Nest sanitation as the evolutionary background for egg ejection behaviour and the role of motivation for object removal

Egg Retrieval as a Cognitive Indicator in Cuckoo Hosts

Egg retrieval behavior in hosts within avian brood parasitism systems was found to be regulated by the motivation to reject parasitic eggs. However, due to the limitations in the research systems, there is a lack of effective validation regarding the adaptation mechanisms of cuckoo hosts to the conflict between retrieving their own eggs outside the nest and rejecting parasitic eggs. This study uses Daurian redstarts (Phoenicurus auroreus), a secondary cavity-nesting host parasitized by common cuckoos (Cuculus canorus), to verify the adaptive decision-making of the host between egg retrieval and egg rejection by simulating the occurrence of eggs outside the nest. The results showed that Daurian redstarts ignored 60.6% of highly mimetic conspecific eggs, with a retrieval rate of only 18.2%. Additionally, Daurian redstarts rejected 21.2% of conspecific eggs. However, non-mimetic budgerigar (Melopsittacus undulatus) and white model eggs were more likely to be directly rejected (75% and 86.4%, respectively) with no retrieval events. Our findings suggest that egg retrieval behavior in Daurian redstarts is likely influenced by the cognitive process of rejecting parasitic eggs, leading to occasional over-identification and difficulty in decision-making between egg retrieval and egg rejection, especially in the context of conflicting motivations.

Egg Rejection and Nest Sanitation in an Island Population of Barn Swallows (Hirundo rustica): Probability, Response Latency, and Sex Effects

Bird nests function as vessels for eggs and nestlings, and an environment for rearing offspring. However, foreign objects falling into bird nests and nestling eggshells may be harmful. Moreover, the smell of fecal sacs increases the risk of detection by predators. Many bird species have evolved nest sanitation to prevent damage to their nests. Furthermore, egg rejection evolved in some birds to thwart brood parasites that lay eggs in their nests. We studied 133 nests of barn swallows (Hirundo rustica) in an island population through a nest content manipulation experiment to determine nest sanitation and egg rejection behaviors and their relationship. Swallows rejected non-egg foreign objects more frequently (100% vs. 58.6%) and sooner than parasite eggs, which supports the hypothesis that nest sanitation is a pre-adaptation to egg rejection. However, nest sanitation did not increase egg rejection, either in probability or latency. Furthermore, both sexes incubated the eggs, cleaned the nests, and removed parasite eggs, implying that both are confronted with natural selection related to nest sanitation and brood parasitism. However, females invested more time in these behaviors than males. This provides evidence for the evolutionary relationship of nest sanitation and egg rejection behaviors in barn swallows.

Nest sanitation as an effective defence against brood parasitism

Egg rejection is a crucial defence strategy against brood parasitism that requires the host to correctly recognise the foreign egg. Rejection behaviour has, thus, evolved in many hosts, facilitated by the visual differences between the parasitic and host eggs, and driving hosts to rely on colour and pattern cues. On the other hand, the need to recognise non-egg-shaped objects to carry out nest sanitation led birds to evolve the ability to discriminate and eject objects using mainly shape cues. However, little is known regarding the evolutionary significance of rejection behaviour in general and the cognitive processes underlying it. Here, we investigated the response of the barn swallow (Hirundo rustica) during pre-laying and laying stages to four objects types that differed in shape (eggs vs stars) and colour/pattern (mimetic vs non-mimetic) to investigate (1) what cognitive mechanisms are involved in object discrimination and (2) whether egg rejection is a direct defence against brood parasitism, or simply a product of nest sanitation. We found that swallows ejected stars more often than eggs in both stages, indicating that swallows possess a template for the shape of their eggs. Since the effect of colour/pattern on ejection decisions was minor, we suggest that barn swallows have not evolved a direct defence against brood parasitism but instead, egg ejection might be a product of their well-developed nest sanitation behaviour. Nonetheless, the fact that mimetic eggs were ejected especially in the pre-laying stage shows that nest sanitation could be an effective defence against poorly timed brood parasitism.

