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.

Defensive adaptations to cuckoo parasitism in the black-browed reed warbler (Acrocephalus bistrigiceps): recognition and mechanism

Hosts of avian brood parasites suffer a high cost of reproductive loss due to parasitism, driving them to evolve a variety of anti-parasitic defenses. These defenses comprise a series of components, including the recognition of brood parasites and the eggs laid by the parasites, cues used for recognition, and the mechanisms on which these behaviors are based. In this study, we conducted egg recognition and nest intruder experiments to examine these components of anti-parasitic behavior in the black-browed reed warbler (Acrocephalus bistrigiceps), a rare host of the common cuckoo (Cuculus canorus). We found that the host possessed strong recognition capacity, rejecting 100% of parasitic eggs, and used a template-based mechanism for egg recognition. The host birds also rejected 80% of their own eggs on which artificial markings were added to the blunt pole; however, they accepted all eggs with the same manipulation on the sharp pole, implying that the blunt pole was an important recognition cue. Furthermore, the host exhibited stronger aggression to cuckoos than to harmless controls; a behavior specific to the incubation stage rather than the nestling stage. Therefore, the host was able to distinguish the cuckoo from other nest intruders as being a brood parasite. These results together help explain the near absence of cuckoo parasitism in black-browed reed warblers and provide new information concerning anti-parasitic defenses in this host species.

Coevolution between Himalayan cuckoos and 2 sympatric Pycnonotidae hosts

Selection due to cuckoo parasitism is responsible for the evolution of anti-parasitism defenses in hosts. Different host species breeding sympatrically with a single parasitic cuckoo may evolve different strategies to reduce the risk of counter cuckoo parasitism, resulting in different interactions between cuckoos and hosts in areas of sympatry. Here, we studied the coevolutionary interactions between Himalayan cuckoos Cuculus saturatus and 2 sympatric and closely related potential hosts belonging to the family Pycnonotidae, the brown-breasted bulbul Pycnonotus xanthorrhous and the collared finchbill Spizixos semitorques. We investigated parasitism rates and nest-site selection (nest height, nest cover, human disturbance, perch height, forest distance, and degree of concealment) related to parasitism risk, nest defense against a cuckoo dummy, and egg rejection against cuckoo model eggs. Bulbuls used specific nest sites that were further away from forests than those of finchbills, and they behaved more aggressively toward cuckoos than finchbills. In contrast, bulbuls possessed moderate egg rejection ability, whereas the finchbill rejected 100% of cuckoo model eggs. We suggest that selection of a nest site away from forests by the bulbul explains the absence of parasitism by Himalayan cuckoos. We suggest that these interspecific differences in nest-site selection and nest defense indicate alternative responses to selection due to cuckoos.

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.

Persistent fine-tuning of egg rejection based on parasitic timing in a cuckoo host even after relaxation of parasitism pressure

Egg rejection is one of the most effective defenses to avoid avian brood parasites by hosts, and reflects avian cognition during parasite-host coevolution. However, egg rejection varies with different populations and species, or under different contexts. Here we studied the egg recognition behavior in a non-parasitized population of the red-whiskered bulbul (Pycnonotus jocosus), which has been recorded as a host of the banded bay cuckoo (Cacomantis sonneratii). By comparing its egg rejection frequency and latency between egg-laying and egg-incubation stages with a variety of nest ages, which represent different costs of cuckoo parasitism, the results show that the hosts rejected parasite eggs more frequently in the egg-laying stage than in the egg-incubation stage without a difference in latency, and accepted parasite eggs with an increase of nest age. These findings indicated that the hosts are able to adjust the intensity of their anti-parasitic responses according to the parasitic timing. A relaxation of parasitism risk and the tradeoff between parasitism risk and egg rejection cost may contribute to explaining our results.

Nestling odour modulates behavioural response in male, but not in female zebra finches

Studies investigating parent offspring recognition in birds led to the conclusion that offspring recognition is absent at the early nestling stage. Especially male songbirds were often assumed to be unable to discriminate between own and foreign offspring. However, olfactory offspring recognition in birds has not been taken into account as yet, probably because particularly songbirds have for a long time been assumed anosmic. This study aimed to test whether offspring might be recognised via smell. We presented zebra finch (Taeniopygia guttata) parents either the odour of their own or that of foreign nestlings and investigated whether the odour presentation resulted in a change in the number of head saccades, i.e. the rapid horizontal turning of the head, with which birds scan their environment and which can be used as a proxy of arousal. Our experiment indicates that male zebra finches, in contrast to females, differentiate between their own and foreign offspring based on odour cues, as indicated by a significant differences in the change of head saccadic movements between males receiving the own chick odour and males receiving the odour of a foreign chick. Thus, it provides behavioural evidence for olfactory offspring recognition in male zebra finches and also the existence of appropriate phenotypic odour cues of the offspring. The question why females do not show any sign of behavioural response remains open, but it might be likely that females use other signatures for offspring recognition.

