Why do all host species not show defense against avian brood parasitism: evolutionary lag or equilibrium?

Avian brood parasitism reduces the reproductive success of hosts and is therefore expected to select for host defenses against parasitism, such as an ability to reject parasitic eggs. Field studies have shown that some hosts recognize and reject parasitism, whereas others do not, and the degree of the defense varies from population to population. One long-standing debate concentrates on the differences in the distribution of host defenses observed in hosts parasitized by the brown-headed cowbird and the common cuckoo. The cowbird's hosts show either few or nearly perfect defenses, whereas the cuckoo's hosts have defenses varying from none to complete, with most falling in between the two extremes. To explore the mechanisms underlying this pattern, I constructed a mathematical model in which host defense is assumed to be genetically determined and analyzed how the host defense is established under parasitic pressure. The model shows that differences in the defense-level distribution can be attributed to the difference in the parasite's breeding strategy, generalized or specialized: hosts parasitized by generalists show perfect, none, or intermediate levels of the defense depending on the host abundances, whereas hosts parasitized by specialists always exhibit either none or intermediate levels of the defense if the parasite lacks counter defenses such as egg mimicry. This result provides a testable explanation for the existence of accepter species of the brown-headed cowbird, which might reconcile the previously conflicting hypotheses.     

Rapid increase of host defence against brood parasites in a recently parasitized area: the case of village weavers in Hispaniola

Passerine hosts of brood parasitic birds usually vary in their ability to discriminate and reject alien eggs. Two main hypotheses have been suggested to explain the persistence of acceptor individuals in species that are exploited by brood parasites. The evolutionary lag hypothesis postulates that some hosts have not yet evolved the ability to discriminate against alien eggs. Once the ability to recognize the parasitic egg has appeared by mutation, it spreads because of the selective advantage of rejection. Parasites are then selected to produce more mimetic eggs, in order to escape host discrimination, which eventually ends up in an arms race between the parasite and the host. The evolutionary equilibrium hypothesis is based on the assumption that rejection behaviour is costly in the absence of parasitism, because of recognition errors. Acceptor hosts can persist when parasitism rate fluctuates or is consistently low. Indirect evidence for costs of rejection in the absence of parasitism has been provided by Cruz and Wiley, who reported low rejection rate for a population of village weavers (Ploceus cucullatus) introduced from Africa to Hispaniola (West Indies) more than a century ago. In Africa the species is parasitized by Chrysococcyx cuckoos and shows high levels of egg discrimination. Since no brood parasite was present in Hispaniola, Cruz and Wiley suggested that rejection was selected against in the absence of parasitism due to recognition costs. Introduction of village weavers in Hispaniola, therefore, provided a unique opportunity to test the decline of an adaptation. During the past century the shiny cowbird (Molothrus bonariensis) has expanded its range-invading most of the West Indies from South America. It was first observed in Hispaniola in 1972, and it started to exploit village weavers as a host. Given that shiny cowbirds substantially reduce the reproductive success of weavers, we should expect higher rejection rates nowadays compared to those reported by Cruz and Wiley 16 years ago. In agreement with this prediction, we found a high rejection rate of cowbird model eggs (89.3%, 95% CI = 81.1 to 97.5%), moderate levels of rejection of non-mimetic weaver model eggs (67.5%, 95% CI = 52.5 to 82.5%) and rather low levels of rejection of mimetic weaver model eggs (25%, 95% CI = 4 to 46%). The rejection rate of artificial cowbird eggs has therefore increased from 13.8% (95% CI = 5 to 22.6%) to 89.3% in 16 years. To check whether this rapid increase in host resistance is compatible with a genetic microevolutionary change, we built a population dynamics model where, as an upper bound, resistance is inherited by the progeny with a probability of one. This simple model shows that observed changes of rejection rate are compatible with a genetic microevolutionary shift only under the most favourable scenario for rejecters to spread. Relaxing one or several of these assumptions (e.g. high parasitism rate, absence of rejection costs) considerably lengthens the period needed for rejecters to spread. We suggest that both genetic and learning processes might be involved in the observed changes.     

The role of kin recognition in the evolution of conspecific brood parasitism

Conspecific brood parasitism (CBP) is a common strategy in several species of birds. Currently, some studies suggest that relatedness between host and parasite enhances CBP, since indirect fitness benefits could select for acceptance of related eggs by hosts. Conversely, parasites should avoid laying eggs in nests of relatives if this is costly for the host. Based on the latter argument, kinship should not promote brood parasitism. A recent model clarified this relationship, and showed that kinship can promote brood parasitism, assuming kin recognition. However, in that model kin recognition was assumed perfect. Here we present a model that addresses the role of relatedness and kin selection in CBP, when kin recognition is not perfect and hosts do not always detect parasitism. We consider both the indirect fitness of the parasite and the possible responses of the host. Our results indicate that the existence and accuracy of a kin recognition system is crucial to the final outcome. When CBP represents a cost to the host, a parasitic female that has the choice should avoid parasitizing relatives, unless (1) the costs are not too high and (2) hosts can accurately enough recognize eggs laid by relatives, rejecting them less often than eggs laid by nonkin. But if ‘parasitism’ enhances the direct fitness of the host (which is possible in species with precocial young) parasites should choose relatives whenever possible, even if hosts do not recognize kin eggs. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.     

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.     

Community-level patterns of population recruitment in a generalist avian brood parasite, the brown-headed cowbird

The brown-headed cowbird (Molothrus ater) is a generalist brood parasite that typically parasitizes many host species in a single bird community. Population recruitment in a generalist parasite should be diverse with respect to host species; however, host-specific rates of cowbird recruitment have not been reported in any host community, and the determinants of host quality are poorly known. We studied the combined influence of parasitism level, nest abundance, and host quality on community-level patterns of cowbird recruitment in New Mexico, USA. Our objectives were to: (1) evaluate patterns of host use and quality; (2) compare cowbird egg investment and recruitment among host species; (3) identify host species of most importance to cowbird recruitment. Cowbirds parasitized 11 host species, with five “major” hosts experiencing high parasitism levels (≥1 cowbird egg/nest) and six minor hosts experiencing low parasitism levels (<0.3 cowbird eggs/nest). Parasitism level was not correlated with host species abundance, host mass, host nestling period length, or host success at fledging cowbirds. However, tree-nesting hosts were parasitized more than ground-nesters, and foliage-gleaners more than sally-foragers and ground-foragers. Average estimated survival to fledging of cowbird eggs laid in active host nests was 0.19. Cowbird recruitment was diverse with respect to hosts but was less evenly distributed across the host community than was cowbird egg investment because western tanagers (Piranga ludovicianus) fledged cowbirds more successfully than other hosts. This success in western tanagers was due to high cowbird survivorship in tanager nests and may be associated with the larger body size of tanagers relative to other hosts.     

Does superparasitism improve host suitability for parasitoid development? A case study in the Microplitis rufiventris-Spodoptera littoralis system

Superparasitism refers to the oviposition behavior of parasitoid females who lay their eggs in an already parasitized host. Recent studies have shown that allocation of additional eggs to an already parasitized host may be beneficial under certain conditions. In the present work, mortality of Microplitis rufiventris wasps was significantly influenced by both host instar of Spodoptera littoralis larvae at parasitism and level of parasitism. In single parasitization, all host instars (first through sixth) were not equally suitable. Percentage of emergence success of wasp larvae was very high in parasitized first through third (highly suitable hosts), fell to 60% in the fourth instar (moderate suitable) and sharply decreased in the penultimate (5th) instars (marginally suitable). Singly parasitized sixth (last) instar hosts produced no wasp larvae (entirely unsuitable), pupated and eclosed to apparently normal adult moths. The scenario was different under superparasitism, whereas supernumerary individuals in the highly suitable hosts were almost always killed as first instars, superparasitization in unsuitable hosts (4th through 6th) had significant increase in number of emergence success of wasp larvae. Also, significantly greater number of parasitoid larvae successfully developed in unsuitable hosts containing three wasp eggs than counterparts containing two wasp eggs. Moreover, the development of surplus wasp larvae was siblicidal in earlier instars and nonsiblicidal gregarious one in the penultimate and last “sixth” instars. It is suggested that the optimal way for M. rufiventris to deal with high quality hosts (early instars) is to lay a single egg, while the optimal way to deal with low quality hosts (late instars) might be to superparasitize these hosts.     

Adaptations in the common cuckoo (Cuculus canorus) to host eggs in a multiple-hosts system of brood parasitism

