Responses of great reed warblers Acrocephalus arundinaceus to experimental brood parasitism: the effects of a cuckoo Cuculus canorus dummy and egg mimicry

Egg rejection behaviour towards parasitic eggs was studied in a great reed warbler Acrocephalus arundinaceus population in central Hungary, which was heavily (about 65%) parasitised by the common cuckoo Cuculus canorus . Clutches were experimentally parasitised during the egg-laying period with artificial, moderately mimetic cuckoo eggs or with conspecific eggs that were good mimics of the hosts' eggs. Great reed warblers rejected 76.2% of the artificial cuckoo eggs, mainly by ejection, but accepted most of the conspecific eggs (87.5%). Cuckoo eggs in naturally parasitised clutches were rejected at a lower rate (32%). When, in addition to the egg mimicry experiments, a stuffed cuckoo was placed near the nest, accompanied by the recording of a female cuckoo call, hosts' rejection rate of the artificial cuckoo egg increased from 76% to 96%. The sight of the cuckoo, on the other hand, did not influence host's rejection behaviour when a conspecific egg was used in the experiment. A stuffed collared dove Streptopelia decaocto , accompanied by its call, was used as a control, and did not cause any increased rejection. Great reed warblers were more aggressive towards the cuckoo than to the dove dummy. When the cuckoo eggs in naturally parasitised clutches were exchanged with artificial cuckoo eggs, we observed no increase in the rejection rate. We conclude that great reed warblers in our heavily parasitised population are capable of detecting brood parasitism in their clutch by identifying the parasitic egg. The efficiency of this identification depends mainly on the mimicry of the foreign egg. The sight of the cuckoo at the nest may increase rejection rate by stimulus summation, and this conditional effect is mainly affected by the degree of mimicry of the parasitic egg.     

The importance of nest cleaning in egg rejection behaviour of great reed warblers Acrocephalus arandinaceas

We tested the importance of nest cleaning in egg rejection behaviour of the great reed warbler Acrocephalus arundinaceus in a highly parasitised population in which about 64% of nests are parasitised by the common cuckoo Cuculus canorus . Three types of objects of the same weight, texture and colour but with different shapes (dummy cuckoo eggs, sticks and disks) were placed into great reed warbler nests. We investigated the response of hosts in two stages of breeding: pre-incubation when the risk of brood parasitism is high, and during incubation when the risk of parasitism is low. The dummy cuckoo eggs were rejected less often than the other objects in both breeding stages, although we did not find any difference in the frequency of rejection between pre-incubation and incubation. We integrate these results into current views on the evolution of host–parasite interactions, and propose a hierarchical concept to understand egg rejection behaviour: (1) hosts reject all non-egg shaped objects as a general cleaning mechanism; (2) adaptations for the hosts' ability to recognise their own eggs allows them to distinguish these eggs from similar objects and parasitic eggs.     

No change in common cuckoo Cuculus canorus parasitism and great reed warblers’ Acrocephalus arundinaceus egg rejection after seven decades

The coevolutionary process among avian brood parasites and their hosts involves stepwise changes induced by the antagonistic selection pressures of one on the other. As long‐term data on an evolutionary scale is almost impossible to obtain, most studies can only show snapshots of such processes. Information on host behaviour, such as changes in egg rejection rates and the methods of rejection are scarce. In Hungary there is an interesting case between the common cuckoo Cuculus canorus and the great reed warbler Acrocephalus arundinaceus, where the level of parasitism is unusually high (around 50%). We compared host rejection rates and methods of rejection from within our own project to that of an early study carried out and published almost 70 yr ago in the same region. Our comparisons revealed high and stable rates of parasitism (range: 52–64%), and marked fluctuations in the ratio of multiply parasitized nests (range: 24–52%). No difference was revealed in egg rejection rates after 7 decades (34–39%). Linear mixed‐effects modelling revealed no year effect on the type host responses toward the parasitic egg(s) during the years of study (categorized as acceptance, ejection, burial, and nest desertion). Cuckoo egg rejection was primarily affected by the type of parasitism, as more cuckoo eggs were rejected during single parasitism than from multiply parasitized nests. Our comparison did not reveal any directional changes in this cuckoo–host relationship, except a slight decrease in the frequency of multiple parasitism, which is likely to be independent from coevolutionary processes.     

