Restoration of body mass in King Penguins after egg abandonment at a critical energy depletion stage: early vs late breeders

In fasting‐incubating seabirds, it has been proposed that egg abandonment and refeeding should be induced when a low body mass (BM) threshold is attained, thus ensuring adult survival at the expense of immediate breeding. In the context of life‐history trade‐offs in long‐lived birds, we have tested this hypothesis by comparing short‐term survival and restoration of BM in King Penguins Aptenodytes patagonicus that abandoned their egg to those that were relieved normally by their mate at the end of the first incubation shift. Since King Penguins have an extended laying period, the possible influence of seasonal factors was also examined by comparing early and late breeders. Forty incubating males were experimentally forced to fast until egg abandonment by preventing relief by the female. At egg abandonment of both early and late breeding males, BM was below the BM threshold, fasting duration was eight days (about 30%) longer than for relieved birds, and plasma uric acid level was elevated (signature of increased body protein catabolism, phase III of fasting). All abandoning birds survived and came back from sea at a BM similar to that of relieved penguins. The duration of the foraging trip of abandoning early breeders was the same as that of relieved birds, and some abandoning birds engaged in a new breeding attempt. Abandoning late breeders, however, made foraging trips twice as long as those of relieved males. This difference can be explained by time constraints rather than nutritional constraints, abandoning early breeders having enough time left in the breeding season to engage in a new breeding attempt in contrast to abandoning late breeders. These observations lend support to the suggestion that not only BM but also an internal clock intervene in the decision to engage in breeding or not. By preventing a lethal energy depletion ashore and by acting at a fasting stage where the capacity to restore BM at sea is unaffected, abandonment at a low body condition threshold plays a major role in the trade‐off between adult penguin survival and reproduction.     

The adaptive value of energy storage and capital breeding in seasonal environments

Timing of reproduction in a seasonal cycle is a life history trait with important fitness consequences. Capital breeders produce offspring from stored resources and, by decoupling feeding and reproduction, may bend the constraints caused by seasonality in food or predation. Income breeders, on the other hand, produce offspring from concurrent food intake, with the disadvantage of less flexibility, but with high efficiency and no inventory costs of carrying stores. Here, we assess relative profitability of capital and income breeding in herbivorous zooplankton inhabiting seasonal, high-latitude environments. We apply a state-dependent life history model where reproductive values are used to optimise energy allocation and diapause strategies over the year. Three environmental scenarios were modelled: an early, an intermediate, and a late feeding season. We found that capital breeding was most important in the early season. Capital breeding ranged from 7–9% of the eggs produced but, because of the high reproductive value of early eggs, capital breeding ranged from 9–30% when measured in terms of reproductive value. The main benefit of capital breeding was reproduction prior to the feeding season – when the reproductive value of an egg peaked. In addition, capital breeding was also used to increase egg production rates at times of income breeding. For individuals born late in the season the model predicted a two-year cycle instead of the typical annual life cycle. These individuals could then reap the benefits of early reproduction and capital breeding in their second year instead of income breeding late in the first year. We emphasize the importance of evaluating reproductive strategies such as capital and income breeding from a complete life cycle perspective. In particular, knowing the seasonality in offspring fitness is essential to appreciate evolutionary and population-level consequences of capital breeding.     

Spontaneous egg or chick abandonment in energy-depleted king penguins: a role for corticosterone and prolactin?

Various exogenous or endogenous factors may induce an emergency response in birds, redirecting current activity towards survival. In fasting, breeding penguins, the achievement of a critical energy depletion was suggested to induce egg abandonment and departure to sea for re-feeding. How such a behavioral shift is hormonally controlled remains unknown. The possible involvement of corticosterone and prolactin was examined by characterizing the nutritional and hormonal states of king penguins at egg abandonment. Further, we tested if these states differ according to whether an egg or a chick is abandoned, and according to the timing of breeding. In every case of abandonment, birds were in phase III fasting characterized by accelerated protein catabolism. However, body condition at egg abandonment was lower in early than in late breeders, suggesting that king penguins are willing to tolerate a larger energy depletion when their potential breeding success is high. At egg and chick abandonment, plasma corticosterone levels were, respectively, increased by 2- and 4-fold, whereas plasma prolactin levels were, respectively, depressed by 3- and 1.4-fold. The increase in plasma corticosterone and the decrease in plasma prolactin could be involved in the control of abandonment by, respectively, stimulating the drive to re-feed and diminishing the drive to incubate or brood. The smaller decrease in prolactin levels and the greater increase in corticosterone levels observed at chick vs egg abandonment suggest that, in addition to nutritionally-related stimuli, tactile or audible stimuli from the egg or chick could intervene in the endocrine control of abandonment.     

