Experimental demonstration of the insurance value of extra eggs in an obligately siblicidal seabird

A variety of organisms regularly produce more offspring thanthey raise. Despite the apparent energetic waste of such areproductive tactic, overproduction may be favored by naturalselection in some cases. One such case is when surplus offspringcan serve as replacements, or insurance, for failed siblings.We tested the Insurance Egg Hypothesis (IEH) as an explanationfor the overproduction of offspring in an obligately siblicidal seabird, the Nazca booby (Sula grant)i, which fledges a maximumof one nestling regardless of its clutch size. We manipulatedclutch sizes within the range of natural variation encounteredin this species (one-two eggs). The IEH predicts that parentswith two-egg clutches should have higher reproductive successthan those with one-egg clutches because the second egg canprovide a nestling when the first egg fails to hatch, or when the first chick dies young. Consistent with the IEH, naturalone-egg clutches that were enlarged to two eggs produced morehatchlings and fledglings than control one-egg clutches did,and natural two-egg clutches that were reduced to one egg producedfewer hatchlings and fledglings than control two-egg clutchesdid. We also evaluated aspects of the Individual Optimization Hypothesis, which proposes that individual optimal clutch sizesdiffer, as an explanation for clutch size variation in thisspecies. In Nazca boobies, selection driven by replacementvalue appears to favor clutches larger than one even thoughfinal brood size is invariably one. One-egg clutches may be produced by parents experiencing some proximate limitation,such as a lack of food.     

FACTORS AFFECTING THE EGG SIZE OF RED-BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

The factors influencing the egg size of the Red-billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two- and three-egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one-, two- and three-egg clutches declined significantly as the season progressed. The size of eggs from single-egg clutches tended to be smaller than eggs from two-egg clutches laid at the same time. There were correlations between the proportions of one-egg and of three-egg clutches being laid at a given period and the mean egg volume of two-egg clutches. When the mean egg volume of two-egg clutches increased there was a corresponding increase in the proportion of two- and three-egg clutches laid. When the mean egg volume of two-egg clutches decreased there was an increase in the proportion of single-egg clutches laid. The egg size of the Red-billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time. Two-year-old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females. It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.     

Why don't all siblicidal eagles lay insurance eggs? The egg quality hypothesis

Several species of birds lay second eggs that are eliminatedby the siblicidal behavior of the first-hatched chick. A widelyaccepted explanation for the occurrence of these second eggsis insurance against complete nest failure. However, if insuranceis seen as an important breeding strategy for two-egg (c/2)layers, the question arises why single-egg species do not layinsurance eggs. The insurance-egg hypothesis predicts that extraeggs should occur where hatch failure is not trivial, whichmay be particularly prevalent in dense populations. Neitherprediction was supported for siblicidal l Wahlberg's eaglesAquila wahlberge Neither could food constraints or allometricrelationships explain the small one-egg clutch (c/1) of thisspecies Instead, clutch size was experimentally shown to berelated to optimal brood size: parents given two young wereunable to rear them, and subsequent breeding opportunities weresignificantly curtailed. Since clutch and brood size are similarlyrelated in c/2 eagles, insurance may be an exaptation of thesecond egg. One-egg spedes, however, appear to trade second(insurance) eggs for large, high-quality eggs, which enhancehatchability and chick viability. This was borne out by comparisonof the world's c/1 eagles, which lay significantly (p<.01)larger eggs than c/2 eagles of the same body size. Large Wahlberg'seagle eggs also showed significantly (p=.02) greater hatchabilitythan small eggs, and other studies show enhanced survival/qualityfor chicks from large eggs. Because only longer-lived eaglestraded two eggs for single, large eggs, this is consistent withthe idea of selection for offspring quality in long-lived species.I condude that higher hatchability of single, large eggs decreasesthe need for an insurance egg and simultaneously enhances viabilityof resultant chicks in sibliddal eagles and possibly sulids.     