Specific responses of cuckoo hosts to different alarm signals according to breeding stage: a test of the offspring value hypothesis

The selective pressure exerted by avian brood parasites forces their hosts to evolve specific defense strategies. When subject to brood parasite attack, avian hosts will often emit alarm calls. To date, few studies have examined whether and how host responses to different alarm calls indicative of different enemies vary with the host's breeding stage. We carried out alarm call playback experiments during both the egg and nestling stages of the oriental reed warbler Acrocephalus orientalis, a host of the common cuckoo Cuculus canorus. The playback exemplars were selected from recorded alarm calls of the warbler to the presence of common cuckoos, sparrowhawks Accipiter nisus, and oriental turtle doves Streptopelia orientalis, which represented brood parasite, predator, and harmless control, respectively. The results showed that the oriental reed warblers did not discriminate alarm calls issued to different intruder types, but the intensity of the response was significantly higher in the nestling stage than in the egg stage. Attack behavior related to sparrowhawk alarm calls was absent in the egg stage, but aggressive behavior increased dramatically and exceeded the attack frequency in response to the cuckoo alarm call in the nestling stage, implying a shift in the tradeoff between the parents' own survival and the loss of offspring. Alarm calls attracted a larger number of conspecifics than members of other species. In general, the oriental reed warbler had consistently stronger responses to different alarm calls in the nestling stage than in the egg stage, supporting the offspring value hypothesis.

Egg polymorphism and highly sensitive egg recognition of cross-phenotypes in rufescent prinias Prinia rufescens as effective defenses against brood parasitism

Whether escalating egg polymorphism increases the efficiency to reduce the fitness costs of brood parasitism is not supported by empirical studies. Rufescent prinias (Prinia rufescens) laying tetramorphic eggs are highly sensitive to conspecific foreign eggs with different phenotypes that they reject perfectly at 100%, and suffer only 1.4% of plaintive cuckoo (Cacomantis merulinus) parasitism. Our study provided empirical evidence showing that escalation of egg polymorphism in hosts increases their fitness because it reduces successful cuckoo parasitism rate.

Egg rejection and egg recognition mechanism of chestnut thrushes (Turdus rubrocanus)

Recognizing and rejecting foreign eggs is one of the most effective anti-parasite strategies for hosts in avian brood parasitism. Studies have shown that most cuckoo (Cuculus spp.) hosts have evolved egg recognition abilities. Although some open-nesting birds, especially thrushes belonging to the family Turdidae, are rarely parasitized by cuckoos, they still have high egg recognition ability. This evolutionary selection pressure on egg recognition is currently controversial. Previous studies on egg recognition of thrushes have mainly been carried out in Europe in a single-cuckoo system. In this study, chestnut thrushes (Turdus rubrocanus), which are distributed in a multiple-cuckoo system in China, were used to test their egg rejection and egg recognition mechanism. Our results showed that chestnut thrushes had a rejection rate of 54 % for non-mimetic blue model eggs, showing moderate egg recognition ability. Their egg recognition was true recognition, which relied on a memory template; chestnut thrushes could accurately reject foreign eggs in their nests. This study added the second case to report the egg recognition mechanism of thrushes in the Turdidae family and showed that the evolution of egg recognition ability of chestnut thrushes was likely a retained anti-parasitic strategy because of being parasitized by cuckoos in the past.

Female and male rufous horneros eject shiny cowbird eggs using a mental template of the size of their own eggs

Hosts of interspecific brood parasites often evolve antiparasitic defences, like the recognition and rejection of parasite's eggs. Most hosts use differences in coloration and maculation to discriminate between their own and parasitic eggs, but there are a few cases of hosts using the size of eggs as a cue. To recognize parasite eggs, hosts may require the presence of their own eggs and use a discordancy rule or may use a mental template of their own eggs. Females are responsible for egg rejection in hosts in which they incubate alone, but if incubation is shared, males can also reject parasitic eggs. The rufous hornero, Furnarius rufus, a host of the shiny cowbird Molothrus bonariensis, ejects parasite eggs using egg size as a cue. We studied the cognitive mechanism underlying the recognition and ejection of parasitic eggs by this host. We experimentally parasitized hornero nests with eggs of different size, with and without the presence of host eggs and determined which sex was responsible for the ejection. We found that horneros ejected parasitic eggs using the size of the egg as a cue and did not need to compare parasitic eggs with their own eggs, which is consistent with the hypothesis of a mental template. Females and males ejected eggs at similar frequencies. We also found that cowbird eggs laid in hornero nests were longer and wider than those laid in nests of other host in the same area, which is consistent with the hypothesis of host-specific female cowbird lineages evolving larger eggs to deceit horneros from recognizing and ejecting their eggs.