Nest sanitation behavior does not increase the likelihood of parasitic egg rejection in herring gulls

Birds’ behavioral response to brood parasitism can be influenced not only by evolution but also by context and individual experience. This could include nest sanitation, in which birds remove debris from their nests. Ultimately, nest sanitation behavior might be an evolutionary precursor to the rejection of parasitic eggs. Proximately, the context or experience of performing nest sanitation behavior might increase the detection or prime the removal of parasitic eggs, but evidence to date is limited. We tested incubation-stage nests of herring gulls Larus argentatus to ask whether nest sanitation increased parasitic egg rejection. In an initial set of 160 single-object experiments, small, red, blocky objects were usually rejected (18 of 20 nests), whereas life-sized, 3D-printed herring gull eggs were not rejected whether red (0 of 20) or the olive-tan base color of herring gull eggs (0 of 20). Next, we simultaneously presented a red, 3D-printed gull egg and a small, red block. These nests exhibited frequent nest sanitation (small, red block removed at 40 of 48 nests), but egg rejection remained uncommon (5 of those 40) and not significantly different from control nests (5 of 49) which received the parasitic egg but not the priming object. Thus, performance of nest sanitation did not shape individuals’ responses to parasitism. Interestingly, parents were more likely to reject the parasitic egg when they were present as we approached the nest to add the experimental objects. Depending on the underlying mechanism, this could also be a case of experience creating variation in responses to parasitism.

Impact of nest sanitation behavior on hosts' egg rejection: an empirical study and meta-analyses

Egg rejection in birds is a specific adaptation toward avian brood parasitism, whereas nest sanitation is a general behavior for cleaning the nest and avoiding predation. However, both behaviors refer to the action of ejecting objects out of the nest, and nest sanitation has been proposed as a pre-adaptation for egg rejection. Here, we tested the eliciting effect of nest sanitation on egg rejection in the red-whiskered bulbul Pycnonotus jocosus, a potential host species that are sympatric with parasitic cuckoos. We conducted meta-analyses of previous studies on both nest sanitation and egg rejection, in order to evaluate the consistency of our conclusions. Our results showed that nest sanitation did not elicit egg rejection in P. jocosus. The conclusions concerning such an eliciting effect from previous studies were mixed, whereas the methodologies were inconsistent, making the studies unsuitable for comparisons. However, the ejection frequency of nest sanitation was consistently higher than the frequency of egg rejection across different host species or populations. These results suggest that nest sanitation, which is an ancient behavior, is more fundamental than egg rejection, but the effect of the former on the latter is complex and needs further study. Standardized methodologies and the integration of behavior, physiology, and modeling may provide better opportunities to explore the relationship between nest sanitation and egg rejection.

Variations in tree sparrow (Passer montanus) egg characteristics under environmental metal pollution

Environmental metal pollution is known to adversely affect bird reproduction, for which the variations of egg characteristics are considered very important. Our study explored whether variations in egg characteristics, such as egg volume, eggshell spotting pattern, eggshell coloration, and eggshell thickness, were correlated with heavy metal levels (Cu, Zn, Pb, and Cd) and Ca levels in the eggshells of tree sparrows (Passer montanus), a widespread passerine species. Eggs were collected from a long-term heavy metal polluted area (Baiyin, BY, northwest China) and a relatively unpolluted area (Liujiaxia, LJX, northwest China). Our results showed that the embryonated (eggshell: Cu: p = 0.003, Pb: p = 0.002) and non-embryonated (egg contents: Pb: p = 0.044, Ca: p = 0.045) eggs collected from BY contained relatively higher metal concentrations than those from LJX. Eggs from BY were smaller in volume (p < 0.01) and thinner in eggshell thickness (p < 0.01) than those from LJX. Mean egg volume increased with clutch size in BY (p = 0.017), which was also accompanied by an increase in the within-clutch coefficient of variation (CV) for egg volume (p = 0.045). Clutches with a higher CV for egg volume tended to contain higher concentrations of Zn and Pb (Zn: p = 0.084; Pb: p = 0.081) in the eggshells from BY. No differences were found in the eggshell spotting coverage ratio of eggs; however, eggshells were much darker in BY than in LJX. A more aggregated eggshell spotting distribution indicated higher eggshell Zn and Pb levels (BY: Zn: p = 0.040, Pb: p = 0.076; LJX: Pb: p = 0.066). The results demonstrate that the egg characteristics of tree sparrows can be used as indicators of metal pollution, especially for the within-clutch CV for egg volume, eggshell spotting pattern and eggshell coloration.

