Determinants of Chick Survival in the Lesser Black-Backed Gull: Relative Contributions of Egg Size and Parental Quality

The fitness‐related consequences of egg size, independent of the influences of parental quality, are poorly understood in altricial birds. Not only can egg size and parental quality influence growth and survival, but each could influence the development of condition‐dependent plumage coloration in offspring. The Eastern Bluebird Sialia sialis is an altricial, multi‐brooded, cavity‐nesting passerine in which juveniles display dichromatic UV‐blue plumage. Previous research suggests that plumage coloration acts as a signal of individual quality among juvenile and adult Eastern Bluebirds. Here, we separate the effects of egg size and parental quality (defined by egg size laid) on nestling growth and plumage ornamentation by exchanging clutches of large eggs with clutches of small eggs. Nestlings were significantly larger immediately post‐hatching when hatched from a large egg, but to maintain a larger size, nestlings needed to have hatched from a large egg and to have been reared by high‐quality parents. Nestlings were brighter when reared by high‐quality parents and this relationship was strongest later in the breeding season. Nestlings exhibited greater UV chroma if hatched early in the season, but UV chroma was not significantly affected by egg size or parental quality. These findings demonstrate varying influences of both egg size and parental quality on offspring growth and plumage ornamentation but suggest that quality of post‐hatching investment is more influential than pre‐hatching investment.

Effects of egg size and parental quality on lapwing Vanellus vanellus chick survival were studied in southwestern Sweden over 6 years. Chicks from large eggs were heavier at hatching and survived significantly better than those from small eggs. To control for the confounding effect of parental quality on egg size and chick survival, we performed a cross-fostering experiment during 2 years, exchanging clutches between nests with large and small eggs. In control clutches, chicks from large eggs survived better than those from small eggs, but we found no significant difference in chick survival between exchanged clutches. Thus, egg size did not affect chick survival independently of parental quality. Fledging success increased with parental age and/or experience, and with female body mass. Hence, both egg size and parental quality affect chick survival in the lapwing.

1. There is little unequivocal evidence to date in support of a positive relationship between egg size and offspring fitness in birds. Although 40 studies (of 34 species) have considered the effect of variation in egg size on chick growth and/or survival up to fledgling only 12 studies have controlled for other characters potentially correlated both with egg size and offspring fitness. Of these only two have reported a significant residual effect of egg size on chick growth (in the roseate tern and European blackbird) and three a residual effect on chick survival (all in seabirds: common tern, lesser black-backed gull and kittiwake). 2. More consistent evidence exists, though from fewer studies, for a positive relationship between egg size and offspring fitness early in the chick-rearing period; chick growth and chick survival being dependent on egg size in 8 of 10 studies and 4 of 5 studies respectively. It is suggested that the most important effect of variation in egg size might be in determining the probability of offspring survival in the first few days after hatching. 3. Egg size explains on average 66% of the variation in chick mass at hatching (n = 35 studies) but only 30% of the variation in chick body size (n = 18). When effects of hatching body size are controlled for chick mass remains significantly correlated with egg size, though the reverse is not true. This supports the hypothesis that large eggs give rise to heavier chicks at hatching, i.e., chicks with more nutrient (yolk) reserves, rather than structurally larger chicks. 4. Egg composition increased isometrically with increasing egg size in about half the studies so far reported (n equals approximately 20). However, in seabirds, and some passerines, larger eggs contain disproportionately more albumen, whilst in some waterfowl percentage yolk content increases with increasing egg size. Changes in albumen content largely reflect variation in the water content of eggs, but changes in yolk content involve variation in lipid content, and therefore in egg 'quality.' The adaptive significance of variation in egg composition is considered; females may adjust egg composition facultatively to maximise the benefits to their offspring of increased reproductive investment. 5. Considerations for future research are discussed with particular emphasis on experimental studies and the application of new techniques.

