Intraclutch Variation Research Articles

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.

Maternal Transfer of Contaminants to Eggs in Common Grackles (Quiscalus quiscala) Nesting on Coal Fly Ash Basins

Coal combustion is a major source of trace elements to the environment. Aquatic disposal of wastes from this process can result in reproductive failure in many wildlife species, but little is known regarding impacts on avian fauna. Individual eggs were collected from common grackles (Quiscalus quiscala) nesting in association with coal fly ash settling basins and a reference site to determine if females from the contaminated site transfered trace elements to their eggs. Whole clutches were also collected from both sites to examine inter- and intra-clutch variability of maternally transferred contaminants. Selenium was the only trace element found in significantly higher concentrations in ash basin eggs (x = 5.88 +/- 0.44 microg/g DW) than in reference eggs (x = 2.69 +/- 0.13 microg/g DW). Selenium concentrations in eggs from the ash basins were above background levels, but did not exceed higher proposed Se toxicity thresholds. Inter- and intra-clutch variation was higher for ash basin clutches than reference clutches. The relationship between selenium concentrations and laying order (estimated by egg mass) was not statistically significant, but increased Se concentration in the second egg of most ash basin clutches followed by declining concentrations in subsequent eggs suggested that further examination of this pattern might be warranted.

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.

Seasonal change in egg-volume variation within a clutch in the Bull-headed Shrike, Lanius bucephalus

I studied the seasonal change in egg volume in Bull-headed Shrikes, Lanius bucephalus, to explore the importance of egg-volume variation within a clutch. The mean egg volume did not change over a season; however, the coefficient of variation in egg volume in six-egg clutches significantly increased in 1994 and 1995 but did not change in 1996. Peaks in arthropod biomass occurred early in the breeding seasons in 1994 and 1995, but late in the season in 1996. Higher food availability was related to a reduction in variation in egg volume within a clutch. A significant difference in egg volume was found within six-egg clutches, and the first egg was the smallest. Nestlings that hatched from small eggs early in the hatching order suffered lower mortality rates than nestlings that hatched from large eggs laid late in the hatching order. The duration of time between settlement of a female in a male territory and clutch initiation decreased with date. Intraclutch variation in egg volume may be caused by a trade-off between the time to develop an egg and the volume of the first egg within a clutch. Because eggs that hatch first do not need to be large for the hatchling to survive, the Bull-headed Shrikes may advance the clutch-initiation date at the cost of reducing the volume of the first egg.

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.

Some Aspects of Laying, Incubation and Hatching in the Great Reed-Warbler

During the breeding season of 1998, breeding ecology of the Great Reed-Warbler (Acrocephalus arundinaceus orientalis) was studied at Yangsoo-ri and Yongdam-ri of the Yangpyung-gun, Kyunggi province, Korea. Egg-weight (CV: 6.25) was more variable than either length or breadth, and breadth was the least variable of the measures. Significant variations in overall egg-weight occurred between clutches, and that more of the total variation in egg-weight and shape are due to inter-clutch variation as to intra-clutch variation when the data were pooled. The last egg tends to be larger than the remaining eggs in the clutch of the Great Reed-Warbler, suggesting the Great Reed-Warbler may adopt the brood-survival strategy. When method 3 was used, the most common incubation period is 12 days. In the Great Reed-Warbler, the length of the incubation period was related to clutch-size when method 1 (r=0.485, p<0.05) and method 2 (r=0.621, p<0.01) were employed, but not related to egg weight. The averagee number of days of hatching asynchrony was 2.5, raging 0.5~2.5. Asynchronous hatching was related to the clutch size (r=0.66, p<0.01). Hatching sequence was closely related to the laying sequence (r=0.93, p<0.001), suggesting Great Reed-Warblers incubate their eggs before clutch completion. The effect of egg weight on hatching asynchrony was found in Great Reed-Warblers (t-test, p<0.01).

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.

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.