The common cuckoo Cuculus canorus parasitism greatly reduces the reproductive success of its hosts and imposes strong selection pressure for hosts to evolve defences against parasitism, such as the ability to recognize and reject dissimilar parasitic eggs, which, in turn, selects for better egg mimicry by the cuckoo. In the co-evolutionary interaction, however, it remains unknown how the cuckoo successfully expanded its range of host usage and how they developed egg mimicry. Most previous studies were conducted in areas where a very few number of host species (i.e. one or two at most) are sympatric with the cuckoo. Several host species, however, breed sympatric with the cuckoo and have been parasitized in the study site in Nagano, central Japan. Such a multiple-hosts system will provide valuable insights for understanding the cuckoo–hosts interactions in the past. In the present study, we report quantitative profiles of eggs based on spectrometer reflectance for four major host species and the corresponding cuckoo gentes. The hosts include the oriental reed warbler ( Acrocephalus orientalis ), bull-headed shrike ( Lanius bucephalus ), azure-winged magpie ( Cyanopica cyana ), and black-faced bunting ( Emberiza spodocephala ). We show that (1) egg morphs of each host and corresponding cuckoo gens can be categorized by two chromatic components of reflectance spectra and (2) there is a significant difference in a particular chroma component between hosts and the cuckoo. We suggest that the cuckoo parasitism in central Japan originated from parasitism on the black-faced bunting.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 291–300.     

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.     

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.     

Cuckoos versus reed warblers: Adaptations and counteradaptations

At study sites in Cambridgeshire, England, the percentage of reed warbler, Acrocephalus scirpaceus, nests parasitized by cuckoos, Cuculus canorus, in 2 years was 22·5% and 9·1%. The warblers rejected cuckoo eggs at 19% of parasitized nests. Parasitized clutches suffered less predation than unparasitized clutches, suggesting that the cuckoo itself was the major predator, plundering nests too advanced for parasitism so that the hosts would re-lay. The cuckoos laid a mimetic egg, parasitized nests in the afternoons during the host laying period, usually removed one host egg, laid a remarkably small egg and laid very quickly. Nests were experimentally parasitized with model eggs to study the significance of this procedure. Experiments showed that host discrimination selects for: (1) egg mimicry by cuckoos (poorer matching model eggs were more likely to be rejected); (2) parasitism during the laying period (mimetic eggs put in nests before host laying began were rejected); (3) afternoon laying (mimetic eggs were less likely to be accepted in the early morning than in the afternoon, when hosts were more often absent from the nest); (4) a small egg (large eggs, typical of non-parasitic cuckoos, were more likely to be rejected); (5) rapid laying (a stuffed cuckoo on the nest stimulated increased rejection of model eggs), and (6) sets a limit to host egg removal by cuckoos (if more than one or two are removed desertion may occur). Mimicry may also be selected for because it reduced the chance that second cuckoos can discriminate the first cuckoo's egg from the host's clutch. Predation did not select for mimicry; nests with a non-mimetic egg did not suffer greater predation than those with a mimetic egg. Host rejection of model eggs did not depend on: (1) stage of parasitism once host egg laying had begun (nevertheless cuckoos were more likely to lay early in the host laying period probably to increase the chance the cuckoo chick hatched); (2) removal of a host egg (however, this reduced the incidence of unhatched eggs so cuckoos may remove a host egg so as not to exceed the host incubation limit). There were two costs of rejection, an ‘ejection’ cost (own eggs ejected as well as the cuckoo egg) and, with mimetic eggs, a ‘recognition’ cost (own eggs ejected instead of the cuckoo egg). Reed warblers did not discriminate against unlike chicks (another species) and did not favour either a cuckoo chick or their own chicks when these were placed in two nests side by side. Possible reasons why the hosts discriminate against unlike eggs but not unlike chicks are discussed.     

Susceptibility of Nezara viridula (L.) (Hemiptera: Pentatomidae) Egg Masses of Different Sizes to Parasitism by Trissolcus basalis (Woll.) (Hymenoptera: Platygastridae) in the Field

Egg masses of Nezara viridula (L.) are commonly parasitized by Trissolcus basalis (Woll.), and we investigated the role of size of egg masses on parasitization by T. basalis. Sentinel egg masses were exposed to parasitism in the field for 6–7 days, when they were collected for evaluation of parasitoid emergence. We recorded the number of eggs per egg mass, the number of emerged hosts, and the number of empty and parasitized eggs. We calculated the proportion of attacked host egg masses (DE), the proportion of parasitized eggs per attacked egg mass (PE), and total parasitism (PI). The total number of egg masses exposed to parasitism was 330. The minimum, mean, and maximum egg mass sizes were 25, 75.2, and 111, respectively. DE and PE varied widely between different fields, and they were independent of egg mass size. In 14.2% of all parasitized egg masses, we found simultaneous emergence of T. basalis and N. viridula independently of host egg mass size. PE exhibited low variability compared with PI and DE, which were linearly related. PI and DE values from other field studies are consistent with the linear relationship, suggesting that PI is mostly related to the proportion of the DE. This also suggests that total parasitism is independent of egg mass size, of possible differences in plant species, and T. basalis density and strains.     