The influence of age on laying date,clutch size,and egg size of the white-fronted tern,Sterna striata

Abstract

The age at first breeding and the influence of age on laying date, clutch size, and egg size of white-fronted terns were studied at the Kaikoura Peninsula, New Zealand, between 1971 and 1976. Of the 134 banded birds recovered breeding at Kaikoura, 63% were marked as nestlings at Kaikoura; the remainder came from colonies within 104 km. The bird is extremely capricious in its choice of nesting locality, and there are indications that it is not consistently philopatric. A small number bred as 3-year-olds, but the majority did not commence breeding until after they were 6 years old. Most pairs (73%) were of partners with an age difference of 1 year or less. Laying date and egg size varied with the age of the parent, but clutch size showed no significant change in relation to age. Mean egg volume did not vary between one-egg and two-egg clutches, but in two-egg clutches the first egg laid was significantly the larger in length, breadth, and volume. Single-egg clutches were the most common, but as the season progressed the proportion of two-egg clutches increased. There was no significant seasonal change in egg size.     

Relationship between clutch size,egg volume and hatching success in a Yellow-legged Gull Larus michahellis colony in south-eastern Tunisia

This study aimed to test the hypothesis that clutch size covaries with egg volume and hatching success in the Yellow-legged Gull Larus michahellis. We determined clutch size and egg volume in a sample of 131 nests, and we used the data to check whether egg volume varied among nests according to clutch size, while taking into account the effects of egg laying order. We also estimated hatching success rate and investigated the relationship between hatching success and clutch size. Egg volume varied among clutches according to clutch size, with eggs being larger in three-egg clutches than in two-egg clutches. Moreover, three-egg clutches showed higher daily survival rates, and hence hatching success, than two-egg clutches. Overall, our results suggest that in the Yellow-legged Gull clutch size covaries with egg volume and hatching success, which could possibly reflect an age effect through different mechanisms. Indeed, older females could be hypothesised to exhibit greater breeding performance than younger females because of their higher experience in tapping energy resources for egg formation and defending nests from dangers. Moreover, due to their age, older females are likely to have lower residual reproductive potential and should invest more heavily in current breeding attempts.     

Maturation increases early reproductive investment in Adélie Penguins Pygoscelis adeliae

Correlations between female investment in egg production and age, breeding experience and laying date have been reported in several seabird species. In general, clutch and egg sizes increase with female age and breeding experience but decrease with laying date. Positive correlations of clutch and egg size with age and breeding experience can be caused by an increase in reproductive investment with maturation or they may be an artefact of lower survival rates for individuals with poor-quality phenotypes. Negative correlations of clutch and egg size with laying date might signal an adaptive reduction in egg production or be due in part to variation among individuals. We examined the interactions of female age, breeding experience, laying date and clutch and egg size in Adélie Penguins Pygoscelis adeliae . Breeding experience strongly affected clutch size with 87.3% of all one-egg clutches laid by first-time breeders. In addition, increasing age had a positive influence on egg size and was associated with earlier laying dates. However, there was little evidence to suggest that either clutch or egg sizes are influenced by laying date. Laying dates and clutch and egg sizes did not affect a female's probability of returning to breed in the following year, indicating that increased investment is a product of maturation and not of the loss of poor-quality breeders from the population. Our results suggest that as female Adélie Penguins gain foraging and breeding experience they are able to initiate breeding earlier, to lay complete clutches of two eggs and to lay larger eggs.     

Effects of clutch size and egg-laying order on the breeding success in the Little Tern Sterna albifrons on the Nakdong Estuary, Republic of Korea

Survivorship of Little Tern Sterna albifrons eggs and chicks was followed on an islet in the Nakdong Estuary, Republic of Korea, in 1995 and 1996. Mean egg size and incubation period were significantly different between the 2 years. The maximum clutch size was three eggs, and the second egg in the clutch often hatched earlier than the first, while most of the third eggs hatched last. In 1996, when the fate of 249 eggs from 106 nests was followed for 40 days, hatching success, fledging success and breeding success were 77%, 40% and 31%, respectively. High mortality occurred in the early chick stage, mostly because of rain and predation by Weasels Mustela sibirica. The breeding success per egg was 14% in one-egg clutches, 28% in two-egg clutches and 34% in three-egg clutches. This difference was mainly attributed to the lower hatching success in the smaller clutches. In three-egg clutches, the third egg showed significantly lower breeding success than siblings. The main foods of the Little Tern were Tridentiger obscurus, Engraulis japonicus, Hyporhamphus intermedius, Acanthogobius flavimanus (all fish), Palaemon sp. and Crangon affinis (shrimps). The feeding frequency was, apparently, not affected by time of day and age of chicks but was probably influenced by weather conditions. Newly hatched chicks failed to eat 25% of the prey brought to them, although this decreased with the age of the chicks.     