Plasma Amino Acid Concentrations as an Indicator of Protein Availability to Breeding Herring Gulls (Larus Argentatus)

AbstractIn a captive-feeding study using Herring Gulls (Larus argentatus), plasma amino-acid concentrations increased in response to an increase in dietary protein. Plasma amino-acid concentrations were also measured in wild Herring Gulls captured during incubation at eight Laurentian Great Lakes colonies. Those concentrations were used as an indicator of protein availability at those locations. Significant differences in amino acid concentrations were observed among colonies. Lower amino acid levels, particularly of the essential amino acids, were measured in gulls nesting on Lake Superior, whereas values in gulls captured on Lake Ontario and Lake Erie were greater. Those geographic differences in protein availability likely reflected spatial differences in availability of high quality prey (e.g. fish). Geographic differences in prey availability probably affected diet composition. Comparison of amino-acid levels in wild birds to reference values obtained through the captive feeding study indicated that gulls nesting on Lake Superior may have been protein limited. Colony-wide estimates of adult female body condition, intraclutch variation in egg size, and productivity were correlated with an index of plasma amino-acid concentrations.

Intraclutch egg‐size variation in acanthosomatid bugs: adaptive allocation of maternal investment?

If there are differences in predation risk among the offspring within a clutch, parents may allocate less resources to the offspring facing higher risk. I examined parental investment in terms of egg size within clutches in five species of stink bugs (Heteroptera, Acanthosomatidae). In subsocial Elasmucha and Sastragala species, the female guards her eggs and first‐instar nymphs against invertebrate predators by covering her clutch with her body. Large differences in survival from predation between offspring at the centre and offspring at the periphery of the clutch have been reported in such subsocial insects. I found that Elasmucha and Sastragala females laid significantly smaller eggs in the peripheral (and thus more vulnerable) part of the clutch. Phenotypic trade‐offs between egg size and clutch size were detected in these subsocial species. Egg size was positively correlated with hatched first‐instar nymph size: smaller nymphs hatched from smaller peripheral eggs. In asocial Elasmostethus humeralis, however, no significant difference in size was detected between the eggs at the centre of and those at the periphery of the clutch. Thus, in subsocial acanthosomatid bugs, females seem to allocate their resources according to the different predation risks faced by the offspring within the clutch.

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

Experimental (tamoxifen-induced) manipulation of female reproduction in zebra finches (Taeniopygia guttata).

Experimental manipulation of reproductive phenotype is a potentially powerful approach for understanding the fitness relationships of traits such as egg size, egg composition, and egg number. In this study, I investigated the effect of the antiestrogen tamoxifen on multiple, estrogen-dependent reproductive traits in female zebra finches (Taeniopygia guttata). Short-term tamoxifen treatment of egg-laying females (two daily injections before laying) had no effect on the timing or the pattern of egg laying compared to sham controls. However, tamoxifen treatment caused (1) a marked, but transient, decrease in egg size; (2) increased within-clutch egg-size variation; (3) a reduction in plasma vitellogenin (VTG) levels; and (4) lower dry yolk and yolk protein content of tamoxifen-treated females. Plasma levels of the second yolk precursor, very low density lipoprotein (VLDL), were not affected by tamoxifen, and tamoxifen appeared to have no effect on oviduct function in egg-laying females. These results are consistent with tamoxifen blocking estrogen receptors in the liver, suppressing VTG production, and decreasing the plasma pool of yolk precursors below a level required to maintain yolk formation at the normal rate. Tamoxifen treatment can therefore be used successfully to manipulate several components of the female reproductive phenotype (egg composition, intraclutch egg-size variation) to further explore the fitness consequences of these traits.

ESTABLISHMENT AND PERSISTENCE OF MASS HIERARCHIES IN BROODS OF THE BROWN PELICAN

I investigated the roles of intraclutch variation in egg volume and hatching asynchrony in the establishment of mass hierarchies in broods of the Brown Pelican (Pelecanus occidentalis). The second egg averaged 2.6% larger than the first and 1.9% larger than the third. In unmanipulated control broods, first eggs hatched an average of 25 h before second eggs, which hatched 40 h before third eggs. On the day the last egg hatched, first chicks averaged 16% heavier than second chicks and 30% heavier than third chicks. Although chick mass at hatching was strongly correlated with egg volume, differences in mass within broods were almost entirely the result of hatching asynchrony. In broods in which I experimentally reduced hatching intervals, initial nestling mass differences were significantly smaller and strongly correlated with differences in egg volume but not with hatching asynchrony. Intrabrood mass differences remained lower in experimental than control broods throughout the first 2 weeks of the nestling period. Furthermore, mass ranks established at hatching were less likely to persist through this period in experimentally synchronized broods than in asynchronous controls. These results indicate that hatching asynchrony promotes the establishment of a more stable size hierarchy. However, the adaptive significance of nestling size hierarchies in Brown Pelicans remains unresolved.