No evidence for recognition errors in Acrocephalus warblers

Failure to recognise own eggs (recognition errors) may be an important selective force behind acceptance of parasitic eggs, leading to a balance between rejecters and acceptors in a host population (the equilibrium hypothesis). We predicted that recognition errors should occur frequently among host species with intermediate rejection rates, whose rejection behaviour shows many conditional responses. The reed warbler Acrocephalus scirpaceus and great reed warbler A. arundinaceus fulfil these requirements. These two species were therefore used in an experiment where host birds were exposed to a common cuckoo Cuculus canorus dummy, either <2 m or 5–10 m from the nest, at fishponds in southern Moravia (Czech Republic). The hosts responded to the cuckoo dummy, great reed warblers being much more aggressive than reed warblers, and both species being more aggressive towards the dummy when it was close to the nest than when it was farther away. We furthermore predicted that there should be more eggs rejected (ejected or nest abandoned) due to recognition errors among hosts exposed to a dummy close to the nest than among both those exposed to a dummy farther away from the nest and towards controls not exposed to cuckoo dummies. When comparing egg loss between groups of birds that were exposed to a cuckoo dummy with those that were not, we found no significant difference. However, partial egg loss was frequent among hosts in the studied population, most probably due to cuckoo depredation. We discuss why there were no detectable recognition errors in the studied population, when other researchers have claimed to have found such errors in host populations elsewhere.     

Explaining variation in brood parasitism rates between potential host species with similar habitat requirements

Host specialization evolved in many parasite-host systems. Evolution and maintenance of host specificity may be influenced by host life-history traits, active host selection by the parasite, and host anti-parasite strategies. The relative importance of these factors is poorly understood in situations that offer parasites a choice between hosts with similar habitat requirements. The common cuckoo Cuculus canorus is a generalist parasite on the species level, but individual females prefer particular host species. In reed beds of the Yellow River Delta, China, two potential hosts with similar nest characteristics, Oriental reed warblers Acrocephalus orientalis and reed parrotbills Paradoxornis heudei, breed in sympatry. We found that warblers were parasitized at much higher rates than parrotbills. Both hosts recognized and rejected non-mimetic model eggs well, indicating that they have been involved in an arms-race with cuckoos. Cuckoo eggs closely resembled warbler eggs, and such eggs were mostly accepted by warblers but rejected by parrotbills. Only warblers recognized adult cuckoos as a specific threat. Both hosts were equally good at raising cuckoo chicks. Low nest density, partial isolation by breeding time, small scale differences in nest and nest site characteristics, and high rejection rates of natural cuckoo eggs are likely cumulatively responsible for the low current parasitism rate in parrotbills. This study emphasizes the importance of integrating the study of general host life-history characteristics and specific anti-parasitism strategies of hosts across all breeding stages to understand the evolution of host specificity.     

Rejection of artificial cuckoo (Cuculus canorus) eggs in relation to variation in egg appearance among reed warblers (Acrocephalus scirpaceus)

Passerines that are exposed to brood parasitism can evolve reduced intraclutch variation in egg appearance to facilitate recognition and rejection of the parasitic egg. This has been shown to be true for European passerine species that are assumed to have participated in an evolutionary arms race with the cuckoo (Cuculus canorus). However, few investigations have been carried out with the aim of finding out whether there is a relationship between these two traits within a species. In this study, we compare the level of intraclutch variation in egg appearance and the rejection of an unlike parasitic egg within a population of reed warblers (Acrocephalus scirpaceus) in the south-eastern part of the Czech Republic. We parasitized reed warbler nests with an artificial non-mimetic cuckoo egg, and then monitored the reaction of the hosts. In 27 out of 48 nests (56.3%) the parasitic egg was rejected. The rejecter pairs had a statistically significantly lower intraclutch variation in egg appearance than the acceptor pairs. We discuss possible explanations for the observed relationship between rejection of unlike eggs and intraclutch variation in egg appearance within this population of reed warblers. The results are consistent with the evolutionary arms race hypothesis, but the intermediate rejection rate found in this population could also be maintained by an equilibrium between acceptors and rejecters due to rejection costs.     