BREEDING SUCCESS IN SOME AFRICAN EAGLES RELATED TO THEORIES ABOUT SIBLING AGGRESSION AND ITS EFFECTS

Brown, L. H., Gargett, V. & Steyn, P. 1977. Breeding success in some African Eagles related to theories about sibling aggression and its effects. Ostrich 48:65-71. Previous explanations for fatal inter-sibling strife in eagles (lack of food, extra aggressiveness which enhances survival, and an expression of the innate aggressiveness or territoriality of raptors) can not be supported by recent evidence. The latest theory, that the second egg acts as a “reserve”, is examined. If so, eagles normally laying two eggs should have better reproductive success than eagles laying a single egg. However there was no significant difference between the mean reproductive success of five African eagle species that lay two eggs and that of five African eagle species laying one egg. Even excluding inequalities due to sample size, and other factors, the overall finding is the same. In Verreaux's Eagle Aquila verreauxi for which the largest number of records is available there was a significantly higher total loss in clutches recorded as c/l than in c/2 clutches, but the reasons for the laying of a one-egg clutch, and its success or failure, have not been investigated. In three out of 120 c/2 the second-laid egg acted as a “reserve”, but in 110 cases the second-laid egg was “wasted”. Direct evidence and a comparison of reproductive success between two-egg and one-egg African eagles does not support the latest theory, therefore inter-sibling strife which effectively reduces the clutch of two to one, is still inexplicable.     

Nonsiblicidal behavior and the evolution of clutch size in bethylid wasps

Parent-offspring conflict over clutch size may lead to siblicidal behavior between juveniles. In parasitoid wasps, selection for siblicide in small broods is predicted to produce a dearth of gregarious broods with few eggs. Here we document the clutch size distribution in the Bethylidae, a large family of aculeate parasitoids. Small gregarious clutches are the most common. Further data suggest that the most common gregarious clutches in the parasitoid Hymenoptera as a whole contain only a few eggs. Across bethylid species, both clutch size and wasp size increase with host size. Within genera clutch size is more closely related to host size, but between genera or larger clades wasp size is more closely related to host size. The volume of the emerging wasp brood does not depend on whether a species lays single- or multiple-egg clutches once host size is taken into account. These data suggest that clutch size in bethylid wasps is best described by traditional optimality models and that siblicide plays little role in this and possibly other families. We propose several ecological reasons for the rarity of siblicide in bethylids: ectoparasitism, idiobiosis, and a suite of characteristics associated with high within-brood relatedness.     

Patterns of variation in size of Boat-tailed Grackle Quiscalus major eggs

Boat-tailed Grackle Quiscalus major eggs averaged 8^.09 g wet weight. Mean egg weight represented 8 05 % of female body weight. Based on egg weight, Grackle eggs have a shorter incubation period than the general passerine pattern.
Egg weight varied significantly with sequence of laying and between clutches in both two-egg and three-egg clutches. Last laid eggs weighed less than first laid eggs. The mean weight of the first two eggs laid in three-egg clutches did not differ from the mean egg weight of two-egg clutches. The average clutch of two and three eggs weighed 16.33 g and 24.16 g, respectively.
Mean egg weight varied between study colonies and with season. Grackles at East Lake laid eggs that weighed less than grackles at either Alligator Lake or North Lake. Eggs laid during March averaged less than eggs laid later in the season. The locality variation reflects the different timing of nesting between sites rather than food abundance. As nutrient provisioning increases with an increase in egg weight, the seasonal change in egg weight probably reflects improved feeding conditions for the female. This suggests that selection favours beginning laying before reserves of the female are sufficient to lay the largest egg possible.
The size of the young at hatching was correlated with egg size. Newly hate had young averaged 79.6% of fresh egg weight. Egg weight did not determine the probability of hatching or starvation of the young. Females do not appear to adjust egg size facultatively depending on the sex of the young. Eggs producing males and females did not vary significantly in weight, whether comparisons were made within or between clutches.     

