Persistence of egg recognition in the absence of cuckoo brood parasitism: pattern and mechanism

Broad ecological shifts can render previously adaptive traits nonfunctional. It is an open question as to how and how quickly nonfunctional traits decay once the selective pressures that favored them are removed. The village weaverbird (Ploceus cucullatus) avoids brood parasitism by rejecting foreign eggs. African populations have evolved high levels of within-clutch uniformity as well as individual distinctiveness in egg color and spotting, a combination that facilitates identification of foreign eggs. In a companion study, I showed that these adaptations in egg appearance declined following introductions of weavers into habitats devoid of egg-mimicking brood parasites. Here, I use experimental parasitism in two ancestral and two introduced populations to test for changes in egg rejection behavior while controlling for changes in egg appearance. Introduced populations reject foreign eggs less frequently, but the ability of source and introduced populations to reject foreign eggs does not differ after controlling for the evolution of egg color and spotting. Therefore, egg rejection behavior in introduced populations of the village weaver has been compromised by changes in egg appearance, but there has been no significant decline in the birds' ability to recognize foreign eggs. This result reconciles earlier studies on this system and provides insights into the ways behavior can change over generations, especially in the context of recognition systems and the avoidance of brood parasitism.     

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.     

Obligate brood parasites as selective agents for evolution of egg appearance in passerine birds

Many passerine host species have counteracted the parasite egg mimicry in their coevolutionary arms race with the common cuckoo (Cuculus canorus) by evolving increased interclutch and reduced intraclutch variation in egg appearance. Such variations make it easier for hosts to recognize a foreign egg, reduce the possibility of making recognition errors, and reduce the ability of the cuckoo to mimic the eggs of a particular host. Here, we investigate if such clutch characteristics have evolved among North American passerines. We predict that due to the absence of brood parasites with egg mimicry on this continent, these passerines should (1) not show any relationship between rejection rates and intra- or interclutch variation, and (2) intraclutch variation should be lower and interclutch variation higher in European hosts exposed to cuckoo parasitism as compared to North American hosts parasitized by cowbirds. Here we present data that show support for most of these and other predictions, as well as when controlling statistically for effects of common descent. However, the effect of continent on intraclutch variation was less than predicted and we discuss a possible reason for this. All things considered, the results demonstrate that parasitism by a specialist brood parasite with egg mimicry is a powerful selective force regarding the evolution of egg characteristics in passerine birds.     

Disturbance of the rainforest has the potential to enhance egg parasitism of lepidopteran noctuid stemborers in Kisangani,DR Congo

Landscape context influences population dynamics of insects and impacts biological processes within communities. It was expected that anthropogenic disturbances of the rainforest landscape in DR Congo would lead to a decreased level of noctuid stemborer egg parasitism as a consequence of a decoupling between stemborers and their naturally occurring parasitoids through dispersal. To test this hypothesis, noctuid egg batches were collected in maize fields along an anthropogenic disturbance gradient to assess change in the rates of eggs parasitism and maize plant infestation with noctuid egg batches. Our results showed that, in contrast to what was initially expected, egg parasitism increased from less to highly disturbed landscape whereas maize infestation had an inverse tendency. Discovery efficiency and mean egg parasitism were 1.416 and 1.392 times higher, respectively, in the most than in the less disturbed landscape. The numbers of eggs and egg batches per 100 maize plants were 0.55 times and 0.532 times the value in the less disturbed landscape, suggesting a dilution of the stemborer population within a large habitat patch encompassing cultivated fields and the surrounding wild host plants. It was concluded that the presence of suitable host plants enhances noctuid stemborers egg parasitism in adjacent maize fields.     

Importance of plant size, distribution of egg masses, and weather conditions on egg parasitism of the European corn borer, Ostrinia nubilalis by Trichogramma ostriniae in sweet corn