Using 3D modelling and printing to study avian cognition from different geometric dimensions

Studying animal cognition is meaningful because it helps us understand how animals adapt to the natural environment. Many birds build nests, clean their nests and reject foreign objects from their nests, which provide an optimal opportunity for studying their cognition toward foreign objects in nests. However, hand-made models used in previous studies have many deficiencies that considerably constrain our capacity to understand the evolution of avian cognition of foreign objects because they are unquantifiable and dependent on different features. We established a 3D modelling and printing method to manipulate one geometric dimension of a model while controlling for others, which allowed us to investigate avian cognition for different dimensions independently. Here we introduce this method, conduct an empirical study as an example, and discuss its applications to further studies.

Intraclutch eggshell colour variation in birds: are females able to identify their eggs individually?

BACKGROUND

One possibility suggested regarding female post-mating strategies is differential allocation into offspring investment. Female birds produce not only the largest, but also most colourful eggs of all oviparous taxa. Larger eggs provide space for bigger embryos, or more nutrition for their development, but the question why eggs are more colourful and why there is variation in eggshell colouration remains. In this context, the focus of interest has been to explain inter-clutch variation but in many bird species, eggshell colouration also varies within a clutch. Surprisingly, less attention has been paid to this phenomenon. Therefore, we propose the "female egg recognition" hypothesis, suggesting that mothers use colour characteristics to interpret egg attributes and allocate further investment into each egg accordingly. To evaluate the feasibility of the hypothesis, we tested several underlying predictions and examined their suitability using a dataset from our tree sparrow (Passer montanus) study. We predict (i) substantial within-clutch variation in eggshell colouration which, (ii) should be related to laying sequence, (iii) reflect egg quality and, (iv) should stimulate a female response.

METHODS

Eggshell coloration data were obtained via digital photography under standardized conditions, taken after clutch completion. Lightness (L*), representing the achromatic properties of an egg has been chosen as the most important predictor in dark cavities and was related to egg quality and position in the nest.

RESULTS

In our tree sparrows, first and mainly last eggs were less pigmented, providing information about laying order. Egg volume, which predicts chick quality, positively correlates with eggshell coloration. Finally, we could show that female tree sparrows placed darker, but not bigger, eggs into more central incubation positions.

DISCUSSION

All basic prerequisites for the "female egg recognition" hypothesis are fulfilled. In this context practicability and feasibility of the hypothesis and alternative explanations are discussed. However future work is necessary to determine a direct effect on offspring condition.

A test of the nest sanitation hypothesis for the evolution of foreign egg rejection in an avian brood parasite rejecter host species