A large number of studies have reported a positive relationship between the egg size of birds and the subsequent growth and/or survival of nestlings, but such effects may partly be due to confounding variables, e.g. parental quality. In order to evaluate the potential effects of egg size, and of parental quality, on early nestling growth in the Antarctic petrel, we performed an experiment in which eggs of different size were swapped between nests. From a sample of 300 nests with eggs of known size, we selected eggs belonging to the lower quartile (small eggs), and those belonging to the upper quartile (large eggs), with respect to volume. Half of the small eggs were exchanged with small eggs from other nests, and the other half with large eggs. A similar procedure was used for large eggs. Growth and survival of the nestlings were recorded until 12 days old. Hatching success was positively related to egg size. Egg size influenced nestling body mass until the age of 3 days, and tarsus length was affected until 12 days old. However, these effects were not due to an effect of egg size on growth rates, but reflected instead the influence of egg size on hatchling size. In contrast to most previous studies, we found no effect of parental quality (as reflected in the size of own eggs) on foster nestling size or growth until 12 days old. This could be because egg size does not reliably reflect parental quality in the species, or because parental effects become evident only at later nestling stages. We discuss why egg size variation is maintained in this and other species where egg size influences parental fitness through the survival of eggs or nestlings.

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 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). 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.

Summary 1 Variation in the quality of the offspring environment can lead to the evolution of variable offspring provisioning. For variable offspring provisioning to evolve, the magnitude of provisioning per offspring must have effects on offspring and parental fitness. 2 Females of the quacking frog, Crinia georgiana, produce clutches of eggs in which egg size varies between individuals in the population and also within clutches, independent of female size or condition. A trade-off between egg size and number exists. 3 Using microsatellite markers to trace offspring to parents, and therefore clutch type, we tested the performance of tadpoles from clutches of large, small and variable-sized eggs in ponds in the field. 4 Clutches of large eggs resulted in the highest parental fitness and clutches of small eggs resulted in lower parental fitness values. 5 The parental fitness indicated that conditions in these ponds were harsh. Clutches with variable egg sizes had intermediate parental fitness but may be of benefit as a bet-hedging strategy when the qualities of ponds are unpredictable. 6 This study provides new empirical evidence of important offspring and parental fitness consequences of variable maternal provisioning under natural conditions in the field and demonstrates the importance of moving experimental life-history studies out of the laboratory into the field where realistic selection pressures act on developing offspring.

As is the case for most avian species, there is considerable variation in the egg size of Continental Black-tailed Godwits Limosa l. limosa breeding in The Netherlands. It is interesting that egg size has costs and benefits yet varies considerably at the population level. To better understand this variation in egg size, we tested its relationship to a suite of individual and environmental factors. We found that egg size can decrease up to 2.8% throughout a breeding season and that egg size increases with clutch size by 1.4% with each additional egg in the clutch. Female body mass and body size explained 5% of the total variation in egg size observed across the population. Furthermore, females wintering south of the Sahara laid 3% smaller eggs than those wintering north of the Sahara. We also found that egg size increases with age, which may indicate age-related differences in the endogenous and/or exogenous conditions of females. The variation in egg size was, however, mostly the result of consistent differences among individuals across years (repeatability = 0.60). A comparison of daughters with mothers suggested that most of this individual repeatability reflects heritable variation (heritability = 0.64). The actual individual traits that underlie this heritable variation among individuals remain mostly undetermined. Smaller eggs did have a slightly lower chance of hatching, but we found no relationship between egg size and chick survival. Finally, nest and chick survival were strongly correlated with lay date. Thus, in Black-tailed Godwits, lay date may actually reflect a female's endogenous and/or exogenous condition at the moment of egg-laying. This finding may be general across birds, since food supplementation experiments usually result in advanced laying and larger clutch sizes rather than in larger eggs.