The evolution of acceptance and tolerance in hosts of avian brood parasites

Avian brood parasites lay their eggs in the nests of their hosts, which rear the parasite's progeny. The costs of parasitism have selected for the evolution of defence strategies in many host species. Most research has focused on resistance strategies, where hosts minimize the number of successful parasitism events using defences such as mobbing of adult brood parasites or rejection of parasite eggs. However, many hosts do not exhibit resistance. Here we explore why some hosts accept parasite eggs in their nests and how this is related to the virulence of the parasite. We also explore the extent to which acceptance of parasites can be explained by the evolution of tolerance; a strategy in which the host accepts the parasite but adjusts its life history or other traits to minimize the costs of parasitism. We review examples of tolerance in hosts of brood parasites (such as modifications to clutch size and multi‐broodedness), and utilize the literature on host–pathogen interactions and plant herbivory to analyse the prevalence of each type of defence (tolerance or resistance) and their evolution. We conclude that (i) the interactions between brood parasites and their hosts provide a highly tractable system for studying the evolution of tolerance, (ii) studies of host defences against brood parasites should investigate both resistance and tolerance, and (iii) tolerance and resistance can lead to contrasting evolutionary scenarios.     

Evidence for adaptive self-superparasitism in the dryinid parasitoid Haplogonatopus atratus when conspecifics are present

Self-superparasitism – ovipositing in or on a host that was previously parasitized by the same individual – had generally been considered maladaptive for solitary parasitoids. Recently, however, some researchers have suggested that self-superparasitized hosts produce higher fitness returns per host than singly parasitized hosts when these hosts are parasitized by conspecifics. The present study is the first to verify this possibility in the parasitoid Haplogonatopus atratus . Fitness returns from the first and second ovipositions under triple parasitism were compared with those from the first oviposition under double parasitism. Fitness returns were assessed in terms of survival rate, adult size, and developmental time. No differences were found between the two groups in adult size and developmental time. When the interval between the second and third ovipositions was 1 h, the total survival rate of the first and second comers under triple parasitism was higher than the survival rate of the first comer under double parasitism, and as high as the total survival rate of two first comers under double parasitism. This suggests that the value of self-parasitized hosts is as high as that of unparasitized hosts. When the second-to-third interval of triple parasitism was 24 h, self-superparasitism produced no fitness returns. Moreover, fitness returns from ovipositing into self-superparasitized hosts were similar to those from ovipositing into singly parasitized hosts.     

Brood parasitism and egg matching in the Red-chested Cuckoo Cuculus solitarius in southern Africa

Host usage and relative rates of egg matching were investigated in the Red-chested Cuckoo Cuculus solitarius in southern Africa, using nest record cards and museum collections. Eighteen host species were found to be parasitized at varying degrees of intensity (0.14–12.5%). The most commonly recorded parasitized host, the Cape Robin Cossypha caffra , had a relatively low rate of parasitism (2.46%). The host species experiencing the most recorded pressure from parasitism was the Bearded Robin Erythropygia quadrivirgata , with 12.5% parasitism. Human perception of cuckoo/host egg matching was assessed for parasitized clutches of host species in museum egg collections. Eggs of three different cuckoo egg morphs were scored as matching those of the host species on a 1–5 scale. Perfect/good matching was recorded for eggs found in Chorister Cossypha dichroa , Heuglin's Cossypha heuglini and Natal Robins' Cossypha natalensis clutches. Poor and very poor matching was evident for cuckoo eggs found in four host species' clutches: the Cape Robin, Stonechat Saxicola torquata , Cape Rockthrush Monticola rupestris and Black Flycatcher Melaenornis pammelaina . Available evidence suggests that the Red-chested Cuckoo parasitizes hosts in a particular environment (low vegetation and trees). Good to intermediate matching was recorded with only 47% of host eggs, and with only 28.5% of Cape Robin clutches. A relatively high degree of host specificity, however, is suggested by the nest record card data, which indicate that species with large numbers of records are not those with the highest rates of parasitism.     

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.     

