Egg Appearance Research Articles

How is host egg mimicry maintained in the cuckoo (Cuculus canorus)?

To investigate the evolutionary mechanism (host specificity vs. random searching) maintaining mimicry between cuckoo egg appearance and that of different European cuckoo Cuculus canorus hosts, we studied the level of mimicry between the appearance of C. canorus eggs and that of their hosts’ eggs in different habitats in southern Finland by using ultraviolet-visible reflectance spectrophotometry. In the main habitat used by C. canorus for reproduction, eggs laid in nests of different host species differed in appearance. Host use by C. canorus was not related to the abundance of hosts, and the level of mimicry was not related to host abundance in the habitat. Furthermore, a close match between C. canorus egg appearance and that of host eggs within habitats was detected after removing the potentially confounding effect of host abundance. In the only two suitable host species nesting in trees (namely chaffinch Fringilla coelebs and brambling Fringilla montifringilla) we detected changes in C. canorus egg appearance that paralleled those of the two host species. Thus our findings suggest the existence of a correlation between the appearance of C. canorus eggs and that of their hosts’ eggs within different habitat types, and suggest that mimicry is maintained by strict host preferences by each C. canorus female when laying. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 57–68.

Rejection of parasitic eggs in relation to egg appearance in magpies

The coevolutionary process between avian brood parasites and their hosts predicts that low intraclutch variation in egg colour appearance favours egg discrimination of parasite eggs by hosts. Low intraclutch variation would also result in high interclutch variation, which would increase the difficulty of evolution of mimicry by the cuckoo, because many host colour patterns might coexist in the same host population. We explored this possibility using an experimental approach in the common magpie, Pica pica, and great spotted cuckoo, Clamator glandarius, system. We artificially parasitized magpie nests with great spotted cuckoo model eggs to assess host response in two populations in Spain (Guadix and Doñana) in relation to intraclutch variation in egg appearance, measured by ultraviolet–visible reflectance spectrophotometry. Individuals that rejected model cuckoo eggs had higher intraclutch variation than accepters, suggesting that an increase, rather than a decrease, in intraclutch variation in magpie egg appearance was advantageous for cuckoo egg discrimination.

Egg characteristics are unreliable in determining maternity in communal clutches of guira cuckoos Guira guira

Egg characteristics have been used to determine egg maternity for birds in situations where two or more females lay eggs in a single nest (as in communal breeders and intraspecific parasites). We assessed the applicability of egg morphometry and eggshell appearance in ascribing egg ownership in communal clutches of guira cuckoos Guira guira, a species where up to seven females may lay eggs in a joint nest. We used both combined variables (including egg mass, length, width, shape and two eggshell variables), and a shape index to test whether eggs laid by each female were similar but different from eggs laid by other females. Also, we conducted discriminant function analyses to verify if eggs could be correctly classified to their mothers based on their characteristics. The correct maternity was determined by yolk protein electrophoresis of freshly‐laid eggs. Individual female chutches were separated through egg characteristics or shape alone in 29% and 41% of the groups tested, respectively. Differences were mostly due to a single female that differed from her nest‐mates in a unique egg variable. On average, 55% of the eggs analyzed were not assigned to the correct mother using egg dimensions and eggshell speckling pattern. In conclusion, the egg characteristics used do not reliably indicate maternity in guira cuckoo communal clutches. We therefore recommend a protein‐based verification of egg appearance characteristics for assigning maternity in other species in which multiple female laying occurs.