This study quantifies the influence of the persistent variation in maternal investment in individual offspring on larval fitness in a population of the oriental fire—bellied toad Bombina orientalis. Offspring fitness was evaluaed by exposing newly hatched larvae to interspecific tadpole predators and assessing the effect of ovum size on the probability of surviving predation. The experiment incorporates natural variation in temperature that occurs during development. Embryos that developed in colder ponds took longer to hatch and hatched at a larger size. Larger size afforded larvae significant protection from predation. In addition, Iarvae that developed from large eggs in cold ponds had a higher probability of surviving predation than larvae that developed from small eggs. A significant interaction between egg size and developmental temperature, however, resulted in larvae that developed from large eggs in warm ponds having a lower probability of surviving predation. Large eggs resulted in large snout—to—vent lengths at all temperatures. In warm ponds, however, tails of larvae from large and small eggs were of equal length. Thus, diminished locomotory ability due to larger amounts of inert yolk can be invoked to explain the increased susceptibility to predation of larvae that developed from large eggs in warm ponds. In cold ponds tail length was substantially greater than in warm ponds, and larvae from large eggs tended to have longer tails than those that developed from small eggs. This allowed larvae from large eggs to achieve the advantages of increased body size. Thermally induced plasticity during early development resulted in an uinpredictability regarding the adaptive value of a particular egg size. This result fits within a growing theoretical literature that indicates that environmental uncertainty can result in the natural selection of development plasticity in egg size, even though the fitness ramifications are realized in subsequent stages. Thus, increased maternal investment in offspring is not necessarily associated with an increase in offspring fitness, and the evolution of developmental plasticity in egg size may be the result of environmental uncertainty during the larval stages of life.

Egg size variation among female mosquitoes (Aedes aegypti) is documented. Females hatching from large eggs grow faster, attain a larger adult size, take larger blood meals, lay more and larger eggs than females hatching from small eggs. This cycle is self-perpetuating; offspring of large females have a high probability of attaining a large size. Three factors oppose selection for large egg size by this apparently deterministic cycle: (1) Smaller and slower-growing females may also produce large eggs under some circumstances; (2) genetic differences among sibships are more important than differences in egg size; (3) there is no effect of body size on the reproductive success of males, so males pass on genetic material which has not been size-selected. It does not appear that there is an optimal egg size within the limits of observed egg sizes. INTRODUCTION Wilbur (1977) investigated the interrelationship between propagule size and number for two genera of long-lived iteroparous organisms adapted to uncertain reproductive success. Although these two groups (milkweed plants and salamanders) are taxonomically distant, the species considered are similar in that the total energy allotted to reproduction is constant. Variation among species within each group occurs in the way that the total energy is divided among individual offspring. In this article I look at the relationship between propagule (egg) size and number for an insect which differs from the previous examples in that the total energy invested is not constant. The female of the yellow fever mosquito Aedes aegypti L. takes at least one blood meal in order to mature each clutch of eggs. Blood meal mass varies (see Steinwascher, 1982, and references cited therein); for this reason, energy investment is not constant among females or among clutches for a single female. The number of eggs an individual female will lay in the first ovarian cycle depends on its body mass (Steinwascher, 1982), the mass and number of blood meals (Lea et al., 1978; Feinsod and Spielman, 1980; Steinwascher, 1982) and the blood source (Woke, 1937; Bennett, 1970). Furthermore, among eggs collected from a colony of Aedes aegypti, there are visible differences in size. In order to determine the interrelationships among egg size, number of eggs and the probabilities of reproductive success for this short-lived, iteroparous invertebrate, I did four sets of experiments. METHODS Variation in egg size. -A cohort of pupae, part of the seventh generation after colonization, was allowed to emerge in a screened cage (17 x 17 x 35 cm) with 10% sugar water available. The original individuals for the colony were collected from tires in Dade Co., Florida. Week-old females were removed from their emergence cage, anesthetized and weighed to the nearest .01 mg. They were held separately overnight without sugar water and each was allowed access to a mouse the next day. Engorged females were immediately anesthetized and weighed to the nearest .01 mg. Each was placed in a separate cylindrical paper cage (diam = 17 cm, height = 18 cm) with a soaked raisin (as a sugar source) and an oviposition vial containing 10 ml distilled water and lined with paper towelling. After 2 weeks, females were dissected and University of Florida, Institute of Food and Agricultural Sciences, Journal Series No. 4522.