Breeding success of common cuckoos Cuculus canorus parasitising four sympatric species of Acrocephalus warblers

We investigated the level of parasitism, egg mimicry and breeding success of cuckoos parasitising four sympatric species of Acrocephalus warblers in southern Moravia, Czech Republic. The parasitism rate was highest in the marsh warbler Acrocephalus palustris (44.8%) followed by great reed warbler A. arundinaceus (33.8%), sedge warbler A. schoenobaenus (26.5%) and reed warbler A. scirpaceus (11.6%). Although the cuckoo eggs showed a high level of mimicry the eggs of the marsh warbler this host species rejected 72% of the cuckoo eggs, resulting in a cuckoo breeding success of only 4.3%. Cuckoo eggs laid in great reed warbler and reed warbler nests showed a similar hatching success, but the cuckoo chicks survived better in great reed warbler nests, resulting in a breeding success of 30.4%, as compared to 16.4% in nests of the reed warbler. The relationship between the level of parasitism, host rejection of cuckoo eggs, cuckoo chick survival and breeding success is discussed for the four host species.     

REED WARBLER HOSTS FINE‐TUNE THEIR DEFENSES TO TRACK THREE DECADES OF CUCKOO DECLINE

Interactions between avian hosts and brood parasites can provide a model for how animals adapt to a changing world. Reed warbler (Acrocephalus scirpaceus) hosts employ costly defenses to combat parasitism by common cuckoos (Cuculus canorus). During the past three decades cuckoos have declined markedly across England, reducing parasitism at our study site (Wicken Fen) from 24% of reed warbler nests in 1985 to 1% in 2012. Here we show with experiments that host mobbing and egg rejection defenses have tracked this decline in local parasitism risk: the proportion of reed warbler pairs mobbing adult cuckoos (assessed by responses to cuckoo mounts and models) has declined from 90% to 38%, and the proportion rejecting nonmimetic cuckoo eggs (assessed by responses to model eggs) has declined from 61% to 11%. This is despite no change in response to other nest enemies or mimetic model eggs. Individual variation in both defenses is predicted by parasitism risk during the host's egg‐laying period. Furthermore, the response of our study population to temporal variation in parasitism risk can also explain spatial variation in egg rejection behavior in other populations across Europe. We suggest that spatial and temporal variation in parasitism risk has led to the evolution of plasticity in reed warbler defenses.     

Common cuckoo Cuculus canorus parasitism,antiparasite defence and gene flow in closely located populations of great reed warblers Acrocephalus arundinaceus

In Hungary an unusually high rate of parasitism on the great reed warbler Acrocephalus arundinaceus by the common cuckoo Cuculus canorus has been maintained for at least the last one hundred years. We evaluated parasitism rate, antiparasite defence and genetic differentiation among Hungarian great reed warblers at three sites located 40–130 km from each other, where hosts suffered from a high (41–68%), moderate (11%), and almost no (<1%) parasitism. We were especially interested in whether the level of antiparasite defence was related to the local parasitism rate, and, if not, to understand why. There was no difference among the three sites in the responses to experimental parasitism by non‐mimetic model cuckoo eggs (rejection rate 71–82%), which can be explained by strong gene flow between populations: there was low level of philopatry and no genetic differentiation in the region. Reproductive success of the host in the heavily parasitised site was about 54% of that in the unparasitised site, indicating that long‐term persistence of host populations in highly exploited areas depends on continuous immigration.     