CORRELATES OF EGG REJECTION IN HOSTS OF THE BROWN-HEADED COWBIRD

We conducted a comparative analysis of eight potential correlates of egg rejection in hosts of the parasitic Brown-headed Cowbird (Molothrus ater) to test the evolutionary equilibrium and evolutionary lag hypotheses as explanations for the acceptance of cowbird parasitism. The analyses generally supported evolutionary lag. Historic contact with cowbirds may explain why hosts that have recently come into contact with cowbirds accept parasitism, but it does not account for acceptance by hosts with long histories of contact with cowbirds. Egg predation by hosts, nest sanitation, population size, and egg appearance were not correlated with rejection. Larger species that typically build larger nests were more likely to reject. Large hosts may have been parasitized more frequently in the past, possibly due to their more easily found nests or superiority as hosts, and as a result, may have had more opportunity to evolve rejection. Rejection was also correlated with taxonomic affiliation, suggesting that once rejection evolves it is maintained, which implies that rejection is not costly and thus argues against an evolutionary equilibrium. Not surprisingly, hosts with large bills were more likely to reject. This may be a corollary of the tendency for large hosts, which tend to have larger bills, to reject. An evolutionary equilibrium may exist for hosts with eggs that resemble cowbird eggs, depending on the costs to host reproductive success and the likelihood of committing recognition errors. Nevertheless, some hosts have been in contact with cowbirds for a long time, build large nests, have large bills, have a “favorable” phylogeny, and lay eggs that differ from cowbird eggs, yet accept cowbird parasitism. Chance may play a role in the accumulation of the necessary recombinants and mutations necessary for the evolution of rejection. Factores Correlacionados con el Rechazo de Huevos por parte de Hospederos de Molothrus ater Resumen. Realizamos un análisis comparativo de ocho factores que potencialmente podrían estar correlacionados con el rechazo de huevos por parte de hospederos del parásito de cría Molothrus ater para poner a prueba las hipótesis de equilibrio evolutivo y de demora en la respuesta evolutiva propuestas para explicar la aceptación del parasitismo. Los análisis generalmente apoyaron la hipótesis de la demora en la respuesta evolutiva. El contacto histórico con Molothrus podría explicar por qué hospederos que sólo recientemente han entrado en contacto con estas aves aceptan el parasitismo, pero no explica por qué hay hospederos que tienen largas historias de contacto con el parásito y aceptan ser parasitadas. La depredación de huevos por parte del hospedero, el comportamiento de aseo del nido, el tamaño poblacional y la apariencia de los huevos no estuvieron correlacionados con el rechazo de huevos parásitos. Las especies grandes, que típicamente construyen nidos más grandes, presentaron una probabilidad más alta de rechazar huevos. Los hospederos más grandes podrían haber sido parasitados más frecuentemente en el pasado, posiblemente debido a que sus nidos se encuentran con mayor facilidad o a que son mejores hospederos y, como resultado, podrían haber tenido mayores oportunidades evolutivas para desarrollar el comportamiento de rechazar huevos. El comportamiento de rechazo también estuvo correlacionado con la filiación taxonómica, lo que sugiere que una vez que éste evoluciona se mantiene, implicando que no es costoso, lo que a su vez es un argumento en contra de la hipótesis del equilibrio evolutivo. De forma poco sorprendente, los hospederos con picos grandes fueron más propensos a rechazar huevos. Esto podría ser un corolario de la tendencia de los hospederos grandes (los cuales tienden a tener picos más grandes) a rechazar huevos. Es posible que exista un equilibrio evolutivo para los casos de hospederos que tienen huevos que se asemejan a los de Molothrus, dependiendo de los costos sobre el éxito reproductivo del hospedero y la probabilidad de cometer errores de reconocimiento. Sin embargo, algunos hospederos han estado en contacto con Molothrus por períodos prolongados, construyen nidos grandes, tienen picos grandes, tienen afinidades filogenéticas “favorables” y ponen huevos que difieren de los del parásito, y aún así aceptan el parasitismo. El azar podría jugar un papel importante en la acumulación de los recombinantes y mutaciones necesarios para que el comportamiento de rechazo pueda evolucionar.

Meadow pipit (Anthus pratensis) egg appearance in cuckoo (Cuculus canorus) sympatric and allopatric populations

Host populations tend to show less ability to discriminate against parasites when living in their absence. However, comparison of rejection rates among sympatric and allopatric host populations does not allow determination of whether the greater tolerance in allopatric populations reflects a genetic change or phenotypic plasticity. Here we test the existence of changes in a host’s adaptation to brood parasitism in the absence of parasitism by studying intraclutch variation in egg appearance, which is a genetically determined component of host defence favouring discrimination of parasitic eggs. We investigated egg phenotypes of a common host of the European cuckoo, Cuculus canorus , in the presence and in the absence of cuckoos. By using objective spectroradiometry techniques of colour assessment we compared intraclutch variation between populations of meadow pipit, Anthus pratensis , sympatric (England) and allopatric (Iceland and Faeroe Islands) with C. canorus . Allopatric populations of A. pratensis showed greater intraclutch variation in egg appearance in the ultraviolet part of the spectrum than did a population sympatric with C. canorus . Two possible alternative mechanisms explaining these findings are discussed. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 543‐549.