Disappearance of eggs from nonparasitized nests of brood parasite hosts: the evolutionary equilibrium hypothesis revisited

The evolutionary equilibrium hypothesis was proposed to explain variation in egg rejection rates among individual hosts (intra‐ and interspecific) of avian brood parasites. Hosts may sometimes mistakenly reject own eggs when they are not parasitized (i.e. make recognition errors). Such errors would incur fitness costs and could counter the evolution of host defences driven by costs of parasitism (i.e. creating equilibrium between acceptors and rejecters within particular host populations). In the present study, we report the disappearance of host eggs from nonparasitized nests in populations of seven actual and potential hosts of the common cuckoo Cuculus canorus. Based on these data, we calculate the magnitude of the balancing parasitism rate provided that all eggs lost are a result of recognition errors. Importantly, because eggs are known to disappear from nests for reasons other than erroneous host rejection, our data represent the maximum estimates of such costs. Nonetheless, the disappearance of eggs was a rare event and therefore incurred low costs compared to the high costs of parasitism. Hence, costs as a result of recognition errors are probably of minor importance with respect to opposing selective pressure for the evolution of egg rejection in these hosts. We cannot exclude the possibility that low or intermediate egg rejection rates in some host populations may be caused by spatiotemporal variation in the occurrence of parasitism and gene flow, creating a variable influence of opposing costs as a result of recognition errors and the costs of parasitism.     

Superparasitism behavior and host discrimination of Campoletis chlorideae (Ichneumonidae: Hymenoptera) toward Mythimna separata (Noctuidae: Lepidoptera)

The oriental army worm, Mythimna separata Walker (Lepidoptera: Noctuidae), was adopted as a host to keep Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae) populations in the laboratory, because it can be mass reared. However, wasp cocoon production in this system via single parasitism was not satisfactory. To improve the rearing efficiency of C. chlorideae on M. separata, we studied the effects of superparasitism on the offspring production of C. chlorideae, as well as the host discrimination ability of the adult wasps in the laboratory. The results showed that, compared with single parasitism, both double and quintuple parasitism significantly increased C. chlorideae cocoon production in M. separata without a significant effect on emergence, weight, or sex ratio of adult wasps at the P < 0.05 level. Host selection experiments suggested that C. chlorideae preferred to lay eggs on unparasitized hosts rather than freshly parasitized hosts but could distinguish neither the 24-h postparasitization hosts from the unparasitized hosts nor the freshly self-parasitized hosts from the conspecifically parasitized hosts. No-choice behavior tests indicated that C. chlorideae took significantly longer time to accept the freshly parasitized hosts containing more than two eggs compared with the unparasitized hosts. In the end, superparasitism behavior and its application in the mass rearing of this endoparasitoid are discussed.     

Community-wide patterns of parasitism of a host “generalist” brood-parasitic cowbird

The brown-headed cowbird (Molothrusater) is a generalist obligate brood parasite. Despite intensive study and growing concern over the negative impact of cowbird parasitism on populations of many hosts, very little is known about the factors influencing community-wide patterns of cowbird parasitism. Using systematic nest searches, nest parasitism was studied over two breeding seasons at a study site in northeastern Illinois encompassing grassland, forest-edge, and forest habitat, supporting a diverse avian community. Parasitism was observed for 18 out of 34 altricial bird species found nesting at the study site. A total of 299 cowbird eggs and nestlings were found in 191 of a total of 593 nests. Analyses revealed several ecological and behavioral factors associated with frequency of parasitism and the resulting distribution of cowbird eggs. Much higher frequencies of parasitism were found in edge and forest habitats than in grassland. Within the edge habitat, open nests were parasitized significantly more often than cavity nests. Among open nests in the edge habitat, the two largest species were never parasitized. Host behavior, particularly egg-ejection behavior, was associated with a reduced observed frequency of parasitism, but at least three species known to eject cowbird eggs were sometimes parasitized. For six common hosts capable of rearing cowbirds, we found no correlation between level of parasitism and host nest-survivorship, suggesting that fine-grained assessments of host quality by female cowbirds do not influence patterns of parasitism among acceptable host species, or that differences in host quality are not great and/or predictable enough for such fine-grained assessments. Our results suggest that when a variety of possible nests are available, the level of parasitism on a particular species is a balance between a␣cowbird's preference for a particular species and the effectiveness of host species' defenses. A conceptual model was developed that incorporates the observed correlation of cowbird eggs or nestlings with habitat, nest-type, host species' body mass, and host behavioral defenses. Additional community-wide studies of cowbird parasitism will test if this model is applicable to other avian communities. Received: 20 December 1996 / Accepted: 17 May 1997     

大杜鹃在荒漠伯劳巢中寄生繁殖

2010年5～7月在安西极旱荒漠国家级自然保护区发现3巢荒漠伯劳被大杜鹃寄生,寄生率10.34%.荒漠伯劳产白色和粉红色两种颜色的卵,大杜鹃产白色寄生卵于卵色为白色的宿主巢中.寄生卵均产于6月,当月寄生率达42.86%.大杜鹃雏鸟19日龄离巢,离巢时体重66.24 g.