Does the Polyphagous Egg Parasitoid <Emphasis Type="Italic">Trichogramma platneri</Emphasis> Nagarkatti (Hymenoptera: Trichogrammatidae) Display Behavioral Plasticity When Parasitizing Different Hosts?

Trichogramma platneri oviposition behavior on Amorbia cuneana egg masses was investigated under laboratory conditions. No relationship was detected between host surface area, number of edge turning on host eggs, and parasitoid clutch size. It was also observed that females parasitized the egg mass randomly by drilling into the egg mass and parasitizing individual eggs without using stereotypical behaviors to assess individual hosts (i.e. drumming and turning). However, clutch size did change due to ovipositional experience as naïve wasps with little or no ovipositional experience (<6 eggs parasitized), allocated significantly more progeny per host than wasps with longer ovipositional experience (24-h oviposition experience on a single egg mass). Moreover, naïve wasps parasitized significantly more eggs on the outer edge of the egg mass than experienced wasps. We contend that the physical characteristics of A. cuneana eggs and egg masses preclude T. platneri from completely discriminating between individual eggs. However, because T. platneri may be using kairomones from the egg mass, this described oviposition strategy remains effective.     

The role of spider cocoons in controlling desiccation

Summary The abilities of the cocoons of the spiders Mecynogea lemniscata and Argiope aurantia to protect the enclosed egg and spiderling stages from desiccation were investigated in the laboratory under controlled humidities, and in the field under ambient conditions. For M. lemniscata, which has a relatively small clutch (8–30 eggs) and remains in the cocoon for approximately 9–10 months, removal of the cocoon had no effect on water loss from the egg stage, nor did it adversely affect hatching or molting success. Cocoon removal did, however, significantly affect water loss and, consequently, survival in the spiderling stage at all humidities in the laboratory and in the field. The importance of the cocoon for survival is probably related to the unusually long time M. lemniscata spiderlings spend in the cocoon overwintering. For A. aurantia, which has a substantially larger clutch size (300–1400 eggs) and remains in the cocoon for a shorter 6–7 months, cocoon removal had no effect on water loss, egg hatching success, molting success, nor spiderling survival. The lack of an effect suggests that other factors (e.g., relative humidity at the oviposition site, or a large clutch size) may be more important in controlling water loss for A. aurantia.     

Egg maturation dynamics of the parasitoid Microplitis rufiventris: starvation speeds maturation in early life

The number of mature eggs carried by a female parasitoid at any given moment (egg load) is a fitness‐related parameter affecting reproductive potential and impacting upon host population dynamics. Microplitis rufiventris Kokujev (Hymenoptera: Braconidae) is a solitary koinobiont endoparasitoid wasp of several noctuid pests, including Spodoptera littoralis. The number of mature eggs carried by females at emergence is approximately 50. The rate of egg maturation is strongly affected both by feeding status and access to host larvae. In early adult life, egg maturation rates are lower for 6–72 h in fed wasps compared with food‐deprived wasps. When given access to hosts, honey‐fed wasps live for approximately 9 days with high lifetime fecundity (226 eggs). By contrast to early adult life, the total realized fecundity is positively affected by feeding status, where water‐fed and starved females have 140 and 107 eggs, respectively. Egg resorption is most pronounced in the later life of females. The results suggest, in addition to confirming the effect of honey‐feeding on total fecundity, that fecundity of starved wasps includes rapid egg maturation early in life, which potentially could improve the performance of the parasitoid as a biological control agent.     

Clutch size, development and wing morph differentiation of Melittobia digitata

Melittobia digitata Dahms (Hymenoptera, Eulophidae) is a wing polymorphic, gregarious ectoparasitoid of bees and wasps. In nature, females of this wasp start to produce their progeny after feeding on the host. The first adults to develop will emerge as short wing morphs (SWM). Mother and daughters will lay eggs in the same hosts, and the following females to develop will emerge as long wing morphs (LWM). We evaluated the effects of the clutch size on the development, egg load, and wing morph differentiation of M. digitata. Developmental time was shorter when insects developed in small clutches, and immature survivorship was reduced in the largest clutch (400 eggs/host). Morph differentiation was also affected by the clutch size. SWMs developed in small clutches (5 and 25 eggs/host) while LWMs in large clutches (200 and 400 eggs/host). However, both morphs developed in intermediate clutches (50 and 100 eggs/clutch), with a decreasing number of SWMs developing with the increase in the clutch. The size and egg load of each morph was affected with the increase in the clutch size, although the forewing length/hind-tibia length ratio was kept constant for each morph developing from different clutch sizes. Egg load of LWMs was also reduced when compared to the SWMs that developed from the same clutch, although LWMs females were larger (longer tibia length). We discuss the possible mechanisms inducing the morph differentiation in M. digitata, as well as the physiological, behavioral, and ecological changes facing each morph.     

New Insights into Non-Avian Dinosaur Reproduction and Their Evolutionary and Ecological Implications: Linking Fossil Evidence to Allometries of Extant Close Relatives

It has been hypothesized that a high reproductive output contributes to the unique gigantism in large dinosaur taxa. In order to infer more information on dinosaur reproduction, we established allometries between body mass and different reproductive traits (egg mass, clutch mass, annual clutch mass) for extant phylogenetic brackets (birds, crocodiles and tortoises) of extinct non-avian dinosaurs. Allometries were applied to nine non-avian dinosaur taxa (theropods, hadrosaurs, and sauropodomorphs) for which fossil estimates on relevant traits are currently available. We found that the reproductive traits of most dinosaurs conformed to similar-sized or scaled-up extant reptiles or birds. The reproductive traits of theropods, which are considered more bird-like, were indeed consistent with birds, while the traits of sauropodomorphs conformed better to reptiles. Reproductive traits of hadrosaurs corresponded to both reptiles and birds. Excluding Massospondylus carinatus , all dinosaurs studied had an intermediary egg to body mass relationship to reptiles and birds. In contrast, dinosaur clutch masses fitted with either the masses predicted from allometries of birds (theropods) or to the masses of reptiles (all other taxa). Theropods studied had probably one clutch per year. For sauropodomorphs and hadrosaurs, more than one clutch per year was predicted. Contrary to current hypotheses, large dinosaurs did not have exceptionally high annual egg numbers (AEN). Independent of the extant model, the estimated dinosaur AEN did not exceed 850 eggs (75,000 kg sauropod) for any of the taxa studied. This estimated maximum is probably an overestimation due to unrealistic assumptions. According to most AEN estimations, the dinosaurs studied laid less than 200 eggs per year. Only some AEN estimates obtained for medium to large sized sauropods were higher (200-400 eggs). Our results provide new (testable) hypotheses, especially for reproductive traits that are insufficiently documented or lacking from the fossil record. This contributes to the understanding of their evolution.     

Nutritional implications for sexual cannibalism in a sexually dimorphic orb web spider

Models predicting mechanisms driving sexual cannibalism in spiders with sexual size dimorphism (SSD) often assume that spiders use post‐copulatory food to channel nutrients into eggs and fecundity is altered through changes in clutch size or egg mass. I tested these assumptions using an orb web spider with extreme SSD, Argiope keyserlingi. I fed mated female spiders prey of either high protein‐low energy or low protein‐high energy composition. I measured egg energy density; a measure of the relative volumes of yolk and albumen. I predicted that if A. keyserlingi increase their egg energy density upon feeding on prey of a specific nutrient composition, they could enhance their fecundity by investing in more energy dense eggs. However, if the egg energy densities are dissimilar to their post‐copulatory prey they must be drawing energy from their somatic reserves to invest in eggs. In a further experiment I allowed female spiders to mate with and cannibalize males to determine if cannibalism induces similar effects on egg energy density. Male spider protein energy ratio was measured and found to resemble the high protein‐low energy prey. I found disagreement between the composition of post‐copulatory food and eggs in both experiments. Additionally, spiders fed high protein‐low energy prey lost weight indicating that they draw on their energy reserves to invest in eggs. I thus concluded that spiders that feed on high protein‐low energy prey or on males increase their egg energy density and, possibly, fecundity. However, the nutrient content of the prey or males cannot provide for investment in eggs. The energy invested in eggs is drawn from somatic reserves, probably induced by an as yet undescribed physiological trigger.     

Spider predators of lepidopteran eggs in south Texas field crops

Observations were made of spiders attacking lepidopteran eggs in south Texas field crops (cotton, corn, and soybean) from 2001 to 2004. Twelve species of spider from seven families were observed feeding on the eggs during the 4 years. These spiders were primarily cursorial hunting spiders, and they were observed feeding on eggs most frequently in cotton, representing 26.6% of all observations in cotton over the 4 years. Spider predation on eggs was proportionally less frequent in corn and soybean with 6.3% and 15.4% of observed predation in those crops, respectively. Four species of spider were responsible for 86.1% of the predation by spiders. The anyphaenid Hibana futilis (Banks) was the spider most frequently observed feeding on lepidopteran eggs during the 4 years of this study, constituting 45.1% of all spiders observed. Grammonota texana Banks (Linyphiidae), Hibana arunda Platnick (Anyphaenidae), and Cheiracanthium inclusum (Hentz) (Miturgidae) were the 2nd, 3rd, and 4th most frequently observed spiders constituting 15.6%, 12.8%, and 11.7% of all spiders observed, respectively. Most spiders represented taxa that are known to forage without a web. However, G. texana was observed feeding on eggs independent of a web, which is uncharacteristic of linyphiids. Other cursorial hunting spiders feeding on eggs included members of the Clubionidae, Corrinnidae, and Salticidae. Ninety-eight percent of all observations of egg predation by spiders were nocturnal; only the Salticidae were diurnal. It is likely that previous studies of predation in crops have vastly underestimated the importance of spiders as predators of lepidopteran eggs due to inadequate evaluation of nocturnal predation.     

Reproductive success and foraging of the crab spider Misumena vatia

Summary Reproductive success and growth rate data were collected for individually marked crab spiders Misumena vatia (Clerck) in 1980, 1981, and 1982. All measures of reproductive success were found to be quite variable between individuals within years, but did not differ between years. Reproductive effort (mass of clutch/prereproductive mass of female) was the least variable measurement and was not correlated with female weight at reproduction. Clutch weight and number of eggs per clutch were highly correlated with female reproductive weight. Egg weight was not correlated with the number of eggs per clutch. Hatching success did not vary with clutch size and averaged 94.5%. Growth rates of spiders were highly variable, indicating large variation in feeding rate. In 1981 and 1982, approximately 20% of female spiders were unable to capture enough prey to grow and reproduce. Primary prey species differed in weight and in their contribution to spider egg production. Spiders attacked a larger percentage of bumblebees but captured a larger percentage of honeybees. There was no simple relationship between diet choice and reproductive success. Spiders which selected suboptimal umbels to forage on some or all of the time, however, had significantly lower reproductive success than spiders choosing the best umbels.     

黄褐新圆蛛生物学以及对棉虫的控制作用

黄褐新圆蛛在湖北省以初孵幼虫在卵袋内越冬,一年可完成三个世代。雌蛛交配一次可终生产受精卵,一生最多可产9个卵袋。每个卵袋含卵粒数一般为80—90粒,单雌产卵量平均在450粒以上,最多可产869粒。卵的孵化率在90％以上。 成蛛的寿命较长,温度和食物对寿命有较大影响;有较强的耐饥能力。 在棉田内以结网捕食害虫,可捕食棉铃虫、红铃虫、棉小造桥虫、小地老虎、棉二点叶蝉、棉大卷叶螟和大青叶蝉等害虫。     

Measuring the discrepancy between fecundity and lifetime reproductive success in a pollinating fig wasp

Lifetime reproductive success in female insects is often egg‐ or time‐limited. For instance in pro‐ovigenic species, when oviposition sites are abundant, females may quickly become devoid of eggs. Conversely, in the absence of suitable oviposition sites, females may die before laying all of their eggs. In pollinating fig wasps (Hymenoptera: Agaonidae), each species has an obligate mutualism with its host fig tree species [Ficus spp. (Moraceae)]. These pro‐ovigenic wasps oviposit in individual ovaries within the inflorescences of monoecious Ficus (syconia, or ‘figs’), which contain many flowers. Each female flower can thus become a seed or be converted into a wasp gall. The mystery is that the wasps never oviposit in all fig ovaries, even when a fig contains enough wasp females with enough eggs to do so. The failure of all wasps to translate all of their eggs into offspring clearly contributes to mutualism persistence, but the underlying causal mechanisms are unclear. We found in an undescribed Brazilian Pegoscapus wasp population that the lifetime reproductive success of lone foundresses was relatively unaffected by constraints on oviposition. The number of offspring produced by lone foundresses experimentally introduced into receptive figs was generally lower than the numbers of eggs carried, despite the fact that the wasps were able to lay all or most of their eggs. Because we excluded any effects of intraspecific competitors and parasitic non‐pollinating wasps, our data suggest that some pollinators produce few offspring because some of their eggs or larvae are unviable or are victims of plant defences.     

Facultative blood-feeding in the crabhole mosquito, Deinocerites cancer

1. Most of the female crabhole mosquitoes Deinocerites cancer Theobald entering bait traps had undeveloped ovaries. 2. Nearly half of the D. cancer females that were captured in bait traps, but denied blood, were gravid following a 1 week holding period; they developed relatively small egg clutches autogenously (mean = 7.4 eggs per gravid female). 3. Although both parous and nulliparous D. cancer females were captured exiting from crabholes, only parous females were attracted to and collected in bait traps. 4. Much larger egg clutches (mean = 41.8 eggs per female) were produced anautogenously by mosquitoes that blood-fed on chickens. 5. All D. cancer females from field collected pupae developed eggs autogenously when allowed to mate and sugar feed in the laboratory. These mosquitoes produced an initial egg clutch with a mean size equivalent to that found in blood-fed, parous mosquitoes from bait traps. 6. In the field, mating and sugar feeding rather than blood-feeding seem to stimulate production of the initial egg clutch. 7. D. cancer blood-feeding appears to be delayed until after the first egg clutch has been laid; some parous crabhole mosquitoes are facultative blood feeders and can produce an egg clutch either with or without a blood meal.     

Assessing the Semelparity Hypothesis: Egg-guarding and Fecundity in the Malaysian Treehopper Pyrgauchenia tristaniopsis

According to the semelparity hypothesis, iteroparous insects should provide either no maternal care or less care than related semelparous species. We present field data on reproductive output and maternal care in the Southeast Asian treehopper Pyrgauchenia tristaniopsis (Mt. Kinabalu, Borneo) relevant to a preliminary assessment of the hypothesis. In a mark‐recapture experiment, more females than expected under semelparity were found to have oviposited a second clutch (37%). Female longevity was a of 75 d. Both these estimates were highly conservative. Oviposition was successive resulting in a of 46 eggs per clutch. Females provided care for eggs only, occasionally scraping their legs along the sides of the clutch apparently attempting to deter Brachygrammatella sp. egg parasitoids (Trichogrammatidae). Females straddled their clutch for a of 27 d, i.e. until 8 d after the beginning of first instar hatching. First instars hatched successively over a period of 11 d. When a female deserted her clutch, it contained about 37% yet unhatched eggs. Egg‐guarding effectively reduced egg mortality: the earlier a female was experimentally removed from her clutch the higher the egg mortality. Displacement experiments demonstrated that egg‐guarding is a behaviour actively maintained despite disturbances and specifically directed towards the egg clutch but not to the feeding site. We interpret our findings as being in accordance with the weaker claim of the semelparity hypothesis, i.e. the iteroparous P. tristaniopsis provided less maternal care than semelparous membracid species. Continued female feeding is discussed as a mechanism to display some level of care despite iteroparity.     

