We tested this "egg arrangement hypothesis" in blackbirds (Turdus merula) and song thrush (T. philomelos). Both species are suitable candidates for research on egg rejection, because they show high inter-individual variation and individual repeatability in egg rejection responses. As predicted, using our custom-defined metrics of egg arrangement, rejecter females' clutches showed significantly more consistent patterns in egg arrangement than acceptor females' clutches. Only parameters related to blunt pole showed consistent differences between rejecters and acceptors. This finding makes biological sense because it is already known that song thrush use blunt pole cues to reject foreign eggs. We propose that a disturbance of the original egg arrangement pattern by the laying parasite may alert host females that maintain a consistent egg arrangement to the risk of having been parasitized. Once alerted, these hosts may shift their discrimination thresholds to be more restrictive so as to reject a foreign egg with higher probability. Future studies will benefit from experimentally testing whether these two and other parasitized rejecter host species may rely on the use of consistent egg arrangements as a component of their anti-parasitic defence mechanisms.

• MeSH
• hnízdění * MeSH
• ptáci MeSH
• rozpoznávání (psychologie) MeSH
• rozpoznávání obrazu MeSH
• vejce * MeSH
• velikost snůšky MeSH
• zvířata MeSH
• Check Tag
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

The optimal acceptance threshold hypothesis provides a general predictive framework for testing behavioural responses to discrimination challenges. Decision-makers should respond to a stimulus when the perceived difference between that stimulus and a comparison template surpasses an acceptance threshold. We tested how individual components of a relevant recognition cue (experimental eggs) contributed to behavioural responses of chalk-browed mockingbirds, Mimus saturninus, a frequent host of the parasitic shiny cowbird, Molothrus bonariensis. To do this, we recorded responses to eggs that varied with respect to two components: colour, ranging from bluer to browner than the hosts' own eggs, and spotting, either spotted like their own or unspotted. Although tests of this hypothesis typically assume that decisions are based on perceived colour dissimilarity between own and foreign eggs, we found that decisions were biased toward rejecting browner eggs. However, as predicted, hosts tolerated spotted eggs more than unspotted eggs, irrespective of colour. These results uncover how a single component of a multicomponent cue can shift a host's discrimination threshold and illustrate how the optimal acceptance threshold hypothesis can be used as a framework to quantify the direction and amount of the shift (in avian perceptual units) of the response curve across relevant phenotypic ranges. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

• MeSH
• barva MeSH
• hnízdění * MeSH
• ovum MeSH
• podněty * MeSH
• rozhodování MeSH
• rozpoznávání (psychologie) MeSH
• vnímání barev * MeSH
• zpěvní ptáci parazitologie   fyziologie   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

• MeSH
• biometrie MeSH
• diskriminační analýza MeSH
• ovum MeSH
• Trichuris MeSH

Certain light environments may hinder egg discrimination by hosts of foreign eggs, which could in some circumstances lead to the acceptance of non-mimetic eggs by hosts. We measured light parameters at red bishop (Euplectes orix) nests and used a model of avian visual processing to quantify the detectability of eggs in the light environment in which they are perceived. We found that the overall amount of light was very variable between red bishop nests and always sufficient for colour discrimination. A model of avian visual processing revealed that nest luminosity had no influence on host responses towards eggs which were painted dark brown. Dark eggs do not appear to be cryptic in red bishop nests and can be distinguished with ease, whereas natural red bishop eggs are usually accepted, despite the domed structure of the nest. We found little variation in both chromatic and achromatic contrasts between host and artificial eggs, indicating that there was very little variation in the light quality inside nests. We suggest that nest luminosity is likely to play a role in egg recognition in situations when light reaches threshold values for colour discrimination, i.e. in scotopic as opposed to photopic vision. Rejection rates for dark eggs were higher than for bright (conspecific) foreign eggs. More investigation of domed nest-building species is required, as this type of nest appears to have a highly variable light environment, dependent on both nest structure and habitat.

• MeSH
• barva MeSH
• druhová specificita MeSH
• ekosystém MeSH
• hnízdění fyziologie   MeSH
• interakce hostitele a parazita fyziologie   MeSH
• ovum * MeSH
• paraziti MeSH
• Passeriformes fyziologie   MeSH
• rozpoznávání (psychologie) MeSH
• světlo * MeSH
• vaječná skořápka chemie   MeSH
• zraková percepce fyziologie   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH