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.     

Colony kin structure and host‐parasite relatedness in the barnacle goose

Conspecific brood parasitism (CBP), females laying eggs in the nest of other ‘host’ females of the same species, is a common alternative reproductive tactic among birds. For hosts there are likely costs of incubating and rearing foreign offspring, but costs may be low in species with precocial chicks such as waterfowl, among which CBP is common. Waterfowl show strong female natal philopatry, and spatial relatedness among females may influence the evolution of CBP. Here we investigate fine‐scale kin structure in a Baltic colony of barnacle geese, Branta leucopsis, estimating female spatial relatedness using protein fingerprints of egg albumen, and testing the performance of this estimator in known mother‐daughter pairs. Relatedness was significantly higher between neighbour females (nesting ≤ 40 metres from each other) than between females nesting farther apart, but there was no further distance trend in relatedness. This pattern may be explained by earlier observations of females nesting close to their mother or brood sisters, even when far from the birth nest. Hosts and parasites were on average not more closely related than neighbour females. In 25 of 35 sampled parasitized nests, parasitic eggs were laid after the host female finished laying, too late to develop and hatch. Timely parasites, laying eggs in the host’s laying sequence, had similar relatedness to hosts as that between neighbours. Females laying late parasitic eggs tended to be less related to the host, but not significantly so. Our results suggest that CBP in barnacle geese might represent different tactical life‐history responses.     

Egg morphology fails to identify nests parasitized by conspecifics in common pochard: a test based on protein fingerprinting and including female relatedness

Conspecific brood parasites lay eggs in nests of other females of the same species. A variety of methods have been developed and used to detect conspecific brood parasitism (CBP). Traditional methods may be inaccurate in detecting CBP and in revealing its true frequency. On the other hand more accurate molecular methods are expensive and time consuming. Eadie developed a method for revealing CBP based on differences in egg morphology. That method is based on Euclidean distances calculated for pairs of eggs within a clutch using standardized egg measurements (length, width and weight). We tested the applicability of this method in the common pochard Aythya ferina using nests that were identified as parasitized (39 nests) or non‐parasitized (16 nests) based on protein fingerprinting of eggs. We also analyzed whether we can distinguish between parasitic and host eggs in the nest. We found that variation in MED can be explained by parasitism but there was a huge overlap in MED between parasitized and non‐parasitized nests. MED also increased with clutch size. Using discriminant function analysis (DFA) we found that only 76.4% of nests were correctly assigned as parasitized or non‐parasitized and only 68.3% of eggs as parasitic or host eggs. Moreover we found that MED in parasitized nests increased with relatedness of the females that laid eggs in the nest. This finding was supported by positive correlation between MED and estimated relatedness in female–female pairs. Although variation in egg morphology is associated with CBP, it does not provide a reliable clue for distinguishing parasitized nests from non‐parasitized nests in common pochard.     

Brood parasitism,relatedness and sociality: a kinship role in female reproductive tactics

Conspecific brood parasitism (CBP) is a reproductive tactic in which parasitic females lay eggs in nests of other females of the same species that then raise the joint brood. Parasites benefit by increased reproduction, without costs of parental care for the parasitic eggs. CBP occurs in many egg‐laying animals, among birds most often in species with large clutches and self‐feeding young: two major factors facilitating successful parasitism. CBP is particularly common in waterfowl (Anatidae), a group with female‐biased natal philopatry and locally related females. Theory suggests that relatedness between host and parasite can lead to inclusive fitness benefits for both, but if host costs are high, parasites should instead target unrelated females. Pairwise relatedness (r) in host–parasite (h‐p) pairs of females has been estimated using molecular genetic methods in seven waterfowl (10 studies). In many h‐p pairs, the two females were unrelated (with low r, near the local population mean). However, close relatives (r = 0.5) were over‐represented in h‐p pairs, which in all 10 studies had higher mean relatedness than other females. In one species where this was studied, h‐p relatedness was higher than between nesting close neighbours, and hosts parasitized by non‐relatives aggressively rejected other females. In another species, birth nest‐mates (mother–daughters, sisters) associated in the breeding area as adults, and became h‐p pairs more often than expected by chance. These and other results point to recognition of birth nest‐mates and perhaps other close relatives. For small to medium host clutch sizes, addition of a few parasitic eggs need not reduce host offspring success. Estimates in two species suggest that hosts can then gain inclusive fitness if parasitized by relatives. Other evidence of female cooperation is incubation by old eider Somateria mollissima females of clutches laid by their relatives, and merging and joint care of broods of young. Merging females tended to be more closely related. Eiders associate with kin in many situations, and in some geese and swans, related females may associate over many years. Recent genetic evidence shows that also New World quails (Odontophoridae) have female‐biased natal philopatry, CBP and brood merging, inviting further study and comparison with waterfowl. Kin‐related parasitism also occurs in some insects, with revealing parallels and differences compared to birds. In hemipteran bugs, receiving extra eggs is beneficial for hosts by diluting offspring predation. In eggplant lace bugs Gargaphia solani, host and parasite are closely related, and kin selection favours egg donation to related females. Further studies of kinship in CBP, brood merging and other contexts can test if some of these species are socially more advanced than presently known.     

High rates of conspecific brood parasitism revealed by microsatellite analysis in a diving duck,the common pochard Aythya ferina

Conspecific brood parasitism (CBP) is a reproductive tactic whereby a parasitic female lays its eggs into the nests of other conspecific females. Genetic‐based data on the occurrence of CBP in birds, however, is still relatively scarce. We analysed prevalence of CBP in a ground‐nesting diving duck, the common pochard Aythya ferina, using a set of 17 microsatellites. Compared to related species, our population showed a relatively high level of CBP, with 39% of genotyped pochard eggs laid parasitically and 89% of nests containing ≥ 1 parasitic egg. In addition, we observed relatively high rates of interspecific brood parasitism (13% of eggs), caused predominantly by mallard Anas plathyrhynchos and tufted duck Aythya fuligula. CBP eggs had decreased hatching success compared to host eggs, with 65% of CBP and 95% of non‐CBP genotyped eggs hatching successfully. Our data suggest that this was probably due to improper timing of parasitic egglaying, which compromised synchronised hatching of CBP and host‐eggs. Despite high rates of CBP in our pochard popu lation, fitness costs associated with this reproductive tactic appear to be low for host females since neither clutch size nor host‐egg hatching probability were reduced due to CBP.     

Relatedness and spatial proximity as determinants of host–parasite interactions in the brood parasitic Barrow's goldeneye (Bucephala islandica)

Recent studies, which have found evidence for kin-biased egg donation, have sparked interest in re-assessing the parasitic nature of conspecific brood parasitism (CBP). Since host–parasite kinship is essential for mutual benefits to arise from CBP, we explored the role of relatedness in determining the behaviour of conspecific nest parasites and their hosts in nesting female Barrow's goldeneyes ( Bucephala islandica ), a duck in which CBP is common. The results revealed that the amount of parasitism increased with host–parasite relatedness, the effect of which was independent of geographical proximity of host and parasite nests. Proximity per se was also positively associated with the amount of parasitism. Furthermore, while hosts appeared to reduce their clutch size as a response to the presence of parasitic eggs, the magnitude of host clutch reduction also tended to increase with increasing relatedness to the parasite. Hence, our results indicate that both relatedness and spatial proximity are important determinants of CBP, and that host clutch reduction may be an adaptation to nest parasitism, modulated by host–parasite relatedness. Taken together, the results provide a demonstration that relatedness influences host and parasite behaviour in Barrow's goldeneyes, resulting in kin-biased egg donation.     

A brood parasite selects for its own egg traits

Many brood parasitic birds lay eggs that mimic their hosts'' eggs in appearance. This typically arises from selection from discriminating hosts that reject eggs which differ from their own. However, selection on parasitic eggs may also arise from parasites themselves, because it should pay a laying parasitic female to detect and destroy another parasitic egg previously laid in the same host nest by a different female. In this study, I experimentally test the source of selection on greater honeyguide (Indicator indicator) egg size and shape, which is correlated with that of its several host species, all of which breed in dark holes. Its commonest host species did not discriminate against experimental eggs that differed from their own in size and shape, but laying female honeyguides preferentially punctured experimental eggs more than host or control eggs. This should improve offspring survival given that multiple parasitism by this species is common, and that honeyguide chicks kill all other nest occupants. Hence, selection on egg size in greater honeyguides parasitizing bee-eaters appears to be imposed not by host defences but by interference competition among parasites themselves.     

Relative reproductive success of female moorhens using conditional strategies of brood parasitism and parental care

In a population of moorhens (Gallinula chloropus), at least27% of netting females laid one or more eggs in a neighbor'snest Females laid parasitically under three conditions: 56%of parasitic eggs were from nesting females that preceded layinga dutch in their own nest by a parasitic laying bout, 19% werefrom females whose nests were depredated before clutch completionand that laid the following egg parasiticaDy, and 25% were froma small number of females without territories, "non-nesting"parasites, that each laid a series of parasitic eggs. Clutchsizes varied greatly between females, but nesting females eachlaid a consistent clutch size both within and between seasonsfor a given mate and territory. Nesting females that employeda dual strategy of brood parasitism and parental care producedextra eggs that they laid in the nests of neighbors before layinga dutch in their own nests. Two out of ten females whose dutchesI experimentally removed during the laying period were successfullyinduced to lay their next egg in the nest of a neighbor. Nestingfemales that laid parasitically selected their hosts opportunisticallyfrom among the nests dosest to their territories. An experimentin which parasitic eggs were removed and hosts left to rearonly their own young showed that parasites did not choose hoststhat were better parents than pairs with contemporary neststhat were not parasitized. Females that only laid parasiticaDywithin a given season timed their parasitic laying bouts poorlyand achieved no reproductive success. Parasitic young rarelyfledged, and the mean seasonal reproductive success of nestingbrood parasites did not differ from that of nonparasitic females.However, the variance in reproductive success of nesting broodparasites was significantly higher than that of nonparasiticfemales.     

Intraspecific nest parasitism in the European starling Sturnus vulgaris

Biochemical genetic markers were used along with conventional methods (abnormal laying sequence/clutch size, unusual egg shape/pigmentation) to identify intraspecific nest parasitism at two British nestbox colonies of the European starling. Between 11 and 37% of first clutches were parasitized during 1977–1979. Parasitic females probably comprised all of the following categories: (1) paired females contesting a nestbox occupied by another pair; (2) previously paired females who had laid a clutch but had been unsuccessful; (3) unpaired females who had copulated with males that already had a mate and nest site; and (4) ‘professional’ nest parasites who distributed at lest some of their eggs in one or more nests other than their own. Although parasitized nests had higher clutch sizes, parasitism led to fewer host young fledging per egg laid, mainly through the eviction of eggs and subsequent nest desertion. Number of parasitic young fledged per egg laid was highest when eggs were laid synchronously with the host, when host clutches were larger, or a smaller number of parasite eggs were added to a nest, thus favouring parasites that distribute their eggs amongst a number of nests. A greater pressure on nest sites may have accounted for the higher levels of parasitism at the Aberdeen colony and for the greater number of parasite eggs laid in a nest. Although most parasitic female starlings appeared to be much less successful than non-parasitic ones, nest parasitism in the starling might evolve directly when one or more of the following advantages are present. (1) There are no constraints on the number of eggs a female may lay but there are constraints on the number of young she may feed adequately. (2) Female survival is increased by having fewer or no eggs/young to care for. (3) Current feeding conditions favour the survival of more young than would be produced by the most common clutch size. Intraspecific nest parasitism is considered to be a first stage in the evolution of interspecific nest parasitism.     