Trichogramma ostriniae (Hym: Trichogrammatidae), an egg parasitoid of the European corn borer, Ostrinia nubilalis (Lep: Pyralidae), were released into sweet corn (Zea mays L.) fields to study the effects of weather, plant size and distribution of egg masses on egg parasitism by the wasp. Sentinel European corn borer eggs were stapled onto leaves located in the upper, middle and lower third of sweet corn plants 5 to 35 meters away from the wasp release point in either a radial or grid manner. Weather conditions and plant architecture were monitored during the experiments. Logistic regression was used to analyze the data. The results indicated that percentage of eggs parasitized was negatively related to an increase in leaf area as well as an increase in distance eggs were located from the point of release of wasps. Eggs distributed on plants at different directions from the release point received different levels of parasitism. Eggs that were stapled onto leaves in the upper third of a corn plant received much less parasitism than those on the middle and lower third of the plant. Higher mean temperature adversely affected the level of parasitism during hotter times of the season and conversely, lower temperatures (<17 °C ) reduced the egg parasitism during cooler times of the season. The longer the exposure of eggs to wasps, the higher the level of egg parasitism. However, the levels of egg parasitism for 2 day's exposure were almost the same as that for 3 day's exposure due to the limited longevity and egg-laying behavior of the wasp. These results suggest that inundative releases of T. ostriniae should be made every two to three days, with multiple release points per hectare. In addition, weather conditions and plant architecture, especially temperature, plant height and leaf area must be taken into consideration to optimize levels of parasitism.     

Nest destruction elicits indiscriminate con‐ versus heterospecific brood parasitism in a captive bird

Following nest destruction, the laying of physiologically committed eggs (eggs that are ovulated, yolked, and making their way through the oviduct) in the nests of other birds is considered a viable pathway for the evolution of obligate interspecific brood parasitism. While intraspecific brood parasitism in response to nest predation has been experimentally demonstrated, this pathway has yet to be evaluated in an interspecific context. We studied patterns of egg laying following experimental nest destruction in captive zebra finches, Taeniopygia guttata, a frequent intraspecific brood parasite. We found that zebra finches laid physiologically committed eggs indiscriminately between nests containing conspecific eggs and nests containing heterospecific eggs (of Bengalese finches, Lonchura striata vars. domestica), despite the con‐ and heterospecific eggs differing in both size and coloration. This is the first experimental evidence that nest destruction may provide a pathway for the evolution of interspecific brood parasitism in birds.     

Egg rejection behaviour in the great reed warbler (<Emphasis Type="Italic">Acrocephalus arundinaceus</Emphasis>): the effect of egg type

Egg discrimination in hosts of the common cuckoo Cuculus canorus is frequently studied by experimental parasitism, using model cuckoo eggs. We compared egg rejection behaviour of the great reed warbler Acrocephalus arundinaceus to either model cuckoo eggs made of plastic or painted real host eggs. We simultaneously parasitised host nests by two different egg types to simulate cuckoo parasitism. A previous study revealed very similar, ca. 70%, rejection rates against both of these egg types (beige or bluish background colour maculated with dark brown) when they were used for single parasitism. In the present study we showed 96% average rejection rates against these egg types when they were applied in multiple experimental parasitism, causing a more predictable output for rejection behaviour. Hard plastic eggs and painted real eggs were rejected at similar frequencies, and videotaping revealed that model egg rejection caused extra work for great reed warblers. We revealed a new type of rejection behaviour, when hosts tried to eject hard-shelled model cuckoo eggs: Hosts made little holes in the middle part of these plastic eggs by pecking them several times before ejection, as if seeking the possibility to pierce and hold these eggs in their bills. Painted real eggs were rejected by actually puncturing the eggshell and holding them in the bill during ejection. No instances of grasp ejection were recorded during filming. Most experimental eggs of either type were ejected within 1 day after the introduction of the eggs, indicating that hosts made their rejection decisions quickly. Our observations suggest the lack of plasticity in the mode and timing of ejection behaviour towards experimental cuckoo eggs of different types in great reed warblers.     

The sight of an adult brood parasite near the nest is an insufficient cue for a honeyguide host to reject foreign eggs

Hosts of brood‐parasitic birds typically evolve anti‐parasitism defences, including mobbing of parasitic intruders at the nest and the ability to recognize and reject foreign eggs from their clutches. The Greater Honeyguide Indicator indicator is a virulent brood parasite that punctures host eggs and kills host young, and accordingly, a common host, the Little Bee‐eater Merops pusillus frequently rejects entire clutches that have been parasitized. We predicted that given the high costs of accidentally rejecting an entire clutch, and that the experimental addition of a foreign egg is insufficient to induce this defence, Bee‐eaters require the sight of an adult parasite near the nest as an additional cue for parasitism before they reject a clutch. We found that many Little Bee‐eater parents mobbed Greater Honeyguide dummies while ignoring barbet control dummies, showing that they recognized them as a threat. Surprisingly, however, neither a dummy Honeyguide nor the presence of a foreign egg, either separately or in combination, was sufficient to stimulate egg rejection.     