Cuckoo parasitism in a cavity nesting host: near absent egg‐rejection in a northern redstart population under heavy apparent (but low effective) brood parasitism

Brood parasite – host systems continue to offer insights into species coevolution. A notable system is the redstart Phoenicurus phoenicurus parasitized by the ‘redstart‐cuckoo’ Cuculus canorus gens. Redstarts are the only regular cuckoo hosts that breed in cavities, which challenges adult cuckoos in egg laying and cuckoo chicks in host eviction. We investigated parasitism in this system and found high overall parasitism rates (31.1% of 360 redstart nests), but also that only 33.1% of parasitism events (49 of 148 eggs) were successful in laying eggs into redstart nest cups. The majority of cuckoo eggs were mislaid and found on the rim of the nest; outside the nest cup. All available evidence suggests these eggs were not ejected by hosts. The effective parasitism rate was therefore only 12.8% of redstart nests. Redstarts responded to natural parasitism by deserting their nests in 13.0% of cases, compared to desertion rates of 2.8% for non‐parasitized nests. Our egg parasitism experiments found low rates (12.2%) of rejection of artificial non‐mimetic cuckoo eggs. Artificial mimetic and real cuckoo eggs added to nests were rejected at even lower rates, and were always rejected via desertion. Under natural conditions, only 21 cuckoo chicks fledged of 150 cuckoo eggs laid. Adding to this low success, is that cuckoo chicks are sometimes unable to evict all host young, and were more likely to die as a result compared to cuckoo chicks reared alone. This low success seems to be mainly due to the cavity nesting strategy of the redstart which is a challenging obstacle for the cuckoo. The redstart‐cuckoo system appears to be a fruitful model system and we suggest much more emphasis should be placed on frontline defences such as nest site selection strategies when investigating brood parasite–host coevolution.     

Behaviour of African turdid hosts towards experimental parasitism with artificial red-chested cuckoo Cuculus solitarius eggs

The red-chested cuckoo Cuculus solitarius parasitises many passerines in Africa, but some common species sympatric with this brood parasite are rarely used as hosts. We tested the responses of three turdid hosts to parasitism with artificial cuckoo eggs. The kurrichane thrush Turdus libonyana , which is not regularly parasitized by the cuckoo, rejected 60% of mimetic model eggs and 81% of non-mimetic eggs. We observed female thrush behaviour during the first visit after parasitism, and thrushes appeared to be initially fooled by mimetic eggs in completed clutches in all cases, and incubated. By contrast, in half of the experiments with non-mimetic eggs, these were ejected by the thrushes, with the host grasping the egg and flying away with it. The time spent nest checking prior to ejection was only one third of the time spent nest checking when females decided to incubate the clutch, suggesting that females were immediately aware of a foreign egg in the nest. By contrast, southern olive thrushes T. olivaceous rejected all non-mimetic and accepted all mimetic model eggs, whereas cape robins Cossypha caffra accepted all model eggs, irrespective of whether or not they were mimetic. Our results support the hypothesis that rejection behaviour in these two thrush species evolved as a defence against interspecific nest parasitism, with thrushes appearing to be ahead in this particular host-parasite arms race. The cape robin, by contrast, appears not to reject cuckoo eggs, either because it is to unable to recognize them, or because the costs associated with removal may be too high.     

Blunt egg pole holds cues for alien egg discrimination: experimental evidence

Eggshell colour patterns play a crucial role in avian host–parasite coevolution. In contrast to many experiments investigating general host egg discrimination abilities, studies testing where specific recognition cues are located on the eggshells (on blunt, sharp or both egg poles) are lacking. Previous studies suggested that discrimination cues might be located at the blunt egg pole, where the shell patterning is typically concentrated. We tested this hypothesis experimentally in species subject to interspecific (great reed warblers Acrocephalus arundinaceus, reed warblers A. scirpaceus), and also intraspecific parasitism (song thrushes Turdus philomelos, blackbirds T. merula). We examined host responses towards two types of intraspecific eggs painted non‐mimetic immaculate blue either at blunt or sharp poles. All four species rejected eggs manipulated at the blunt pole at significantly higher rates, indicating that they perceive the critical recognition cues in the same egg part. Thus, the presence of egg recognition cues at the blunt egg pole may be a general phenomenon in birds parasitized by both intraspecific and interspecific parasites.     