Do parasitic common goldeneye Bucephala clangula females choose nests on the basis of host traits or nest site traits?

Conspecific brood parasitism (CBP) is an important alternative breeding strategy for gaining reproductive output in birds. While interactions between hosts and parasites and consequences of CBP to breeding success of both parties have been studied a lot, the roles of host characteristics and nest site characteristics in CBP have received less attention. We studied the relative importance of host‐related traits, such as female condition and breeding experience, and nest‐site‐related factors, such as overall nest site preference and occupation rate, in explaining the occurrence of CBP in a common goldeneye Bucephala clangula population. We used spatially and temporally extensive data sets, analysed the data with generalized linear mixed models that allowed us to account for the non‐independency of individual nesting attempts across females and nesting sites, and used an information theoretic approach in model selection and inference. About half of the nests were parasitized annually during the seven year study period. The occurrence of CBP decreased with advancement of the breeding season but late nests were also frequently parasitized. We found that the occurrence of CBP was better explained by nest‐site characteristics than host traits, implying that parasitic females target a given nest based on factors related to the nest site itself rather than on the host. Our results suggest that more attention should be paid to factors associated with nest site attractiveness and quality when studying laying decisions of parasites and the occurrence of CBP in general.     

Conspecific brood parasitism and population dynamics

Conspecific brood parasitism (CBP), defined as parasitic laying of eggs in a conspecific nest without providing parental care, occurs in insects, fishes, amphibians, and many birds. Numerous factors have been proposed to influence the evolution of CBP, including nest site limitation; effects of brood size, laying order, or parasitic status on offspring survival; randomness of parasitic egg distribution; adult life-history trade-offs; and variation in parental female quality or risk of nest predation. However, few theoretical studies consider multiple possible types of parasitism or the interplay between evolution of parasitism and population dynamics. We review existing theory of CBP and develop a synthetic modeling approach to ask how best-of-a-bad job parasitism, separate-strategies parasitism (in which females either nest or parasitize), and joint-strategies parasitism (in which females can both nest and parasitize) differ in their evolutionary conditions and impacts on population dynamics using an adaptive dynamics framework including multivariate traits. CBP can either stabilize or destabilize population dynamics in different scenarios, and the role of comparable parameters on evolutionarily stable strategy parasitism rate, equilibrium population size, and population stability can differ for the different modes of parasitism.     

To eject or to abandon? Life history traits of hosts and parasites interact to influence the fitness payoffs of alternative anti‐parasite strategies

Hosts either tolerate avian brood parasitism or reject it by ejecting parasitic eggs, as seen in most rejecter hosts of common cuckoos, Cuculus canorus, or by abandoning parasitized clutches, as seen in most rejecter hosts of brown‐headed cowbirds, Molothrus ater. What explains consistent variation between alternative rejection behaviours of hosts within the same species and across species when exposed to different types of parasites? Life history theory predicts that when parasites decrease the fitness of host offspring, but not the future reproductive success of host adults, optimal clutch size should decrease. Consistent with this prediction, evolutionarily old cowbird hosts, but not cuckoo hosts, have lower clutch sizes than related rarely‐ or newly parasitized species. We constructed a mathematical model to calculate the fitness payoffs of egg ejector vs. nest abandoner hosts to determine if various aspects of host life history traits and brood parasites’ virulence on adult and young host fitness differentially influence the payoffs of alternative host defences. These calculations showed that in general egg ejection was a superior anti‐parasite strategy to nest abandonment. Yet, increasing parasitism rates and increasing fitness values of hosts’ eggs in both currently parasitized and future replacement nests led to switch points in fitness payoffs in favour of nest abandonment. Nonetheless, nest abandonment became selectively more favourable only at lower clutch sizes and only when hosts faced parasitism by a cowbird‐ rather than a cuckoo‐type brood parasite. We suggest that, in addition to evolutionary lag and gape‐size limitation, our estimated fitness differences based on life history trait variation provide new insights for the consistent differences observed in the anti‐parasite rejection strategies between many cuckoo‐ and cowbird‐hosts.     

Why is mimicry in cuckoo eggs sometimes so poor?

I propose that the existence of imperfect adaptations (e.g. egg mimicry) in brood parasites and their hosts (e.g. discrimination abilities) could reflect age-dependent territory and nest-site selection patterns of the host. Studies of various passerines indicate that (1) older breeders tend to occupy nest sites of higher quality than do young birds (ideal despotic distribution resulting from interference competition), (2) nest-site selection affects the risk of parasitism in various habitats, (3) egg recognition in passerines has a strong learning component (therefore naive breeders tend to accept whereas older birds tend to reject parasitic eggs). Because young naive birds, who tend to accept parasitic eggs, usually breed in low-quality areas where they are frequently parasitised, while old experienced birds, who tend to reject parasitic eggs, breed in high-quality areas where they are rarely parasitised, the distribution of acceptors and rejecters with respect to the risk of parasitism is non-random, i.e. populations of some host species may consist of heavily parasitised acceptors and weakly parasitised rejecters. Therefore, the selection pressure exerted by the host on the parasite should be weaker than if brood parasitism was randomly distributed among naive and experienced breeders and affect adaptations such as egg mimicry. This could explain the existence of imperfect adaptations in some brood parasite-host systems.     

Brood parasitic European starlings do not lay high-quality eggs

Chicks of obligate brood parasites employ a variety of morphologicaland behavioral strategies to outcompete nest mates. Elevatedcompetitiveness is favored by natural selection because parasiticchicks are not related to their host parents or nest mates.When chicks of conspecific brood parasites (CBPs) are unrelatedto their hosts, they and their parents would also benefit fromtraits that enhance competitiveness. However, these traits mustbe inducible tactics in CBPs, since conspecific brood parasitism(CBP) is facultative. Such tactics could be induced by resourcespassed to offspring through the egg. Thus, females engagingin CBP should allocate to their eggs resources that will enhanceoffspring competitiveness. We tested this prediction in a populationof European starlings (Sturnus vulgaris) breeding in southernSweden. Previous research showed that almost all CBPs in thispopulation are floater females that have yet to breed in thecurrent season. We identified putative brood parasitic eggsthrough monitoring egg laying and verified parasitism usingprotein fingerprinting. We then determined whether parasiticeggs were larger, larger-yolked, or had higher concentrationsof yolk testosterone or androstenedione than control eggs. The14 brood parasitic eggs laid in active nests (those with clutchesof at least two eggs that were eventually incubated) did notdiffer from controls in any of these characteristics. Ten dumpedeggs, laid in nonactive nest-boxes or on the ground, were smallerand smaller-yolked than control eggs but did not differ in yolkandrogen concentrations. The failure of our prediction couldbe the result of high costs of investing in eggs, lack of competition-basedbenefits for chicks, or physiological constraints on egg manipulation.     

FREQUENCY-DEPENDENT FITNESS CONSEQUENCES OF INTRASPECIFIC NEST PARASITISM IN SNOW GEESE

The reproductive efficiency, defined as the number of breeding recruits produced per egg laid; of intraspecific nest parasites; of hosts in parasitized nests; and of unparasitized nesting females, was measured for 14 years for lesser snow geese Anser caerulescens caerulescens nesting near Churchill, Manitoba, Canada. Relative efficiencies were 0.71–0.88, 0.91, and 1.0 for eggs of parasites, hosts, and unparasitized birds, respectively. Differences in the hatching probabilities of the three classes of eggs produced the efficiency differences. Parasitic success was limited by the parasites' failure to place more eggs than expected by chance into nests at the appropriate time relative to host incubation. Host nesting success was lower when more than one parasitic egg was added to the clutch. No differences in gosling survival and breeding recruitment probabilities were detected among any categories of goslings. Thus, hatching parasitic young are at no disadvantage relative to parental young, and there is no support for the hypothesis that increased success of host young at later stages of reproduction might offset negative effects at the egg stage. The hatching efficiency of parasitic eggs declined more rapidly than that of parental eggs as the parasitism rate increased. Efficiencies were similar when 3–4% of the eggs laid per year were parasitic, but relative parasitic efficiency was significantly lower when the parasitism rate was 8–9% or more. Using ancillary information and assumptions about the fecundity, viability, and behavioral flexibility of parasitic and parental females, we conclude that intraspecific nest parasitism could compete with nesting as a reproductive strategy in this population. The conditional use of parasitism by a large component of the population in certain years, however, combined with negative-frequency dependent success, limits the potential spread of a purely parasitic strategy in this population.     