Do Common Whitethroats (Sylvia communis) discriminate against alien eggs?

In a coevolutionary arms race between a brood parasite and its host, both species are expected to evolve adaptations and counteradaptations, such as egg mimicry and egg discrimination. The Common Whitethroat (Sylvia communis) is a regular Common Cuckoo (Cuculus canorus) host in some parts of its breeding range. We studied egg discrimination in the Whitethroat in the south-eastern part of the Czech Republic in an area where the Whitethroat is parasitised only occasionally. To investigate responses to parasitic eggs, in 34 nests either a non-mimetic or a mimetic (conspecific) egg was added. All 21 non-mimetic eggs were ejected. Of the 13 mimetic eggs, 7 were ejected and 6 eggs were accepted. No significant differences were found in intraclutch variation in egg appearance between acceptors and rejecters. Ejected mimetic eggs, however, showed greater contrast with host eggs than did the mimetic eggs that were accepted. Our experiments show that Whitethroats are persistent rejecters of alien eggs at the study site. This fact is discussed in the light of the host selection and host-parasite coevolution hypotheses.

Experimental Manipulation of Intraclutch Variation in the Great Reed Warbler Shows No Effect on Rejection of Parasitic Eggs

AbstractIn the continuing arms race between hosts and brood parasites, hosts are expected to reduce variation in the appearance of their own eggs within clutches, as it facilitates recognition of parasitic eggs. At the same time, by increasing interclutch variation, hosts should make it more difficult for parasites to evolve perfectly mimetic eggs. In this study, we experimentally manipulated intraclutch variation in the great reed warbler, Acrocephalus arundinaceus, in Hungary, where this species is heavily (c. 64%) parasitized by the common cuckoo, Cuculus canorus. We placed artificial cuckoo eggs, which appeared moderately mimetic to humans, in two groups of nests; in one group we increased variability of egg appearance within clutches by exchanging host eggs among nests. These clutches showed a significantly higher intraclutch variability than natural clutches, which we used as a control group. Our results indicate that it has no effect on rejection behaviour in this species, neither when variation was increased experimentally, nor within the natural range of variation displayed by our population. We suggest that when parasitism is high, selection for reduced intraclutch variation may be less important than frequency‐dependent selection for increased variation between individuals within a host population.

European CuckooCuculus canorusparasitism and host's rejection behaviour in a heavily parasitized Great Reed WarblerAcrocephalus arundinaceuspopulation

An unusually high frequency (64%) of European CuckooCuculus canorusparasitism was found in Great Reed WarblerAcrocephalus arundinaceusclutches in central Hungary. Sixty‐four per cent of the parasitized clutches contained one Cuckoo egg, 23% contained two, 10% had three and 3% had four. This means that 58% of the Cuckoo eggs were found in multiply parasitized clutches. In multiple parasitism the laying second Cuckoo removed an egg from the clutch randomly, so preferred neither the host eggs, nor the concurrent Cuckoo egg. Host response towards the parasitic eggs showed 66% acceptance, 12% ejection, 20% desertion and 2% egg burial. We found great variation in both the host and the parasitic egg colour and pattern. This reduces the chance that the parasitic egg's appearance matched that of the hosts’ but, in spite of this, almost perfect mimesis was found in 28% of the Cuckoo eggs. Poorly mimetic Cuckoo eggs were more frequently rejected by Great Reed Warblers than parasite eggs that were very similar to the host eggs. This high level of mimicry sometimes makes it difficult for the observer to identify the parasitic egg, especially when it is similar in size to the host eggs. It is also difficult for the host, as shown by the relatively high recognition error and ejection cost.

How precise is egg discrimination in weaverbirds?