Testing small clutch size models with Daphnia

Life-history theory predicts that for small clutches, variance in egg size (between individuals) should decrease in a predictable invariant manner as clutch size increases. To test this, we studied Daphnia magna at 350 different food treatments and recorded the number of eggs and the volume of each egg for their first clutch. As predicted, we found that the relationship between clutch size and resources devoted to reproduction was linear, variance in egg volume decreased with increasing clutch size, and resources were shared relatively equally between the eggs in a clutch. However, we found that the rate at which the range of egg volumes decreased with clutch size was slower than predicted. We discuss possible explanations for this discrepancy, including a lower limit on the volume of eggs that are produced and selection for smaller eggs when food is abundant. Consistent with this, we found that mean egg volume decreased with increasing clutch size.     

Effect of brood size and hatching sequence on prefledging mortality of Sandwich terns: why lay two eggs?

The mortality of Sandwich tern Sterna sandvicensis chicks held in enclosures was studied in colonies on Griend, in the Dutch Wadden Sea, from 1992 to 1999, and on Hirsholm, in the Danish Kattegat, in 1997. Survival of chicks until fledging was 73% for chicks hatching from first-laid eggs or single-egg clutches and 59–64% for partially hatched two-egg clutches, whereas 6% of second hatchlings survived until fledging. Less than 2% of all two-chick broods actually fledged two chicks. Because 18% of the two-egg clutches only hatched one egg, 7% of fledglings of two-egg clutches originated from a second-laid egg. In nests where both eggs hatched, the number of chicks was usually reduced soon after hatching. Within five days of hatching more than 50% of the second hatchlings died of starvation or were preyed upon. It seems that overproduction commonly occurs in Sandwich terns and that investment in a surplus egg mainly serves as an insurance mechanism. On Griend and Hirsholm, chick productivity of two-egg clutches was somewhat higher than for one-egg clutches. Undernourishment was an important cause of death, either directly by starvation or by selective predation of chicks in poor condition. This, in combination with earlier, studies suggests that Sandwich tern parents on Griend are exposed to severe food stress.     

Incubation capacity contributes to constraints on maximal clutch size in Brent Geese Branta bernicla nigricans

Lack ( 1967 ) proposed that clutch size in species with precocial young was determined by nutrients available to females at the time of egg formation; since then others have suggested that regulation of clutch size in these species may be more complex. We tested whether incubation limitation contributes to ultimate constraints on maximal clutch size in Black Brent Geese (Black Brant) Branta bernicla nigricans. Specifically, we investigated the relationship between clutch size and duration of the nesting period (i.e. days between nest initiation and the first pipped egg) and the number of goslings leaving the nest. We used experimental clutch manipulations to assess these questions because they allowed us to create clutches that were larger than the typical maximum of five eggs in this species. We found that the per‐capita probability of egg success (i.e. the probability an egg hatched and the gosling left the nest) declined from 0.81 for two‐egg clutches to 0.50 for seven‐egg clutches. As a result of declining egg success, clutches containing more than five eggs produced, at best, only marginally more offspring. Manipulating clutch size at the beginning of incubation had no effect on the duration of the nesting period, but the nesting period increased with the number of eggs a female laid naturally prior to manipulation, from 25.4 days (95% CI 25.1–25.7) for three‐egg clutches to 27.7 days (95% CI 27.3–28.1) for six‐egg clutches. This delay in hatching may result in reduced gosling growth rates due to declining forage quality during the brood rearing period. Our results suggest that the strong right truncation of Brent clutches, which results in few clutches greater than five, is partially explained by the declining incubation capacity of females as clutch size increases and a delay in hatching with each additional egg laid. As a result, females laying clutches with more than five eggs would typically gain little fitness benefit above that associated with a five‐egg clutch.     