Egg size is an important determinant of reproductive investment by birds. For many species, total investment in a clutch is limited by the size of stored reserves (Ankney and MacInnes 1978, Esler and Grand 1994a). Egg size determines the unit by which these stored reserves are partitioned. Individual females in most species of waterfowl show a high repeatability for egg size, implying that individuals either cannot, or do not, alter their egg size in response to varying environmental conditions (Batt and Prince 1979, Duncan 1987, Laurila and Hario 1988, Lessells et al. 1989, Flint and Sedinger 1992). Thus differences in egg size appear to represent different reproductive strategies among individuals. Fitness can be measured by the number of offspring an individual contributes to a population. Egg size may be related to fitness in some species of waterfowl as young from larger eggs are better able to survive extreme conditions (Ankney 1980, Thomas and Brown 1988). Birds laying larger clutches are almost always more fit as they fledge more young (Lessells 1986, Rockwell et al. 1987, Flint 1993). These fitness patterns create the potential for a trade-off between clutch size and egg size where females laying large clutches of small eggs have the same fitness as females laying smaller clutches of large eggs. The fact that Northern Pintails (Anas acuta) utilize stored reserves (Mann and Sedinger 1993, Esler and Grand 1994a) and have a high repeatability for egg size (i.e., egg size is fixed) (Duncan 1987), makes them candidates to engage in clutch sizeegg size trade-offs (Rohwer 1988, Rohwer and Eisenhauer 1989). An inverse relationship between egg size and clutch size would be indicative of a phenotypic trade-off among these fitness components. Our goal in this study was to describe egg size variation in Northern Pintails (hereafter pintails) with regard to female age, body size, clutch size, year, initiation date, and nesting attempt. We compare our results to those from other populations of nesting pintails and discuss whether phenotypic clutch size-egg size trade-offs exist for pintails.

Variation in egg and clutch size of the Black Redstart (Phoenicurus ochruros) at the northeastern edge of the Qinghai-Tibetan Plateau

Geographical variation in egg size is well documented for several taxa, but remains insufficiently described for birds in spite of a well‐known latitudinal gradient in clutch size. Several hypotheses have been proposed to explain avian egg size variation; however, they were not tested on a continental scale. Egg size is a key component of reproductive investment that influences offspring fitness. It is thought to vary geographically as one of a set of correlated life‐history traits that are under selection from varying ecological conditions. We completed a comprehensive literature review and calculated egg sizes for the most widespread clade within tyrant flycatchers, describing for the first time the geographical variation in egg size on a continental scale. We examined the relative support for ecological and environmental variables in explaining egg size variation using multi‐model inference and linear mixed models controlled for phylogenetic autocorrelation among species. We tested five hypotheses and found that: larger eggs occur in colder sites, which is consistent with the embryonic temperature hypothesis; medium/long‐distance migrants had smaller eggs than resident species while short‐distance migrants had the largest eggs; neither species clutch size, nor species nest type, nor evapotranspiration seasonality influenced egg size. Avian egg size is larger in Austral and Neotropical America (ANA), where species are resident or short‐distance migrants, and smaller across the medium/long‐distance migrants of the Nearctic region. In addition, while clutch size increases towards higher northern latitudes and is almost invariable across ANA species, egg sizes vary largely across ANA sites, increasing with southern latitudes and higher elevations and being influenced by summer temperature. While the embryonic temperature hypothesis has been usually linked to parental nest attentiveness, we highlight that environmental temperatures also have strong effects in shaping investment in egg size.

Egg size and parental quality in thin-billed prions, Pachyptila belcheri: effects on offspring fitness

Egg size-dependent embryonic development in the desert locust, Schistocerca gregaria

Procellariform seabirds provide a good model for studies of the causes and consequences of variability in avian egg size, because females can only adjust reproductive investment by breeding intermittently, or by altering the size of their single egg. Maternal characteristics such as age, breeding experience and body size, as well as environmental variability, can influence egg size, but the effect of these factors has rarely been assessed simultaneously in the same study. Previous studies in Scotland have shown that the egg size of northern fulmars Fulmarus glacialis increased in relation to breeding experience. At this colony the influence of breeding experience, body size and inter‐annual variability upon egg size was tested simultaneously. Data collected over seven breeding seasons between 1975 and 2002 showed that egg size varied significantly both between years and in relation to the length of the breeding experience of females, but that female body size explained most variation in egg size. Inter‐annual variability in egg size was not related to the winter North Atlantic Oscillation, which had recently been shown to influence other measures of reproductive success at this colony. Larger eggs also seem to be more likely to produce successful fledglings. These findings are discussed in relation to the relative contribution of egg quality and parental quality on increased reproductive success.