Maternal effects of egg size in brown trout (Salmo trutta): norms of reaction to environmental quality

The magnitude of fitness variation caused by maternal effects and, thus, the adaptive significance of maternal traits may depend on environmental quality, generating crossing reaction norms among offspring phenotypes that shape life-history evolution. By manipulating intraclutch variation in egg size and comparing siblings we examined the maternal effects of egg size on offspring performance and tested for the existence of reaction norms to environmental quality using the brown trout Salmo trutta. When sibling groups of small and large eggs were reared separately in a hatchery environment initial size differences disappeared rapidly. However, in semi-natural environments and under direct competition, juveniles from large eggs experienced growth and survival advantages over siblings from small eggs. Moreover, distinct reaction norms existed, with the differences in performance of juveniles from small and large eggs being most pronounced in the poorer growth environments. Our results provide the first direct evidence, to our knowledge, for a causal relationship between egg size and fitness-related traits in fishes, independent of potentially confounding genetic effects. Moreover, they indicate that previous studies have been biased by experimental conditions that excluded competitive asymmetries and environmental variability. The existence of reaction norms indicates a shift in optimal egg size across gradients of environmental quality that probably shapes the evolution of this trait.

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.

Intraclutch Egg-Size Variation in the Eurasian Kestrel: Advantages and Disadvantages of Hatching from Large Eggs

Egg size in birds may be positively correlated with hatching success (Jarvinen and Vaisanen 1983, Magrath 1992a), hatchling mass (Magrath 1992a, Nilsson and Svensson 1993), and nestling growth and survival (Schifferli 1973, Moss et al. 1981, Williams 1990). Hence, egg size could influence offspring fitness (Williams 1994). Egg size varies among and within clutches, the former accounting for most of the variation (Ojanen et al. 1979, Jarvinen and Viisanen 1983, Wiggins 1990). However, variation in egg size within clutches may have greater fitness consequences than that among clutches because young that hatch from smaller eggs compete with larger siblings and have a higher risk of starvation (Nilsson and Svensson 1993). Generally, egg size varies with laying order following patterns that are typical for a species or population; e.g. increasing (Howe 1976, Mead and Morton 1985), decreasing (Parsons 1972, Nisbet and Cohen 1975), or increasing to the middle egg and decreasing thereafter (Arnold 1991, Williams et al. 1993). These patterns may change relative to food supply (Simmons 1994), year (Jover et al. 1993), or clutch size (Vifnuela 1997) and may result from physiological or nutritional constraints on the female and/or be adaptive strategies to improve breeding success. Several constraints could cause intraclutch variation in egg size. For example, egg size may increase with laying sequence because females are gearing up physiologically for starting egg production (Parsons 1976). Last-laid eggs may be smaller because prolactin blood levels, which increase with the onset of incubation, could reduce the size of developing follicles (Leblanc 1987). Egg size also could decline with laying sequence because energy reserves for egg formation are depleted throughout laying (Ryden 1978, Pierotti and Bellrose 1986). Finally, fluctuating environmental conditions at the time of egg laying associated with low temperatures (Ojanen et al. 1981, Jarvinen and Ylimaunu 1986, Magrath 1992b) or with changes in food availability (Jarvinen and Vaisanen 1983) may cause variation in egg size independent of laying order. At least three hypotheses have been proposed to explain the function of intraclutch variation in egg

Intraclutch Variation in Testosterone Content of Red-Winged Blackbird Eggs

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Egg Size Determines Offspring Size in Neotropical Cichlid Fishes (Teleostei: Cichlidae)