The village weaverbird, Ploceus cucullatus, lays eggs of an extremely broad range of appearance between individuals. This variation is thought to have evolved as a counteradaptation to brood parasitism by the diederik cuckooChrysococcyx caprius . The primary objective of our study was to characterize the relationship between egg appearance and egg rejection in the African village weaverbird. We test predictions of three hypotheses in a study in The Gambia, West Africa: (1) interindividual egg variability permits individuals to discriminate between own and foreign eggs by rejecting eggs in proportion to the difference in appearance from their own; (2) village weavers remember the appearance of their own eggs and do not require a discordancy within their clutch, nor even the presence of one of their own eggs, in order to distinguish a foreign egg as such; and (3) colour and speckling contain the signature information by which village weavers can distinguish their eggs from foreign ones; whereas shape and mass, being less reliable, do not. We analysed rejection behaviour according to egg appearance differences by logistic regression. Results supported all three hypotheses. We estimated the predictive efficacy of our model, the amount of explained variation and the relative contribution of various egg appearance factors to discrimination by the host. These results are consistent with the hypothesis that interindividual egg variation in this species facilitates offspring recognition and is a counteradaptation to either interspecific or intraspecific brood parasitism. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

ADAPTATIONS BY GREAT REED WARBLERS TO BROOD PARASITISM: A COMPARISON OF POPULATIONS IN SYMPATRY AND ALLOPATRY WITH THE COMMON CUCKOO

Abstract Egg rejection behaviour, as an antiparasite defence against brood parasitism was compared in two great reed warbler Acrocephalus arundinaceus populations. In central Hungary the host population lives in sympatry with the common cuckoo Cuculus canorus, and it is heavily parasitised (ca 66%). In central Greece the great reed warbler occurs in allopatry with the cuckoo, so it is not parasitised at all. Experimental parasitism with poorly mimetic (often called 'non-mimetic') artificial cuckoo eggs revealed a 100% rejection rate towards the foreign eggs in the Greek population, but the Hungarian hosts accepted 29.4% and rejected 70.6% of the model cuckoo eggs. No other rejection type than ejection was observed. The significantly higher level of rejection in Greece than in Hungary suggests that the Greek great reed warbler population is an abandoned host population of the cuckoo, which kept its egg recognition ability. A computer-based image analysis of host's clutches revealed that interclutch variation in egg colour was greater than intraclutch variation in the Hungarian population, but not in the Greek population. Spottedness of eggs were similar in both populations. Generally, a tendency for a higher interclutch variation was found in Hungary. Intraclutch variation was more similar in the two populations, but for the colour variables it showed a lower value in Hungary than in Greece. The heavy cuckoo parasitism in Hungary is suggested to be the main cause for the increased interclutch variation in relation to intraclutch variation in egg appearance among great reed warbler clutches. The high interclutch variation reduces the chance that the parasite egg's appearance matches that of the hosts', which facilitates hosts' egg discrimination.

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.

The spatial habitat structure of host populations explains the pattern of rejection behavior in hosts and parasitic adaptations in cuckoos

The spatial habitat structure of host populations explains the pattern of rejection behavior in hosts and parasitic adaptations in cuckoos

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

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

Rapid decline of host defences in response to reduced cuckoo parasitism: behavioural flexibility of reed warblers in a changing world

On Wicken Fen and nearby watercourses eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26 and 16 of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively...

A comparative analysis of the evolution of variation in appearance of eggs of European passerines in relation to brood parasitism

Host of brood parasites increase the ability of rejecting cuckoo eggs by production of (1) a clutch with little variation among eggs and (2) a clutch that differs the most from the modal phenotype of the population. These hypotheses have been tested by 0ien et at. (1995), although they did not control for common phylogenetic ancestry. We analyze the evolution of egg color and marking patterns in European passerines, which are potential hosts of die European cuckoo (Cuculus canorus), using Felsenstein's (1985) independent comparative method to control for the effect of common phylogenetic descent We found a significant positive relationship between interclutch variation in appearance of host eggs and parasitism rate, but this relationship disappeared when hole-nesting species were excluded from the analysis; and we found a highly significant multiple regression between rejection rate and intraand interclutch variation in egg appearance, even when hole nesters were excluded from the analysis. The partial correlation coefficients were negative for intraclutch variation and positive with interclutch variation in agreement widi the hypotheses. Therefore, the use of the independent comparative method strengthens the hypothesis that the evolution of egg patterns in hosts is associated with different stages of coevolution with the brood parasite.