Egg discrimination in the Australian reed warbler (Acrocephalus australis): rejection response toward model and conspecific eggs depending on timing and mode of artificial parasitism

In a coevolutionary arms race between an interspecific broodparasite and its host species, both are expected to evolveadaptations and counteradaptations. We studied egg discriminationin the Australian warbler (Acrocephalus australis). This speciesis currently not significantly parasitized by the seven speciesof cuckoo for which it is a suitable host. However, experimentalbrood parasitism in the warbler revealed a fine tuned egg discriminationresponse towards non-mimetic and conspecific eggs, the firstsuch evidence in an Australian passerine: (1) non-mimetic eggswere significantly more often rejected than conspecific eggs;(2) only non-mimetic dummy eggs were rejected selectively,whereas rejection of conspecific eggs entailed a rejectioncost; (3) replacement of a host's egg with a conspecific eggduring egg laying resulted in a significantly higher rejectionrate than after the day of clutch completion; (4) by contrast,rejection rate after addition of a conspecific egg was independentof nest stage; (5) conspecific eggs introduced into a clutchduring the egg laying period led to a significantly highernest desertion rate and a lower egg ejection rate than afterthe day of clutch completion; and (6) addition of a conspecificegg led to egg ejection while egg replacement with a conspecificegg led to nest desertion. The fact that this species respondsdifferentially toward different modes of artificial parasitismsuggests that its egg discrimination has evolved to minimizethe costs of rejection and parasitism. The ability to rejecthighly mimetic conspecific eggs may explain the current paucityof brood parasitism in this species. The significance of thisfor brood parasite-host coevolution is discussed.     

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.     

Low frequency of observed cowbird parasitism on eastern kingbirds: host rejection, effective nest defense, or parasite avoidance?

Eastern kingbird (Tyrannus tyrannus) nests rarely are parasitizedby brown-headed cowbirds (Molotrus ater). Kingbirds are oneof a dozen or so species known to eject cowbird eggs from theirnests. We hypothesized that either kingbirds eject cowbird eggsso quickly that researchers normally do not detect the eggsduring daily nest inspections, or that cowbirds avoid parasitizingkingbirds. We tested these alternative hypotheses by experimentallyintroducing real cowbird eggs into eastern kingbird nests duringthe pre-egg, early laying, late laying, and incubation stages.We recorded the interval between "parasitism" and ejection ofthe cowbird eggs. Although kingbirds ejected 87 of 88 cowbirdeggs placed in their nests, about 40% of the eggs remained innests for more than 24 h. Thus, during daily nest inspectionswe should have observed cowbird eggs if nests were parasitizedat all. In fact, we detected only one parasitized nest amongthe 402 inspected daily. The time for ejection was longest atnests parasitized early in laying, and shorter at nests parasitizedbefore and after. This variation in ejection times may reflectthe time kingbirds require to learn to recognize their own eggs.Although kingbirds defend their nests aggressively, they donot respond to female cowbirds as unique threats and do notguard their nests before sunrise when cowbirds lay. We concludethat cowbirds avoid parasitizing eastern kingbirds because theireggs most likely will be wasted. The rejection behavior persistspossibly because it is almost cost-free (a maximum of 0.07 kingbirdegg lost or damaged per cowbird egg ejected), or it evolvedin response to conspecific rather than cowbird parasitism. Foreignkingbird eggs introduced into nests at different nest stageswere ejected only during the pre-egg stage. This result supportsthe hypothesis that rejection behavior in eastern kingbirdsevolved in response to cowbird parasitism.     

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.     

Parasitism of maternal investment selects for increased clutch size and brood reduction in a host

The allocation of resources to young that will ultimately beleft to die appears counterintuitive. Yet obligate brood reductionhas evolved in a number of species, despite the waste of reproductiveinvestment this may incur. Here we test whether brood parasitismcould be one factor leading to the evolution of obligate broodreduction because surplus eggs in the nest during incubationoffer some protection from the costs of parasitism. Surpluseggs could benefit females in two ways. First, additional eggsmay protect against the direct costs of parasitism by facilitatingrecognition and removal of parasitic eggs with greater accuracy.Second, additional eggs may protect against the indirect costsof parasitism as parasites often damage or remove host eggswhen entering the host nest; surplus eggs may be an essentialinsurance strategy against this damage. We test these possibilitiesin the Montezuma Oropendola (Psarocolius Montezuma), a speciesexperiencing high levels of parasitism by Giant Cowbirds (Scaphiduraoryzivora) throughout their range. Overall rejection rates ofcowbird eggs were high (72%), and experimental addition of parasiticeggs to empty, one-, and two-egg nests demonstrated that recognitionsuccess was unaffected by the presence of additional host eggsfor comparison. However, the value of surplus eggs when oneegg was removed or damaged by a parasite was high; 31.6% ofsuccessful two-egg clutches lost a single egg during incubationand would have failed to produce a chick without a second egg.This was directly attributable to parasitism in at least 33%of all cases. Therefore, despite highly developed host defensesagainst direct costs of parasitism (recognition and removalof parasitic eggs), the associated indirect costs (egg damageand removal) could play an important role in selection for aclutch size that results in more chicks than can be raised.     