Egg retrieval versus egg rejection in cuckoo hosts

Before complex nests evolved, birds laid eggs on the ground, and egg retrieval evolved as an adaptation against accidental displacement of eggs outside the nest. Therefore, egg retrieval is an ancient, and likely ancestral, widespread behaviour in birds. However, it has received little attention in studies of avian brood parasitism, perhaps because most parasitism occurs in species with complex nests, a context in which egg retrieval seems irrelevant. However, for cavity-nesting hosts of avian brood parasites, egg retrieval may still play an important role in the coevolutionary interactions between obligate brood parasites and hosts, because egg retrieval can be considered to be antagonistic to egg rejection behaviour in hosts, yet both may involve cognition to recognize eggs. We hypothesized that (1) cavity-nesting hosts should retrieve misplaced eggs from outside the nest cup, (2) brood parasitism has modulated egg retrieval behaviour in cavity-nesting hosts and (3) hosts use the same visual cues for decision-making during egg recognition in both egg retrieval and egg rejection actions. To test these hypotheses, we performed a series of experiments in a cavity-nesting host, the green-backed tit ( Parus monticolus). Foreign eggs with different levels of mimicry were placed within or outside nest cups of hosts to test their responses. We found that host decisions about whether to retrieve or reject an egg both depended on the degree of mimicry. However, hosts sometimes first retrieved poorly mimetic foreign eggs and then rejected them. Alternatively, hosts sometimes failed to retrieve highly mimetic conspecific eggs. We suggest that egg retrieval in hosts is likely to be a result of the interaction between ancient retrieval behaviour and subsequent adaptation against brood parasitism. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Egg recognition and chick discrimination in colonial breeding birds

Theory predicts that parents will not raise unrelated offsprings. For colonial breeding birds, evolving an ability to recognize their own eggs and chicks can prevent misdirecting parental behaviour. To verify this hypothesis, egg recognition experiments were performed on colonial breeding Chinese pond herons (Ardeola bacchus) and cattle egrets (Bubulcus ibis) in Chengmai and Lingao, tropical Hainan Island, China. Furthermore, a chick discrimination experiment was conducted on Chinese pond herons. In our study area, we did not record conspecific or interspecific brood parasitism in the two heron species. The results showed that Chinese pond herons had similar recognition rates for the four types of experimental eggs and lacked recognition ability. Cattle egrets had variable abilities to recognize different types of foreign eggs and have low egg recognition ability. Chinese pond herons cannot discriminate foreign chicks. Therefore, we suggest that the two species of colonial breeding herons may lack or have low egg recognition ability, and Chinese pond herons show no nestling recognition ability.

Parasitic cuckoo catfish exploit parental responses to stray offspring

Interspecific brood parasitism occurs in several independent lineages of birds and social insects, putatively evolving from intraspecific brood parasitism. The cuckoo catfish, Synodontis multipunctatus, the only known obligatory non-avian brood parasite, exploits mouthbrooding cichlid fishes in Lake Tanganyika, despite the absence of parental care in its evolutionary lineage (family Mochokidae). Cuckoo catfish participate in host spawning events, with their eggs subsequently collected and brooded by parental cichlids, though they can later be selectively rejected by the host. One scenario for the origin of brood parasitism in cuckoo catfish is through predation of cichlid eggs during spawning, eventually resulting in a spatial and temporal match in oviposition by host and parasite. Here we demonstrate experimentally that, uniquely among all known brood parasites, cuckoo catfish have the capacity to re-infect their hosts at a late developmental stage following egg rejection. We show that cuckoo catfish offspring can survive outside the host buccal cavity and re-infect parental hosts at a later incubation phase by exploiting the strong parental instinct of hosts to collect stray offspring. This finding implies an alternative evolutionary origin for cuckoo catfish brood parasitism, with the parental response of host cichlids facilitating its evolution. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

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.