Hosts of avian brood parasites have evolved diverse defenses to avoid the costs associated with raising brood parasite nestlings. In egg ejection, the host recognizes and removes foreign eggs laid in its nest. Nest sanitation, a behavior similar in motor pattern to egg ejection, has been proposed repeatedly as a potential pre-adaptation to egg ejection. Here, we separately placed blue 3D-printed, brown-headed cowbird (Molothrus ater) eggs known to elicit interindividual variation in ejection responses and semi-natural leaves into American robins' (Turdus migratorius) nests to test proximate predictions that (1) rejecter hosts should sanitize debris from nests more frequently and consistently than accepter hosts and (2) hosts that sanitize their nests of debris prior to the presentation of a foreign egg will be more likely to eject the foreign egg. Egg ejection responses were highly repeatable within individuals yet variable between them, but were not influenced by prior exposure to debris, nor related to sanitation tendencies as a whole, because nearly all individuals sanitized their nests. Additionally, we collected published data for eight different host species to test for a potential positive correlation between sanitation and egg ejection. We found no significant correlation between nest sanitation and egg ejection rates; however, our comparative analysis was limited to a sample size of 8, and we advise that more data from additional species are necessary to properly address interspecific tests of the pre-adaptation hypothesis. In lack of support for the nest sanitation hypothesis, our study suggests that, within individuals, foreign egg ejection is distinct from nest sanitation tendencies, and sanitation and foreign egg ejection may not correlate across species.

Eggshell coloration and its importance in postmating sexual selection

Avian eggshell color seems to fulfill multiple functions, some of them being structural and others signaling. In this study, we tested whether or not eggshell coloration may play a role in sexual selection of Tree Sparrows (Passer montanus). According to the "Sexually selected eggshell coloration" hypothesis, eggshell coloration signals female, egg or chick quality and males adjust parental investment according to this signal. Eggs of this species are covered with brown spots and patches, and variation between clutches is high. We found that eggshell coloration correlates with both protoporphyrin and biliverdin, but protoporphyrin concentrations are ten times higher. Eggshell coloration reflects egg and offspring quality, but not female quality. Thus, eggshell coloration may signal female postmating investment in offspring rather than female quality. Furthermore, differential allocation in terms of maternal investment is supported by the fact that females lay more pigmented clutches when mated to males with bigger melanin-based ornaments relative to their own. Moreover, males invested proportionally more to chicks that hatched from more pigmented clutches. Our correlative results thus seem to support a role of sexual selection in the evolution of eggshell coloration in birds laying brown eggs, pigmented mainly by protoporphyrin.

Nest sanitation does not elicit egg ejection in a brown-headed cowbird host

Most passerine birds practice nest sanitation whereby they remove debris from their nest. Nest sanitation has been posited as a pre-adaptation for egg ejection by hosts of avian brood parasites. However, relatively few North American hosts of the brood parasitic brown-headed cowbird (Molothrus ater) eject cowbird eggs to the detriment of their fitness. In this study, I added either a piece of flagging tape or a pine cone bract scale along with an artificial cowbird egg to nests of the red-winged blackbird (Agelaius phoeniceus) to determine whether the act of nest sanitation would elicit egg ejection. All red-winged blackbirds removed the debris within 24 h, but all individuals also accepted the cowbird eggs and this rate of ejection did not differ from that in nests that only received a cowbird egg. While nest cleaning and egg ejection are similar mechanically, they differ cognitively and egg ejection is not elicited in red-winged blackbirds during the act of removing debris from their nests.

Egg rejection in blackbirds Turdus merula: a by-product of conspecific parasitism or successful resistance against interspecific brood parasites?

BACKGROUND

Traditional theory assumes that egg recognition and rejection abilities arise as a response against interspecific brood parasitism (IBP). However, rejection also appears in some species that are currently not exploited by interspecific parasites, such as Turdus thrushes. Recent evidences suggest that rejection abilities evolved in these species as a response to conspecific brood parasitism (CBP). To test these two alternative hypotheses, we performed an experimental study by parasitizing nests of the common blackbird (Turdus merula) with conspecifics or heterospecific eggs under different risk of parasitism (presence of interspecific or conspecific parasites near the nest). Common blackbird is a potential host of the common cuckoo (Cuculus canorus) but suffers low levels of CBP too.