Egg Discrimination in an Open Nesting Passerine Under Dim Light Conditions

Many hosts of avian brood parasites such as the common cuckoo (Cuculus canorus) show refined egg discrimination behaviour. Egg recognition in most open‐nesting hosts seems to be based entirely on differences in colour. However, hole‐ and dome‐nesting hosts may rely largely on luminance contrasts. Here, we studied egg rejection behaviour in nightingales (Luscinia megarhynchos), an open‐nesting species that nests in deeply shadowed positions and lays very specific dark olive‐green eggs. Although being theoretically suitable as hosts of the cuckoo, nightingales are very rarely parasitized and no cuckoo egg morph mimicking nightingale eggs is known. Thus, we predicted high rejection rate of foreign eggs, but because of the dim nesting environments, luminance contrasts would be an important cue in egg rejection decisions, similar to cavity‐ or dome‐nesting species. We experimentally parasitized nightingale nests with two groups of model egg types: ‘bright eggs’ and ‘dark eggs’. Within each group, one of the egg types was an effective match while the other type was a poor colour match (whitish vs. pale blue and olive‐green vs. black).We used a discrimination visual model to quantify host‐model egg similarity and compared egg rejection predicted by the model with the observed rejection pattern. Consistent with a scenario of largely luminance‐based egg recognition, blue and white eggs, which had larger achromatic mismatching, were rejected at a higher relative rate than the better achromatic matching black and green eggs. Nightingales showed strong aggression to a cuckoo dummy, suggesting that they were involved in coevolutionary interactions with the cuckoo in the past. However, because of the highly distinct appearance of nightingale eggs relative to the other sympatrically breeding passerines, and the largely luminance‐based egg recognition, this arms race was likely terminated at an early stage.     

Are blackcaps current winners in the evolutionary struggle against the common cuckoo?

Blackcaps Sylvia atricapilla reject artificial cuckoo eggs, and their eggs vary little in appearance within clutches, whereas among clutches eggs vary considerably. Low variation within clutches facilitates discrimination of parasitic eggs, whereas high variation among clutches makes it harder for the cuckoo to mimic the eggs of a certain host species. These traits have most probably evolved as counteradaptations against brood parasitism by the common cuckoo Cuculus canorus, even though blackcaps are not regularly parasitised today. In this study, we investigated how fine-tuned the rejection of parasitic eggs is in this species by introducing three types of eggs into their nests: a real non-mimetic egg the approximate size of a cuckoo egg, an artificial mimetic egg the size of a cuckoo egg and a real conspecific egg. As the rejection frequency of both mimetic and non-mimetic artificial cuckoo eggs has been shown to be high in previous studies, the variation in rejection behaviour between individuals is low, indicating that most individuals within the population are able to reject parasitic eggs. Thus, we predict that (1) the intraclutch variation in egg appearance should be generally low in all individuals, and that (2) regarding conspecific eggs, rejection decisions should be highly dependent on the degree of mimicry between parasitic and host eggs. We found support for these predictions, which indicates that due to their highly sophisticated countermeasures against brood parasitism, blackcaps can probably be regarded as current winners of the arms race with the common cuckoo. Furthermore, the high and consistent rejection frequency of cuckoo eggs found throughout Europe for this species supports the spatial habitat structure hypothesis, which claims that woodland-nesting species breeding near trees, like blackcaps, presumably experienced a high level of parasitism throughout their range in the past and, therefore, their rejection behaviour, once evolved, spread rapidly to all populations.     