The detection of and responses to experimental intraspecific brood parasitism in eastern kingbirds

Abstract. Eastern kingbird, Tyrannus tyrannus, nests were experimentally parasitized to (1) devise a method for determining the frequency of intraspecific brood parasitism and (2) determine the frequency with which foreign, conspecific eggs are accepted. The data indicate that eggs of different females vary in appearance and that experimental intraspecific brood parasitism significantly increased intra-clutch variation in egg length, width, shape and spot density in the upper third of the egg. Based on a discriminant function analysis of egg traits in control and experimental nests (N = 10 for both), eight of 10 experimentally parasitized nests and seven of 10 parasitic eggs were correctly identified. Examination of physiognomic variation among eggs can thus provide an approximate (minimum) estimate of the frequency of intraspecific brood parasitism in kingbirds. Kingbird responses to experimental intraspecific brood parasitism varied with the stage of the laying cycle. Egg ejection predominated in nests parasitized prior to clutch initiation whether parasitized by control (red-winged blackbird, Agelaius phoeniceus) or foreign kingbird eggs. Once egg laying began kingbirds virtually never rejected a kingbird egg, but did eject most blackbird eggs. Hence, despite significant differences in egg appearance among females, and an ability to eject eggs, females will accept virtually all parasitically laid kingbird eggs once they have begun laying.

Estimating conspecific nest parasitism in the northern masked weaver based on within-female variability in egg appearance

-The Northern Masked Weaver (Ploceus taeniopterus), like many weavers, exhibits a dramatic degree of intraspecific variability in egg appearance. While the rule is that all eggs within a nest look alike, this rule frequently is violated. By observing marked females laying eggs, I determined that within-female variability in egg appearance is both significantly and substantially lower than the variability seen among eggs laid by different females. Because communal breeding does not occur in this species, and because I ruled out parasitism by heterospecifics, nests containing odd-looking eggs must have been victimized by conspecific nest parasites. Based on the maximum variability in egg appearance observed in nonparasitized clutches, and by noting clutches with peculiar laying sequences that are indicative of parasitism (e.g. two eggs appearing on the same day), I determined the rate of conspecific nest parasitism in the Northern Masked Weaver to be between 22.8% and 34.7%. This is one of the highest rates of conspecific nest parasitism reported for passerines. My results support the conclusion that conspecific nest parasitism is a widespread and common behavior among Ploceus weaverbirds. Received 6 December 1990, accepted 13 January 1992. THE Northern Masked Weaver (Ploceus taeniopterus), like many of its congeners, exhibits a dramatic degree of intraspecific egg-color variability (Mackworth-Praed and Grant 1955, Moreau 1960, Freeman 1988, pers. observ.). Eggs range from dark brown to tan to blue-green. They can be either speckled or immaculate and, if speckled, can have various patterns of speckling. Typically, as in other Ploceus species with variable eggs, all the eggs within a nest look alike, suggesting that eggs produced by individual females do not vary in appearance. Initial support for this conclusion comes from studies of a captive colony of Village Weavers (P. cucullatus; Victoria 1972, Collias 1984); eggs laid over a lifetime by individually marked females varied little in either color or speckling. However, the rule that all eggs within a nest look alike is frequently violated in the Northern Masked Weaver, as well as in other weavers. In a study of Ploceus egg collections, Freeman (1988) found that among 16 species with variable eggs, between 2% and 29% of the nests of a species contained an odd-looking egg. If females produce only one egg type, these oddlooking eggs must have been laid by a brood parasite. Communal nesting is unknown among I Present address: Burke Museum DB-1O, University of Washington, Seattle, Washington 98195, USA. Ploceus species and, in this study of the Northern Masked Weaver, I never observed two females tending to the same nest. Freeman (1988) concluded that the parasites were conspecifics, rather than Chrysococcyx cuckoos, common brood parasites of many tropical Old World passerines, because he found no correlation between the incidence of cuckoo parasitism reported in the literature and the incidence of odd-egg clutches. In addition, Jensen and Vernon (1970) incubated 12 oddly colored eggs they discovered in weaverbird nests, and all of them hatched into weavers of the species in whose nest they were found. Finally, Chrysococcyx cuckoos are relatively unimportant brood parasites of the Northern Masked Weaver (Jackson in press a). Thus, conspecific nest parasitism (CNP) appears to be a common occurrence among Ploceus weaverbirds. Conspecific nest parasitism is now known to be widespread and common in birds. In an extensive review of the phenomenon, Rohwer and Freeman (1989) listed more than 140 species that engage in this behavior. The number known to exhibit this behavior has increased dramatically over the years and is likely to continue growing (see Jackson 1990). This growth is the result of improved detection rather than changes in the rates of CNP. Because in most species there is little variation in egg appearance, either within a nest or between nests, researchers rare-