RESULTS

We found that blackbirds were able to recognize and eject heterospecific eggs at high rates whereas most of conspecifics eggs were not recognized and, therefore, accepted. Ejection rates of conspecific eggs did not exceed 13 %, even in situations of high risk of CBP (blackbird female placed near the nest), which contradict the main prediction derived from the CBP hypothesis. Conversely, ejection rates of experimental eggs simulating IBP were much higher (80-100 %). Furthermore, female blackbirds were more aggressive towards cuckoos than towards blackbird dummies.

CONCLUSIONS

Our results considered together support the IBP hypothesis, indicating that recognition and rejection of parasitic eggs in blackbirds have probably evolved due to previous cuckoo parasitism. The current absence of IBP in blackbirds may be due to the highly efficient rejection abilities in this species. Thus, these abilities have been retained in absence of brood parasitism as a consequence of the low costs involved for blackbirds, resulting in a successful resistance against interspecific brood parasitism.

An experimental test on time constraint and sexual conflict over parental care

Because parental care is costly, a sexual conflict between parents over parental investment is expected to arise. Parental care behavior is an adaptive decision, involving trade-offs between remating, and consequently desertion of the brood, and continuing parental effort. If the main advantage of desertion is remating, then this will be a time constraint, because the deserting individual will require a certain minimum period of time to breed again in the same breeding season. So, a short breeding season should force certain individuals to desert the first brood to have enough time to successfully complete their second breeding attempt. The rock sparrow, Petronia petronia, is an unusual species in which brood desertion can occur in both sexes and the breeding season is quite short so it is a good species to investigate the role of time constraint on brood desertion. For 3 years, I investigated the brood desertion modality of the rock sparrow. Then, for 2 years, I removed a group of experimental nest boxes during the autumn. Later, I re-installed the experimental nest boxes after the start of the breeding season (2 weeks after the first egg was laid), mimicking a shortening of the breeding season for the (experimental) pairs that used experimental nest boxes. I found that in the experimental pairs, the percentage of deserting individuals was significantly higher than in the control groups, and the deserting individuals were older females. This experiment adds to our knowledge of timing of reproduction effects on individual decisions to desert by showing that a short and delayed breeding season may have different effects on males and females. To my knowledge, this is the first experimental study that demonstrates a direct link between time constraint and brood desertion.

Nest sanitation elicits egg discrimination in cuckoo hosts

Nest sanitation is a nearly universal behavior in birds, while egg discrimination is a more specific adaptation that has evolved to counter brood parasitism. These two behaviors are closely related with nest sanitation being the ancestral behavior, and it has been hypothesized to constitute a preadaptation for egg discrimination. However, previous studies found little evidence to support this hypothesis. Here, we conducted an empirical test of the association between nest sanitation and egg discrimination in the barn swallow (Hirundo rustica) by inserting a single non-mimetic model egg or a non-mimetic model egg plus half a peanut shell into host nests. Compared to the rejection rate of single model eggs, barn swallows significantly increased egg rejection frequency if a half peanut shell was simultaneously introduced. Our result for the first time shows the impact of nest sanitation on egg discrimination and demonstrates that nest sanitation can elicit egg discrimination in hosts of brood parasites. This study provided evidence for nest sanitation being a preadaptation to egg discrimination by facilitating egg rejection, thereby significantly advancing our understanding of avian cognition of foreign objects. Furthermore, we suggest that egg discrimination behavior in many accepters and intermediate rejecters may be lost or diluted. Such egg discrimination can be elicited and restored after nest sanitation, implying a sensitive and rapid phenotypic response to increased risk of parasitism. Our study offers a novel perspective for investigating the role of so-called intermediate rejecter individuals or species in the long-term coevolutionary cycle between brood parasites and their hosts.