Fixed Rejection Responses to Single and Multiple Experimental Parasitism in Two Fringilla Hosts of the Common Cuckoo

Previous studies have shown that the tendency to reject parasitic eggs among certain hosts is strongly dependent on the degree of similarity with own eggs, whereas other conditional cues do not affect rejection decisions. This paper examines whether two such hosts, the closely related brambling and chaffinch, show a different tendency to reject parasitic eggs if they are multiply parasitized. Some individuals were experimentally parasitized with both a non-mimetic and a low–intermediate contrasting egg in the same breeding attempt. The non-mimetic egg was rejected almost without exception. In chaffinches, the low–intermediate contrasting egg was introduced shortly after rejection of the non-mimetic egg whereas in bramblings, both eggs were introduced simultaneously. A control group consisted of individuals that were parasitized with one low–intermediate contrasting egg. There was no significant difference in the tendency to reject the low–medium contrasting eggs between the experimental and control group in any of the species, implying that the same acceptance threshold is applied to each parasitic egg independently. Moreover, the rejection rate of non-mimetic eggs was high (>90%) regardless of whether the egg was introduced alone or together with a low–medium contrasting egg. The results are discussed in relation to recent studies with great reed warblers Acrocephalus arundinaceus that obtained contrasting results in similar experimental designs. The different degrees of flexibility displayed by Fringilla and great reed warbler hosts are likely to reflect differences in both the perception and action component of the recognition system.     

Do Nest Light Conditions Affect Rejection of Parasitic Eggs? A Test of the Light Environment Hypothesis

Discrimination of foreign eggs is one of the most studied aspects of host defences against avian brood parasites. Although many factors affecting host egg‐recognition processes have already been evaluated, only a few attempts have been made to test the importance of light conditions in microhabitats of host nests. Here, we examined whether the objectively measured nest light environment affects great reed warbler (Acrocephalus arundinaceus) responses towards real common cuckoo (Cuculus canorus) eggs. More specifically, we predicted that parasitic eggs will be rejected with a lower frequency from nests placed in darker conditions than those in lighter conditions. However, we found no effect of the ambient light on egg‐rejection behaviour alone, but the photosynthetically active radiation exhibited a positive interactive effect with chromatic contrast between cuckoo and host eggs. Most rejection events were accomplished when cuckoo eggs of poor mimicry were laid in well‐lit nests. Our study suggests that this phenomenon may have important implications for the evolution of egg mimicry and host egg discrimination. We encourage further testing of the light environment hypothesis in other host species breeding in variable nest microhabitats and light conditions.     

Egg Rejection and Brain Size among Potential Hosts of the Common Cuckoo

Interspecific brood parasitism by the common cuckoo (Cuculus canorus) lowers host fitness, and has selected for discrimination and rejection of parasitic eggs in their commonly parasitized hosts. Cognitive demands needed to discriminate and reject cuckoo eggs may have led to augmentation of relative brain size among passerine hosts parasitized by cuckoos. This hypothesis predicts for across species positive relationships of brain size with rejection rate, host suitability and parasitism level. Here we test these predictions while controlling for phylogenetic, ecological and developmental factors known to affect brain size and egg rejection in a comparative study using the cuckoo and their hosts in Europe as a model system. Contrary to expected the rate of rejection of non‐mimetic cuckoo eggs covaried negatively with relative brain size across bird species. Either suitability as cuckoo host, which reflects long‐time duration of exposure to cuckoo parasitism, and level of parasitism, did not relate to brain size. Our results do not support the hypothesis that cuckoo parasitism was a main direct force affecting brain size variation across passerine hosts.     