Two eggs,two different constraints: a potential explanation for the puzzling intraclutch egg size dimorphism in Eudyptes penguins

Phenotypic plasticity and phenotypic stability are major components of the adaptive evolution of organisms to environmental variation. The invariant two-egg clutch size of Eudyptes penguins has recently been proposed to be a unique example of a maladaptive phenotypic stability, while their egg mass is a plastic trait. We tested whether this phenotypic plasticity during reproduction might result from constraints imposed by migration (migratory carry-over effect) and breeding (due to the depletion of female body reserves). For the first time, we examined whether these constraints differ between eggs within clutches and between egg components (yolk and albumen). The interval between colony return and clutch initiation positively influenced the yolk mass, the albumen mass, and the subsequent total egg mass of first-laid eggs. This time interval had only a slight negative influence on the yolk mass of second-laid eggs and no influence on their albumen and subsequent total masses. For both eggs, female body mass at laying positively influenced albumen and total egg masses. Female investment into the entire clutch was not related to the time in the colony before laying but increased with female body mass. These novel results suggest that the unique intraclutch egg size dimorphism exhibited in Eudyptes penguins, with first-laid eggs being consistently smaller than second-laid eggs, might be due to a combination of constraints: a migratory carry-over effect on the first-laid egg and a body reserve depletion effect on the second-laid egg. Both these constraints might explain why the timing of reproduction, especially egg formation, is narrow in migratory capital breeders.     

Clutch size in the tropical scincid lizard Emoia sanfordi, a species endemic to the Vanuatu Archipelago

The majority of species in the scincid genus Emoia (Squamata: Scincidae) have a fixed clutch size of two eggs per clutch and produce between two and four clutches per year. One lineage within Emoia, the Emoia samoensis species group, consists of 13 species occurring in Melanesia and the islands of the southwestern Pacific Ocean, and exhibits variation in clutch size, with previously reported clutch sizes of two to five eggs. Little is known about reproduction in several members of this lineage including Emoia sanfordi, a large-bodied lizard endemic to the archipelago of Vanuatu in the South Pacific. We analyzed reproduction and clutch size in E. sanfordi females and discovered that there is a substantial amount of intraspecific variation, with clutch size ranging from two to seven eggs, with a modal clutch size of five eggs. Females were reproductively active throughout the study period of June through October and appear to be laying multiple clutches. The variation in clutch size seen in E. sanfordi is congruent with the variation previously reported within other closely related species.     

CLUTCH SIZE, EGG SIZE, HATCH WEIGHT AND LAYING DATE IN RELATION TO EARLY MORTALITY IN RED GROUSE LAGOPUS LAGOPUS SCOTICUS CHICKS

Clutches of Red Grouse eggs were collected from the wild and subsequent hatching and rearing done in standard conditions in captivity. Variations in chick survival from one clutch to another in the same year were related to differences in hatch weight. Hatch weight was determined only partly by egg size. Weight loss between laying and hatching was related to survival independently of egg size. Variation in this weight loss obscured any simple relationship between egg size and survival, except in eggs laid by captive hens. Intrinsic differences amongst hens caused some variations in laying date, egg size, hatch weight and chick survival. Variations in egg size and hatch weight accounted for less than half the variation in survival; other unmeasured intrinsic factors were also important. Big clutches hatched earlier than small ones. The commonest clutches were of seven and eight eggs, with six and nine frequent. Very big clutches of ten or more eggs were infrequent and chicks from them sometimes survived worse than from smaller clutches. As in other species, the commonest clutch sizes were not the most productive. There was no simple relationship between egg size and clutch size.     

Reproductive life history variation among colour morphs of the pygmy grasshopper Tetrix subulata

Individuals of pygmy grasshoppers ( Tetrix subulata [L.] Orthoptera: Tetrigidae) exhibit genetically coded discontinuous variation in colour pattern. To determine whether reproductive performance is likely to be affected by colour pattern, this study investigated variation in body size and reproductive life-history characteristics among individuals belonging to five different colour morphs. The proportion of reproductive females (i.e. females with eggs) declined significantly as the season progressed (from 100% in mid-May to 40% in mid-June), but no such seasonal trend was apparent for body size, clutch size or egg size. Colour morphs differed significantly in body size, and these size differences accounted for most of the variation in clutch size and egg size. Colour morphs also differed in the regression of egg size on clutch size, suggesting that trade-offs between number and size of offspring might vary among morphs. Finally, I found a negative relationship across colour morphs between the proportion of females with eggs and average clutch size. This suggests that individuals belonging to certain colour morphs produce a relatively large number of clutches per unit time, at the expense of fewer offspring in each clutch, compared to other morphs. Collectively, my results indicate that different colour morphs of T. subulata may have different reproductive strategies. These differences may reflect variation in thermoregulatory capacity or differences in probability of survival induced by visual predators.     

