Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50 % bigger, and sometimes twice as large, as the smallest. Generally, approximately 70 % of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts ; the repeatability of egg size is generally above 0n6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size ( 0n5) than for clutch size or laying date ( 0n5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64 %) studies. Where dietary supplements do increase egg size, the effect is never more than 13 % of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15 % of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10 %. Female mass and size rarely explain more than 20 % of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.

We investigated the quantitative matching of plasma yolk precursor supply (the plasma pool) to follicle demand during yolk formation in European starlings (Sturnus vulgaris). Plasma concentrations of the two yolk precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL), were only elevated coincident with rapid yolk development (RYD) and matched variation in total yolky follicle mass. VTG and VLDL were low (<0.4 microg/mL and <4.2 mg/mL, respectively) in nonbreeders and prebreeders with no yolky follicles, and at clutch completion. They increased to 4.02 microg/mL and 19.4 mg/mL in birds with a full follicle hierarchy (F1-F4), and concentrations then remained high and actually increased up to the point where only a single, yolky (F1) follicle remained. However, there was some evidence for mismatching of supply and demand: (a) precursor concentrations increased throughout the laying cycle even though the number of developing follicles decreased. We suggest that this is because of a requirement to maintain a large precursor pool to maintain high uptake rates; and (b) in birds with a full follicle hierarchy, precursor concentrations were negatively correlated with total follicle mass. This suggests that high uptake rates in large follicles can actually deplete circulating precursor concentrations. Plasma concentrations of both yolk precursors increased rapidly in the early morning with (predicted) time after ovulation, consistent with a lack of fine control of precursor concentrations. However, mean plasma VTG concentrations did not differ between morning or evening samples. In contrast, plasma VLDL concentrations were lower in the morning (16.8 mg/mL) than in the evening (22.9 mg/mL). Although there is marked individual variation in plasma VTG and VLDL (four- to eightfold), both precursors were repeatable in the short term (24 h), and plasma VTG was repeatable over a 14-d interval between successive breeding attempts.

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Costs of reproduction might be mediated by a physiological (resource allocation) trade-o¡ between immune function and reproductive e¡ort, and several recent studies have shown that an experimental increase in reproductive e¡ort is associated with decreased immune function. Here we test the complementary prediction of this hypothesis: that increased immune function (speci¢c antibody production) depresses reproductive output. Female European starlings (Sturnus vulgaris) were injected with a nonpathogenic antigen (sheep red blood cells) following completion of laying of their ¢rst clutch, to stimulate an in vivo humoral immune response (primary antibody production). We induced laying of a second clutch by removing the ¢rst clutch, and assessed changes in reproductive performance in individual females preand post-treatment. Injection of sheep red blood cells produced a signi¢cant antibody response in 96% (n 29) of treated females, with titres comparable to previous studies (range 1^7). However, increased antibody production did not decrease primary or secondary female reproductive e¡ort (re-laying interval, egg size, clutch size, chick growth or £edging success), compared with control, saline-injected birds (n 22). These data do not support a simple resource allocation model for the cost of reproduction, based on a reciprocal, negative relationship between resources allocated to immune function and reproduction.

Summary1. Anaemia has been reported in wild animals, typically associated with traumatic events or ill health. However, female birds routinely become 'anaemic' during egglaying; we sought to determine the causes of this reduction in haematocrit. 2. Haematocrit in female European Starlings ( Sturnus vulgaris Linnaeus) decreased between pre-breeding and egg-laying in 3 out of 4 years (the decrease was marginally non-significant in the fourth year). This was independent of changes in ambient temperature altering the metabolic requirements for thermoregulation. 3. There was a positive relationship between haematocrit and plasma levels of the yolk precursor vitellogenin among egg-laying birds, supporting the hypothesis that the initial reduction in haematocrit is caused by increased blood volume associated with osmoregulatory adjustments to elevated levels of yolk precursors. 4 . However, haematocrit did not always recover upon cessation of egg production, remaining low at clutch completion (2 of 4 years), incubation (1 of 2 years) and chick rearing (1 of 4 years), suggesting an additional cause of the prolonged reduction in haematocrit. 5. Given the magnitude and prolonged nature of the changes in haematocrit we report, and the interannual variation in haematocrit even during chick-rearing (47-54%), we suggest that 'anaemia' associated with egg production might have implications for aerobic performance during later stages of breeding.

Summary1. Van Noordwijk & de Jong proposed a model to predict the sign of an intraspecific correlation between two life-history traits, given that a trade-off between the two traits exists. In this paper, the model is adapted to egg size/number trade-offs. 2. The predictions of the model are tested within various clades of waterfowl. Although the relationship between egg mass and clutch size in this group has been examined previously, prior analyses have either failed to take phylogenetic structure into account or relied upon phylogenies that lack resolution. Phylogenetic structure is adjusted for using recently constructed phylogenies and the method of phylogenetically independent contrasts. 3. Van Noordwijk and de Jong's model was successful in explaining betweenclade variation in the relationship between egg mass and clutch size. Even if a trade-off between egg size and number exists, a negative relationship will only be observed if the variation in the pattern of allocation (i.e. few large eggs vs many small eggs) is high relative to the variation in the amount of resources invested in the clutch. Thus, this model may be useful for understanding interspecific relationships as well as intraspecific patterns.

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Summary1. The seasonal decline in clutch size observed in many avian populations may be due to (a) a causal effect of the timing of breeding or (b) variation in individual quality (i.e. lower quality individuals lay fewer eggs and lay later in the season). 2. To distinguish between these two hypotheses, we manipulated the timing of breeding of European starlings ( Sturnus vulgaris L.) by removing first clutches to induce the production of replacement clutches. This experiment was performed at two sites over three breeding seasons. 3. To test whether the within-individual trend in clutch size differed from the seasonal decline in clutch size observed in the population, we devised a novel randomization test that overcomes some of the potential limitations of previously used analyses. 4. We observed no within-female decline in clutch size in any year at either site. Therefore, clutch removal did not appear to manipulate female quality (due to the costs of egg production), a potentially confounding factor in such experiments; only the timing of breeding was manipulated. 5. The within-individual seasonal trend in clutch size differed significantly from the population decline in four of six cases (year-site combinations); there is good reason to believe that the lack of a significant difference in the two other cases was due to low statistical power. Thus, our data support the hypothesis that the season decline in clutch size in this species is due to variation in female quality. 6. The between-year repeatability of clutch size was 0·47 and significantly different from zero, whereas that of relative laying date was much lower (-0·23) and non-significant. The repeatability data are thus consistent with the quality hypothesis: clutch size is characteristic of individuals regardless of their timing of breeding. 7. Female body mass at the beginning of incubation was not related consistently to clutch size or laying date, and therefore does not appear to be an indicator of female quality in this context.