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.     

Brood parasitism selects for no defence in a cuckoo host

In coevolutionary arms races, like between cuckoos and their hosts, it is easy to understand why the host is under selection favouring anti-parasitism behaviour, such as egg rejection, which can lead to parasites evolving remarkable adaptations to ‘trick’ their host, such as mimetic eggs. But what about cases where the cuckoo egg is not mimetic and where the host does not act against it? Classically, such apparently non-adaptive behaviour is put down to evolutionary lag: given enough time, egg mimicry and parasite avoidance strategies will evolve. An alternative is that absence of egg mimicry and of anti-parasite behaviour is stable. Such stability is at first sight highly paradoxical. I show, using both field and experimental data to parametrize a simulation model, that the absence of defence behaviour by Cape bulbuls (Pycnonotus capensis) against parasitic eggs of the Jacobin cuckoo (Clamator jacobinus) is optimal behaviour. The cuckoo has evolved massive eggs (double the size of bulbul eggs) with thick shells, making it very hard or impossible for the host to eject the cuckoo egg. The host could still avoid brood parasitism by nest desertion. However, higher predation and parasitism risks later in the season makes desertion more costly than accepting the cuckoo egg, a strategy aided by the fact that many cuckoo eggs are incorrectly timed, so do not hatch in time and hence do not reduce host fitness to zero. Selection will therefore prevent the continuation of any coevolutionary arms race. Non-mimetic eggs and absence of defence strategies against cuckoo eggs will be the stable, if at first sight paradoxical, result.     

The evolution of obligate interspecific brood parasitism in birds

We present a simple analytical model to investigate the conditionsfor the evolution of obligate interspecific brood parasitismin birds, based on clutch size optimization, when birds canlay more eggs than their optimal clutch size. The results showthat once intraspecific parasitism has appeared (i.e., femalesstart to spread their eggs over their own and other nests) the evolutionarily stable number of eggs laid in its own nest decreases.Two possible ESSs exist: (1) either the evolutionarily stablenumber of eggs laid in its own nest is larger than zero, anda fraction of the total number of eggs is laid parasitically(i.e., intraspecific parasitism); and (2) either the evolutionarilystable number of eggs laid in its own nest is zero and all eggs are laid parasitically. Since all females lay parasitically,this could favor the evolution of obligate interspecific broodparasitism. The key parameter allowing the shift from intraspecificto obligate interspecific parasitism is the intensity of density-dependentmortality within broods (i.e., nestling competition). Strongnestling competition, as in altricial species, can lead toan ESS where all eggs are laid parasitically. Altricial speciesare, therefore, predicted to evolve more easily toward obligate interspecific parasitism than precocial species. These predictionsfit the observed distribution of brood parasitism in birds,where only one species out of 95 obligate interspecific parasitesexhibits a precocial mode of development. Different nestlingsurvival functions provided similar findings (i.e., obligatebrood parasitism is more likely to evolve in altricial species),suggesting that these results are robust with respect to themain assumption of the model.     

How can distinct egg polymorphism be maintained in the rufescent prinia (Prinia rufescens)–plaintive cuckoo (Cacomantis merulinus) interaction—a modeling approach