Maternal defensive behaviors of <Emphasis Type="Italic">Uloborus</Emphasis> sp. (Araneae,Uloboridae): behavioral repertoire and influence of clutch size and female size on female aggressiveness

Maternal care in spiders often involves behaviors associated with the protection of eggs and spiderlings against parasitoids and predators (including conspecifics). The females of several species have been documented to move their egg sacs away from natural enemies or to invest in active defense behaviors against web invaders, such as parasitoid wasps or araneophagic spider species, to protect their brood. In this study, we present observations of protective behavior by Uloborus sp. females carrying egg sacs. We also investigated whether brood size and female size influence female aggressive behaviors and response time against an artificial source of disturbance. Females carrying egg sacs almost immediately perceived and reacted aggressively against the artificial stimulus, whereas females without egg sacs moved away or ran to the web margins, avoiding the source of disturbance. The aggressive response was independent of clutch size and female body size, indicating that all females will risk interacting with potential agents of egg mortality. This systematic response by all females with egg sacs may be important for reducing the incidence of attack by the egg predator wasp Bathyzonus sp. (Ichneumonidae).     

Annual variation in the seasonal shift in egg size and clutch size in Sceloporus woodi

Summary Early and late season clutch parameters were examined over a three year period in the Florida scrub lizard, Sceloporus woodi. Precipitation levels were monitored throughout the study. In the early and late season of 1984 and the early season of 1986 precipitation levels approximated long-term mean levels of precipitation. In 1985 a severe winter drought occurred. Clutch size was positively related to body size in all samples in every year. In 1984 and 1986, egg size was not related to clutch size, whereas, in 1985 egg size was negatively related to clutch size. In 1985, females produced large clutches of small eggs early in the season and small clutches of large eggs late in the season. In 1984, no seasonal changes in egg or clutch size occurred. In the late season of 1986, females produced the largest clutches and the smallest eggs of all the samples, but egg and clutch size were not statistically different from the early season egg and clutch size of 1986. Total clutch dry weight, an estimate of total clutch energy, was not different in any of the six sampling periods. These data do not support current adaptationist models that attempt to explain the control of clutch and egg size in lizards. It is argued in this paper that egg and clutch size may vary in response to past environments that affect a female's physical condition, as well as, current resources that may be important for maintenance and reproduction. Egg and clutch size appear to be plastic traits selected to respond to proximal environmental variation, whereas, the investment of total dry matter/clutch has been optimized.     

拟寄生蜂搜索产卵过程中对寄主的竞争

综述拟寄生蜂搜索产卵过程中对寄主竞争的最新研究进展.这类竞争具有四种方式,即标记寄主、杀卵和杀幼、守护寄主和捕食寄主.（1）标记寄主常涉及寄主标记信息素,这是由雌蜂在产卵前、产卵时或产卵后分泌的化学物质.寄主标记信息素常介导拟寄生蜂对已寄生和健康寄主的辨别、减少过寄生和多寄生、减轻种内和种间竞争压力.（2）雌蜂遇到已寄生寄主时,很多种类杀死前一雌蜂遗留的卵和幼虫,再产下自己的卵.雌蜂使用三种方法杀卵和杀幼,即产卵器穿刺、取食和使用有毒物质.通过杀卵和杀幼,产卵雌蜂清除了前一雌蜂遗留的后代,主动改善了寄主品质,从而有利于自身后代的生存.（3）守护寄主在肿腿蜂科、缘腹细蜂科、金小蜂科、缨小蜂科和茧蜂科中均有报道,守护者驱逐入侵者以保护后代及健康寄主.（4）捕食寄主不仅减少了健康寄主数量,且直接导致已寄生寄主中拟寄生蜂卵和幼虫的死亡.雌蜂一般在体内成熟卵量较少时捕食寄主.讨论了研究拟寄生蜂搜索产卵过程中竞争寄主的理论意义和实际应用价值.     

Oogenesis and ovarian morphology in pollinating and non-pollinating fig wasps: evidence from adult and immature stages

Hymenopteran insects have meroistic polytrophic ovaries characterised by trophocytes associated with oocytes inside the follicles. In pro-ovigenic species, all oocytes mature before emergence and no trace of oogenesis is visible in adult females. Pro-ovigeny is a rare condition among Hymenoptera, but common in pollinating fig wasps. In the present investigation, we studied adult and pupa females of three fig wasp species with different trophic strategies. We demonstrated that females of Pegoscapus aerumnosus and Idarnes spp. have an unusual ovarian organisation (i.e. each ovariole has only one mature egg and no oocyte) that has led to misleading interpretation of fig wasp reproductive anatomy. The ovaries of these studied species have several ovarioles, recognisable by the presence of nuclei of tunica propria cells surrounding them. Each adult wasp ovariole had one mature egg. None of the pupae had mature eggs, but all of them had follicles with oocytes in different developmental stages. The studied fig wasps are pro-ovigenic, irrespective of their trophic strategy, since there were no signs of ovigeny in adult females. We discuss ecological and phylogenetic factors that could play a role in fig wasps reproductive strategies.     