Production of insurance eggs in Nazca boobies: costs, benefits, and variable parental quality

Numerous studies of birds have demonstrated the value of producinginsurance eggs. We have previously found that second eggs provideinsurance against failure of first eggs for Nazca boobies (Sulagranti), which raise only one chick to fledging, yet some femaleslay only one egg. We used an 8-year data set to compare 2 hypothesesfor clutch size variation: one based on trade-offs, predictingdeclining future performance by females that lay a costly secondegg and one based on parental quality, predicting that intrinsicallysuperior females lay 2 eggs both currently and in the future.Clutch size variation did not contribute to the best multistatemark–recapture model of survival, suggesting that clutchsize and survival are unrelated and that any survival cost ofreproduction related to laying second eggs is small. Transitionprobabilities between reproductive states were generally, butnot entirely, inconsistent with fecundity costs of producinga marginal egg. Parental quality effects were apparent, withfemales tending to remain in a given reproductive state acrossseasons. Parents producing a marginal egg had consistently higherbreeding success than did parents of 1-egg clutches, due principallyto the insurance effect and secondarily to differences in parentalquality after hatching. Different model selection approachesgave differing results for logistic regression analyses of breedingsuccess of females that hatched eggs. A conventional significance-testing,iterative-fitting approach excluded insurance state from thetop model, whereas the information-theoretic (I-T) approachdetected an association between current insurance state andsubsequent survival and fecundity. Using the I-T approach, potentiallybiological significant (but not statistically significant underconventional analysis) effects were detected that may otherwisehave been ignored.     

Intraclutch egg-size variation in the Barnacle Goose Branta leucopsis: an egg-removal experiment

We carried out an egg-removal experiment on Barnacle Geese Branta leucopsis to test the hypothesis that tactile contact between the brood patch and eggs accumulating in the nest during laying was causally related to intraclutch egg-size variation, acting through the development of incubation behaviour. Egg removal had no effect on mean egg mass, relaying interval or clutch size: mean (±s.e.) clutch sizes for first clutches were 4.9 ± 0.9 and 4.7 ± 0.9 eggs for control and experimental females, respectively. We therefore conclude that Barnacle Geese have a determinate pattern of egg laying. Daily removal of all eggs from the first-laid egg did not affect the within-clutch pattern of egg-size variation: in both control and experimental clutches the first egg was relatively small, the second or third egg was largest and there was a linear decline in egg size to the smallest, last-laid egg. There was no significant difference in either the absolute or relative size of the last-laid eggs in control and experimental clutches. Tactile contact with eggs in the nest, therefore, is not required for the expression of intraclutch egg-size variation in Barnacle Geese. In this respect, determinate-laying Barnacle Geese differ from species with indeterminate laying patterns (e.g. gulls) where egg removal does affect the normal pattern of within-clutch egg-size variation. If hormonal changes (e.g. prolactin) associated with onset of incubation are causally related to the intraclutch decrease in egg size, then the stimuli involved appear to differ between determinate and indeterminate layers.     

Maternal resource variation across the laying sequence in Canada geese Branta canadensis maxima

Variation in maternal investments to offspring presumably reflects an optimization of resource allocation such that a female's fitness is maximized. In birds, both egg size and yolk constituents are examples of resources that can vary among offspring within a clutch. Egg size and maternally-derived steroid hormone concentrations present in yolk have been characterized for many species that lay small clutches or have altricial young, but little information is available for species that lay moderate to large clutches of precocial young. In this study, we recorded laying position, measured fresh egg mass and determined maternally-derived testosterone and estradiol concentrations present in yolks for whole clutches of free-living Canada geese Branta canadensis maxima to assess variation in maternal resources within clutches. We found that egg size varied non-linearly across the laying sequence such that first laid eggs were small, the largest eggs in the clutch occurred in the second and third positions, and size declined in eggs laid in subsequent positions. Concentration of testosterone in the yolk followed a pattern in which the first and second laid eggs have the highest concentrations within a clutch and declining concentrations in subsequently laid eggs. In contrast, maternally-derived yolk estradiol concentrations (measured in a subset of clutches) did not change across the laying sequence.