In avian brood parasitism, both the host and the parasite are expected to develop various conflicting adaptations; hosts develop a defense against parasitism, such as an ability to recognize and reject parasitic eggs that look unlike their own, while parasites evolve egg mimicry to counter this host defense. Hosts may further evolve to generate various egg phenotypes that are not mimicked by parasites. Difference in egg phenotype critically affects the successful reproduction of hosts and parasites. Recent studies have shown that clear polymorphism in egg phenotype is observed in several host–parasite interactions, which suggests that egg polymorphism may be a more universal phenomenon than previously thought. We examined the mechanism for maintaining egg polymorphism in the rufescent prinia (Prinia rufescens) that is parasitized by the plaintive cuckoo (Cacomantis merulinus) from a theoretical viewpoint based on a mathematical model. The prinia has four distinct egg phenotypes: immaculate white, immaculate blue, white with spots, and blue with spots. Only two egg phenotypes, white with spots and blue with spots, are found in the cuckoo population. We show that the observed prinia and cuckoo phenotypes cannot be at an equilibrium and that egg polymorphism can be maintained either at stationary equilibrium or with dynamic, frequency oscillations, depending on the mutation rates of the background color and spottiness. Long‐term monitoring of the prinia–cuckoo interaction over a wide geographic range is needed to test the results of the model analyses.     

Recognizing odd smells and ejection of brood parasitic eggs. An experimental test in magpies of a novel defensive trait against brood parasitism

One of the most important defensive host traits against brood parasitism is the detection and ejection of parasitic eggs from their nests. Here, we explore the possible role of olfaction in this defensive behaviour. We performed egg‐recognition tests in magpie Pica pica nests with model eggs resembling those of parasitic great spotted cuckoos Clamator glandarius. In one of the experiment, experimental model eggs were exposed to strong or moderate smell of tobacco smoke, whereas those of a third group (control) were cleaned with disinfecting wipes and kept in boxes containing odourless cotton. Results showed that model eggs with strong tobacco scent were more frequently ejected compared with control ones. In another experiment, models were smeared with scents from cloacal wash from magpies (control), cloacal wash or uropygial secretions from cuckoos, or human scents. This experiment resulted in a statistically significant effect of treatment in unparasitized magpie nests in which control model eggs handled by humans were more often rejected. These results provide the first evidence that hosts of brood parasites use their olfactory ability to detect and eject foreign eggs from their nests. These findings may have important consequences for handling procedures of experimental eggs used in egg‐recognition tests, in addition to our understanding of interactions between brood parasites and their hosts.     

The importance of clutch characteristics and learning for antiparasite adaptations in hosts of avian brood parasites

There is considerable variation in rejection rates of parasitic eggs among hosts of avian brood parasites. In this article, we develop a model that can be used to predict host egg rejection behavior in brood parasite-host systems in general, by considering both intra- and interclutch variation in host egg appearance; clutch characteristics that may be important in calculating the fitness of individuals adopting rejecter or acceptor strategies. In addition, we consider the importance of learning the appearance of own eggs during the first breeding attempt and host probability of survival between breeding seasons on evolution of rejection behavior. Based on this model we can predict at which level of parasitism fitness of rejecter individuals is higher than that of acceptor individuals and vice versa. The model analyses show that variation in egg appearance can be a key factor for the evolution of host defense against parasitism. In more detail, analyses show that we should expect to find a prolonged learning period only in hosts that have a high intraclutch variation in egg appearance, because such hosts may potentially experience high costs in terms of recognition errors. Furthermore, learning is in general more adaptive in parasite-host systems in which hosts do have some reproductive success even when parasitized, and when parasitism rates are moderate. By including variables that have not been considered in previous models, our model represents a useful tool in investigations of host rejection behavior in various host-parasite systems.     

Host-parasite kinship in a female-philopatric bird population: evidence from relatedness trend analysis

Conspecific brood parasitism (CBP), an alternative reproductive tactic where some females lay eggs in the nests of other females of the same species, occurs in many animals with egg care. It is particularly common in waterfowl, for reasons that are debated. Many waterfowl females nest near their birthplace, making it likely that some local females are relatives. We analyse brood parasitism in a Hudson Bay population of common eiders, testing predictions from two alternative hypotheses on the role of relatedness in CBP. Some models predict host-parasite relatedness, others predict that parasites avoid close relatives as hosts. To distinguish between the alternatives, we use a novel approach, where the relatedness of host-parasite pairs is tested against the spatial population trend in pairwise relatedness. We estimate parasitism, nest take-over and relatedness with protein fingerprinting and bandsharing analysis of egg albumen, nondestructively sampled from each new egg in the nest throughout the laying period. The results refute the hypothesis that parasites avoid laying eggs in the nests of related hosts, and corroborate the alternative of host-parasite relatedness. With an estimated r of 0.12-0.14, females laying eggs in the same nest are on average closer kin than nesting neighbour females. Absence of a population trend in female pairwise relatedness vs. distance implies that host-parasite relatedness is not only an effect of strong natal philopatry: some additional form of kin bias is also involved.