AVIAN BROOD PARASITISM: INFORMATION USE AND VARIATION IN EGG‐REJECTION BEHAVIOR

Hosts of avian brood parasites often vary in their response to parasitized clutches: they may eject one or several eggs, desert the nest, or accept all the eggs. Focusing on hosts exposed to single‐egg parasitism by an evicting brood parasite, we construct an optimality model that includes all these behavioral options and use it to explore variation in rejection behavior. We particularly consider the influence of egg mimicry and external cues (observations of adult parasites near the nest) on optimal choice of rejection behavior. We find that several rejection responses will be present in a host population under a wide range of conditions. Ejection of multiple eggs tends to be adaptive when egg mimicry is fairly accurate, external cues provide reliable information of the risk of parasitism, and the expected success of renesting is low. If the perceived risk of parasitism is high, ejection of one or a few eggs may be the optimal rejection response even in cases in which hosts cannot discriminate between eggs. This may have consequences for the long‐term outcome of the coevolutionary chase between hosts and parasites. We propose an alternative evolutionary pathway by which egg ejection may first arise as a defense against brood parasitism.     

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.     

Molecular tracking of individual host use in the Shiny Cowbird – a generalist brood parasite

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120‐bp fragment of the mtDNA control region, we reconstructed full‐sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk‐browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.     

Costs and response to conspecific brood parasitism by colonial red-breasted mergansers

Costs of conspecific brood parasitism (CBP) are expected to be influenced by a species’ life history traits. Precocial birds lay large clutches, and clutches that have been enlarged by CBP can affect host fitness through a longer incubation period, displaced eggs, and lower hatching success. We examined costs and response to CBP by hosts in a population of colonial red-breasted mergansers (Mergus serrator; n?=?400 nests over 8 years) within which 29% of parasitized clutches were enlarged considerably (≥?15 eggs). Length of the incubation period did not increase with clutch size. The mean number of eggs displaced from a parasitized nest during incubation (2.8) was 2×?greater than at an unparasitized nest (1.4). Hatching success declined by 2% for each additional egg in the nest. Thus, for a nest with?≥?15 eggs, one or more fewer host eggs hatch relative to an unparasitized nest with the same number of host eggs, assuming equal probability of success for all eggs. Hosts were 40% more likely to desert nests receiving 2 or 6 experimental eggs relative to unparasitized control nests, although it is unknown whether hens deserting a nest renested elsewhere. Our study indicates that costs of CBP to hosts during nesting may be limited to those red-breasted mergansers incubating the largest clutches (≥?15 eggs), and it raises questions about the adaptive significance of deserting a parasitized clutch.     

Why do Red-winged Blackbrids accept eggs of Brown-headed Cowbirds?

Summary Avian brood parasites usually severely depress the reproductive success of their hosts, yet many host species, including those presumably capable of ejecting parasitic eggs, accept them. Female, brood-parasitic, Brown-headed Cowbirds typically remove a host egg when they lay their own and damage some host eggs in the process of ejecting a host egg. Data from a field study of Red-winged Blackbirds show that these costs, which cannot be avoided by ejecting the parasitic egg, account for some of the reproductive losses attributable to parasitism, but part is due to the presence of the cowbird egg in the nest. To assess whether ejection would be favoured under current circumstances, the probable damage a female Redwing could cause to her own eggs by attempting to eject a cowbird egg needs to be determined.