Attractive blue‐green egg coloration and cuckoo−host coevolution

Recent evidence suggests that blue‐green coloration of bird eggshells may be related to female and/or egg phenotypic quality, and that such colour may affect parental effort and therefore the nutritional environment of developing nestlings. Here we suggest that these relationships and the signal function of eggshell coloration would affect the outcome of coevolution between avian brood parasites and their hosts in at least three different non‐exclusive evolutionary pathways. First, by laying blue‐green coloured eggs, cuckoo females may exploit possible sensory biases of their hosts, constraining the evolution of parasitic egg recognition, and thus avoid rejection. Second, because of the relatively high costs of laying blue eggs, cuckoo females may be limited in their ability to mimic costly blue‐green eggs of their hosts because cuckoo females lay many more eggs than their hosts. Furthermore, costs associated with foreign egg recognition errors would be relatively higher for hosts laying blue eggs. Third, cuckoos may use coloration of host eggs for selecting individuals or specific hosts of appropriate phenotypic quality (i.e. parental abilities). We here explored some predictions emerging from the above scenarios and found partial support for two of them by studying egg coloration of European cuckoos (Cuculus canorus) and that of their 25 main hosts, as well as parasitism and rejection rate of hosts. Cuckoo hosts parasitized with more blue, green, and ultraviolet cuckoo eggs, or those laying more blue‐green eggs, were more prone to accept experimental parasitism with artificial cuckoo eggs. In addition, coloration of cuckoo eggs is more variable when parasitizing hosts laying bluer‐greener eggs, even after controlling for the effect of host egg coloration (i.e. degree of egg matching). Globally, our results are consistent with the proposed hypothesis that host egg traits that are related to phenotypic quality of hosts, such as egg coloration, may have important implications for the coevolutionary interaction between hosts and brood parasites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 154–168.     

Size and material of model parasitic eggs affect the rejection response of Western Bonelli's Warbler Phylloscopus bonelli

Given the high costs of brood parasitism, avian hosts have adopted different defences to counteract parasites by ejecting the foreign egg or by deserting the parasitized nest. These responses depend mainly on the relative size of the host compared with the parasitic egg. Small hosts must deal with an egg considerably larger than their own, so nest desertion becomes the only possible method of egg rejection in these cases. The use of artificial model eggs made of hard material in egg‐recognition experiments has been criticized because hard eggs underestimate the frequency of egg ejection. However, no available studies have investigated the effect of softer material. Here, we test the potential effect of size of dummy parasitic eggs in relation to egg‐rejection behaviour (egg ejection and nest desertion rates) in Western Bonelli's Warbler Phylloscopus bonelli, a small host, using plasticine non‐mimetic eggs of three different sizes. In addition, we tested the potential effect of material, comparing ejection and desertion responses between real and plasticine eggs. As predicted, small eggs were always ejected, whereas nest desertion occurred more frequently with large eggs, thus suggesting that nest desertion occurs because of the constraints imposed by the large eggs. We found that plasticine may misrepresent the responses to experimental parasitism, at least in small host species, because this material facilitates egg ejection, provoking a decrease in nest desertion rate. Thus, particular caution is needed in the interpretation of the results in egg‐rejection experiments performed using dummy eggs made of soft materials.     

Nest desertion cannot be considered an egg‐rejection mechanism in a medium‐sized host: an experimental study with the common blackbird Turdus merula

Two main mechanisms of egg rejection, the main defence of hosts against brood parasites, have been described: ejection and desertion. Desertion of the parasitized nest is much more costly and is usually exhibited by small‐sized host species unable to remove the parasitic egg. However, nest desertion is frequently assumed to be an anti‐parasite strategy even in medium or large‐sized host species. This assumption should be considered with caution because: 1) large‐sized hosts able to eject the parasitic egg should eject it rather than desert the nest, and 2) breeding birds may desert their nests in response to different disturbances other than brood parasitism. This problem is especially important in the common blackbird Turdus merula because this is a medium‐sized species, potential host of the common cuckoo Cuculus canorus, in which desertion has been frequently reported as a response to cuckoo egg models. Here, we seek to determine whether nest desertion can be considered a response unequivocally directed to the parasitic egg in medium‐sized hosts using the blackbird as the study species. In an experimental study in which we have manipulated levels of mimicry and size of experimental eggs, we have found that both colour (mimetic and non‐mimetic; at least for human vision) and size (small, medium, and large) significantly affected ejection rates but not nest desertion rates. In fact, although large eggs disproportionally provoked nest desertion more frequently than did small or medium‐sized eggs, cuckoo‐sized parasitic eggs were not deserted allowing us to conclude that desertion is unlikely to be an adaptive response to brood parasitism at least for this species.