Within the family Cichlidae, the size of eggs varies tremendously across species (Breder and Rosen, 1966; Fryer and Iles, 1972; Coleman, 1991). Sizes range from the small eggs (0.9 mm in diameter) of rams (Microgeophagus ramirezi) to the massive eggs (4.5 mm in diameter) of some of the African mouthbrooding cichlids such as Cyphotilapia frontosa (unpubl. data). The hatchlings of the large-egged mouthbrooders are clearly much larger than the hatchlings of the rams and other small-egged species (pers. obs.). However, the relationship between egg size and hatchling size for smaller differences in egg size has not been explored. For example, within the Neotropical cichlids, the range of egg sizes is much smaller: small-egged species such as dimerus have an average egg diameter of 1.3 mm, whereas the largest-egged species, Cichlasoma (Theraps) tuba has an average egg diameter of 2.4 mm (RMC, unpubl. data). Does this relatively small difference in egg size actually translate into a consistent difference in the size of the hatchlings? Furthermore, does the variation in egg size within a single laying account for differences in hatchling size? Understanding the consequences of differential allocation to egg size between eggs in a single clutch and within and among species will help us understand the factors shaping the evolution of egg size. A larger egg may not necessarily translate into a larger hatchling for several reasons. For example, a given measure of egg size may not be a true representation of the energetic and material resources of an egg. This could be the case if larger eggs are simply filled with more water than smaller eggs, and size differences may solely be accounted for by water content. In such a case, the difference in egg size may be insignificant to the resulting hatchling. This is particularly plausible because fish eggs are hydrated and expand to some extent when they leave the female (Kamler, 1992). An ideal measure of egg size may involve a complete biochemical analysis of the contents of an egg, reporting protein, lipid, and carbohydrate content (e.g., Kamler, 1992); however, this is not practical for many studies. Indeed, for situations when the eggs are particularly small and yet must be kept alive, even weighing individual eggs or measuring their volume can be difficult. We investigated the relationship between egg size and hatchling size in 26 species of Neotropical cichlids. We also examined eggs and hatchlings produced by 17 females within one species [convict cichlids, Cichlasoma (Archocentrus) nigrofasciatum], and eggs and hatchlings from within single clutches of two species (convict cichlids and rainbow cichlids, Herotilapia multispinosa) to look at the effects of intraspecific and intraclutch variation in egg size.

Order of oviposition and egg size in the red-eared slider turtle (Trachemys scripta elegans)

We studied intraclutch variation in egg size and shape among 432 eggs from 35 clutches of the red-eared slider turtle (Trachemys scripta elegans) from west-central Illinois. Egg shape varied systematically with respect to oviposition order. The first two and last two eggs laid were slightly but significantly longer than eggs laid between the first two and last two eggs. The last two eggs laid were slightly but significantly narrower than other eggs in a clutch. In contrast, egg mass did not vary systematically with oviposition order. Mean egg mass of the first two eggs laid was slightly but insignificantly greater than that of the last two eggs laid and that of eggs laid between the first two and last two eggs. In this species, follicles are apparently fairly uniform in mass when they are ovulated. Consequently, reproductive output can be adjusted by either ovulating more equal-sized follicles or adding material uniformly to all follicles ovulated in a particular reproductive bout. We suggest an adaptive explanation of differences between intraclutch variation in T. s. elegans and that reported in previous studies of intraclutch variation in sea turtles.

Reduced immunocompetence of nestlings in replacement clutches of the European magpie (Pica pica)

Laying date is one of the most important determinants of reproductive success and recruitment probability in birds. Late breeders usually fledge fewer chicks than individuals with earlier breeding dates, and fledglings produced late in the season have high mortality rates. Food availability and nestling mass have been evoked as the principal mechanistic links between laying date and offspring survival. Here we suggest that another factor may actually account for the difference in survival rate between early and late offspring: immunocompetence. We predicted that nestlings produced later in the season or in replacement clutches should have lower immune responses when challenged with an antigen, than early nestlings or nestlings produced in first clutches. This hypothesis was tested in a population of magpies (Pica pica), in which we experimentally induced breeding failure in a group of nests and compared the immune response of nestlings in replacement clutches with the immune response of first clutch nestlings. Cellular immune response, as measured by wing web swelling (a correlate of T-lymphocyte production after injection of phytohaemagglutinin-P), significantly decreased with hatching date and was significantly lower in nestlings of replacement clutches. Furthermore, coefficients of intraclutch variation in immune response were higher in nestlings of replacement clutches. This experiment demonstrates an inverse relationship between immune responsiveness and breeding date, and reduced recruitment probability of late nestlings may be a direct consequence of their inability to cope with parasites.