Experimental shifts in egg-nest contrasts do not alter egg rejection responses in an avian host-brood parasite system

Obligate brood parasitic birds exploit their hosts to provide care for unrelated young in the nest. Potential hosts can reduce the cost of parasitism by rejecting foreign eggs from the nest. Observational, comparative, and experimental studies have concluded that most hosts use the coloration and patterning of eggshells to discriminate between own and foreign eggs in the nest. However, an alternative hypothesis is that birds use the colour contrasts between eggshells and the nest lining to identify parasitic eggs (egg-nest contrast hypothesis). In support of this hypothesis, we found that the avian perceivable chromatic contrasts between dyed eggs and unmanipulated nest linings significantly and negatively covaried with the rejection rates of different dyed eggs of the great reed warbler Acrocephalus arundinaceus, a frequently parasitized host of the common cuckoo Cuculus canorus. To experimentally test whether egg-nest contrasts influence rejection, we reciprocally dyed both eggs and the nest lining of this host species with one of two colours: orange and green. Contrary to the egg-nest contrast hypothesis, host rejection patterns in response to dyed eggs were not altered by dyeing nests, relative to unmanipulated control eggs and nests. In turn, experimental egg colour was the only significant predictor of egg rejection rate. Our results demonstrate that egg-nest contrast is a collateral, not a causal factor in egg rejection, and confirm the conclusions of previous studies that hosts can rely on the parasitic egg's appearance itself to recognize the foreign egg in the nest.

Nest sanitation behavior in hirundines as a pre-adaptation to egg rejection to counter brood parasitism

Previous studies suggested that nest sanitation behavior may have been a pre-adaptation from which egg rejection of brood parasite eggs evolved. We tested this hypothesis in two swallow species, the red-rumped swallow (Cecropis daurica) and the barn swallow (Hirundo rustica). Our results indicated that the red-rumped swallow, which is an accepter of foreign eggs, rejected a low percentage of non-egg-shaped objects and did so less often than the barn swallow, which is an intermediate rejecter of foreign eggs. Furthermore, the egg rejection rates of the barn swallow increased with the increase in rejection rates of non-egg-shaped objects among different populations. These results showed that nest cleaning behavior could have evolved into a means of reducing the costs of brood parasitism, suggesting that egg recognition ability has evolved from recognition of non-egg-shaped objects. This finding advances our understanding of the evolution of egg recognition behavior in birds.

Host responses to interspecific brood parasitism: a by-product of adaptations to conspecific parasitism?

Background

Why have birds evolved the ability to reject eggs? Typically, foreign egg discrimination is interpreted as evidence that interspecific brood parasitism (IP) has selected for the host’s ability to recognize and eliminate foreign eggs. Fewer studies explore the alternative hypothesis that rejection of interspecific eggs is a by-product of host defenses, evolved against conspecific parasitism (CP). We performed a large scale study with replication across taxa (two congeneric Turdus thrushes), space (populations), time (breeding seasons), and treatments (three types of experimental eggs), using a consistent design of egg rejection experiments (n = 1057 nests; including controls), in areas with potential IP either present (Europe; native populations) or absent (New Zealand; introduced populations). These comparisons benefited from the known length of allopatry (one and a half centuries), with no gene flow between native and introduced populations, which is rarely available in host-parasite systems.

Results

Hosts rejected CP at unusually high rates for passerines (up to 60%). CP rejection rates were higher in populations with higher conspecific breeding densities and no risks of IP, supporting the CP hypothesis. IP rejection rates did not covary geographically with IP risk, contradicting the IP hypothesis. High egg rejection rates were maintained in the relatively long-term isolation from IP despite non-trivial rejection costs and errors.

Conclusions

These egg rejection patterns, combined with recent findings that these thrushes are currently unsuitable hosts of the obligate parasitic common cuckoo (Cuculus canorus), are in agreement with the hypothesis that the rejection of IP is a by-product of fine-tuned egg discrimination evolved due to CP. Our study highlights the importance of considering both IP and CP simultaneously as potential drivers in the evolution of egg discrimination, and illustrates how populations introduced to novel ecological contexts can provide critical insights into brood parasite-host coevolution.