Variation in the Appearance of Swallow Eggs and the Detection of Intraspecific Brood Parasitism

A method sometimes used to study avian intraspecific brood parasitism is to visually examine a clutch of eggs and infer parasitic egg laying if an egg of odd appearance is present in the clutch. Yet the degree to which individual females always lay eggs that look alike has seldom been evaluated quantitatively. We measured variation in egg appearance within and between 33 clutches of Barn Swallows (Hirundo rustica) and 35 clutches of Cliff Swallows (H. pyrrhonota) in southwestern Nebraska. Variables measured were egg length, breadth, shape, total amount of spotting, and degree of spotting in four separate sections of the egg. For both species within-clutch variance was significantly less than between-clutch variance for all variables except upper right and lower right spotting, when a sample size of 26-30 nests was used. Significant differences tended not to occur when small numbers of randomly selected nests (five to seven) were examined. Within-clutch variance for nests known to contain parasitic eggs did not differ from within-clutch variance for nonparasitized nests. Differences in egg appearance probably cannot be used safely to infer brood parasitism in Barn and Cliff swallows. Researchers should quantify within- vs. between-clutch variation in egg appearance before assuming that an individual of a given species always lays eggs that look more alike than do eggs from different individuals.

Cortical reaction in inseminated eggs of the horseshoe crab, Limulus polyphemus L

The egg cortical reaction in Limulus polyphemus is described in four events. Approximately 10 min after insemination, small “pits” are visible in the cortex of the inseminated Limulus egg. These pits progressively enlarge, eventually coalesce, and finally disappear, leaving a smooth-appearing surface approximately 60–90 min after insemination. Based on these visible changes, the following four events comprise the cortical reaction: (I) uninseminated egg—smooth surface, (II) inseminated egg—appearance and growth of pits, (III) inseminated egg—coalescence of pits, and (IV) inseminated egg—appearance of smooth surface. Preparation of these events for SEM studies demonstrated morphological characteristics and sequential development of the pits. Also numerous microvilli are found on the surface of uninseminated and inseminated eggs during the cortical reaction. An increase in their diameter during the reaction is particularly unique.

Geographical Variation in the Nestling Coloration of Parasitic Cowbirds

-Cowbird nestlings show significant geographical variation in rictal flange color. Twenty-four cowbirds from the Southwest, where the race Molothrus ater obscurus breeds, had yellow flanges whereas 73 from the remainder of the United States and Canada, where the races artemisiae and ater breed, had white flanges. There is little or no intralocality variation. Nestling cowbirds with yellow and white flanges apparently are accepted and fed by hosts regardless of the color of the rictal flanges of the host's own young, but I hypothesize that the quality of host care received by a nestling cowbird is influenced by its rictal flange color and that host responses are the selective pressure responsible for the geographical variation. Character displacement in nestling coloration is suggested by the fact that M. a. obscurus is the only race of Molothrus ater that has yellow flanges and it is also the only race that is sympatric with another cowbird, Molothrus aeneus, a species that apparently has white flanges. Character displacement could be caused by competition for host species and is further indicated by other lines of evidence. Received 18 February 1976, accepted 19 February 1977. INTRASPECIFIC variation in egg appearance occurs in many parasitic birds and is most plausibly explained by the selective pressures arising from host responses to parasitic eggs (Payne 1967, Rothstein 1971, Southern 1954). In contrast, the appearance of parasitic nestlings has little known effect on the great majority of hosts (Hamilton and Orians 1965). However, variation in parasitic nestlings could imply that some aspects of the hosts exert a selective pressure on the parasite's nestlings. Indeed, the few known cases of within-taxon variation in parasitic nestlings have been explained in this manner. Lack (1968: 88-89) accounted for intraspecific variation in the juvenal plumage of a parasitic cuckoo by relating each plumage variant to different hosts. Similarly, intrageneric variation in the nestling gape of parasitic finches may represent adaption to different hosts (Payne 1967, 1973: 61). In this paper, I show that nestlings of the parasitic Brown-headed Cowbird (Molothrus ater) have either yellow or white rictal flanges. I also discuss the possible significance of this variation. The nestling variation is especially notable because it correlates with recognized subspecies of the cowbird and the cowbird does not show as strong variation in its eggs or adults.