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.     

Indeterminate laying and flexible clutch size in a capital breeder, the common eider

Clutch size control in capital breeders such as large waterfowl has been much debated. Some studies have concluded that clutch size in ducks is determined before the start of laying and does not change in response to egg additions or removals. The response, however, may depend on the timing of tests, and experiments may have been too late for females to alter the number of eggs. We here study clutch size responses to predation of first and second eggs in the common eider, using protein fingerprinting of egg albumen to verify that the same female continues laying in the nest after predation. Sixty of 79 females with early egg predation (one or both of the two first eggs) deserted the nest. Among the 19 females that stayed and continued laying, the mean number of eggs produced was 4.4, significantly higher than the 3.7 in non-predated nests. The staying females had similar egg size and clutch initiation date as females that deserted, and their body mass and clutch initiation date was similar to that of females whose clutches were not predated. Even capital-breeding common eiders may therefore be indeterminate layers, as many females in which early eggs are removed lay more eggs than others. A previous study has shown that they can reduce their laying if eggs are added. Our results add to increasing evidence that ducks have more flexible egg production than previously thought.     

Host-plant effects on Parasyrphus melanderi (Diptera: Syrphidae) feeding on a willow leaf beetle Chrysomela aeneicollis (Coleoptera: Chrysomelidae)

Abstract.

• 1 Generalist predators are repelled by chrysomelid (Chrysomela spp., Phratora vitellinue L.) larval defensive secretions that are obtained from salicin in their host plants. But little is known about the effect of these secretions on specialist predators.
• 2 In this study, we describe the feeding behaviour of a fly, Parasyrphus melanderi Curran (Diptera: Syrphidae), which feeds on Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae). Parasyrphus melanderi lays its eggs on C.aeneicollis egg clutches, and its larvae consume C.aeneicollis eggs and larvae.
• 3 Chrysomela aeneicollis hatching rates were significantly lower (20%) on clutches with fly eggs than on clutches without them (40%). Half of the clutches with one fly egg had survival rates below 5%, and when two fly eggs were present (four clutches), the entire clutch was consumed.
• 4 In nature, P.melanderi eggs were 3 times more abundant on a salicylaterich willow species S.orestera Schneider, than on the medium-salicylate S.geyeriana Anderss. (1.8 v 0.6 eggs per clutch). On 18% of the S.orestera clones, all the beetle clutches contained fly eggs. In laboratory-choice tests, P.melanderi larvae fed equally rapidly on C.aeneicollis larvae that were chemically defended (feeding on S.orestera) as on larvae that produced no secretion (feeding on the salicylate-poor S.lutea Nutt.). This predator does not appear to be deterred by C.aeneicollis's defensive secretion. We discuss the implications of specialist predators on determining host suitability to herbivorous insects.

     

Egg-size variation and reproductive success in the Herring Gull Lams argentatus: adaptive or constrained size of the last egg?

Herring Gull Larus argentatus eggs from a study colony in the Baltic showed a slight but significant variation in egg size within the laying sequence. Last-laid eggs were only about 5% smaller by volume than first eggs. There was no significant difference in dry yolk weight or dry albumen weight, although possible differences were evident. The chicks had nearly equal hatching weights and equally long tarsi. There was no differential mortality in the third chick in the study colony, and there were no indications of egg-size-mediated mortality. The birds in the colony produced an average of 1.45 fledglings per pair. Compared with several other studied colonies, the difference in egg size within a clutch was low, and a comparison of colonies from northwestern Europe suggests that variance within the clutch is negatively correlated with fledging success, so that a large difference in size between first and last eggs is associated with low fledging success. We suggest that the size of the last egg in the clutch reflects the feeding potential in the environment and is mainly a nonadaptive response to poor feeding conditions during laying.