Do purely capital layers exist among flying birds? Evidence of exogenous contribution to arctic-nesting common eider eggs

The strategy of relying extensively on stored resources for reproduction has been termed capital breeding and is in contrast to income breeding, where needs of reproduction are satisfied by exogenous (dietary) resources. Most species likely fall somewhere between these two extremes, and the position of an organism along this gradient can influence several key life-history traits. Common eiders (Somateria mollissima) are the only flying birds that are still typically considered pure capital breeders, suggesting that they depend exclusively on endogenous reserves to form their eggs and incubate. We investigated the annual and seasonal variation in contributions of endogenous and exogenous resources to egg formation in eiders breeding at the East Bay colony in the Canadian Arctic. We collected prey items along with females and their eggs during various stages of breeding and used two complementary analytical approaches: body reserve dynamics and stable isotope [δ(13)C, δ(15)N] mixing models. Indices of protein reserves remained stable from pre-laying to post-laying stages, while lipid reserves declined significantly during laying. Similarly, stable isotope analyses indicated that (1) exogenous nutrients derived from marine invertebrates strongly contributed to the formation of lipid-free egg constituents, and (2) yolk lipids were constituted mostly from endogenous lipids. We also found evidence of seasonal variation in the use of body reserves, with early breeders using proportionally more exogenous proteins to form each egg than late breeders. Based on these results, we reject the hypothesis that eiders are pure capital layers. In these flying birds, the fitness costs of a strict capital breeding strategy, such as temporary loss of flight capability and limitation of clutch and egg size, may outweigh benefits such as a reduction in egg predation rate.     

The effect of food on laying date and clutch-size in Tengmalm's Owl Aegolius funereus

The breeding of Tengmalm's Owl Aegolius funereus was studied at Umeå, Sweden, during the 1984–85. Mean clutch-size was one egg larger in 1984 than in 1985 despite the later laying in 1984. The difference in clutch-size was related to a better food supply in 1984. Daily weight increase of females during the prelaying period showed a high negative correlation with laying date in 1985, and a high positive correlation with clutch-size independently of laying date in 1984–85. This suggested that food eaten before and during laying had a great and direct influence on both laying date and clutch-size. Many females increased in weight during laying and most others decreased only moderately (relative to egg weight), suggesting that body reserves were not a main source for egg production.
Late breeding females were provided with extra food during the prelaying and laying periods in 1985. Fed females weighed more, bred eight days earlier and laid one more egg than controls. At the same laying dates in mid season, and after heavy snow-fall, clutch-size and female weight were larger in the fed birds than in controls, but this was not so near the end of the laying season. Although the earliest of the fed late breeders weighed more, and probably were less restricted by food availability just before or during laying, they did not lay more eggs than did early breeders. This result suggested some limitation on clutch-size that could not be overcome by the supplementary feeding. Weights of females during laying did not show any consistent relationship with clutch-size during successive laying date intervals, suggesting that clutch-size was not directly related to body condition.     

Interrelationships between breeding frequency,timing and outcome in King Penguins Aptenodytes patagonicus: are King Penguins biennial breeders?

King Penguin Aptenodytes patagonicus chick growth is interrupted by a winter fast which extends the length of the breeding cycle (laying to chick independence) to 14–16 months, so that continuous annual successful breeding appears to be impossible. The 3–month laying period imposes further constraints with respect to timing of breeding attempts in relation to the onset of fasting. By the frequent resighting of individually marked birds at Marion Island, we examined the frequency of breeding and the relationships between timing, outcome and frequency in the same and in consecutive years. A total of 3101 adult King Penguins were banded between 1984 and 1991, yielding continuous breeding histories spanning a maximum of 5 years. Most penguins attempted to breed in consecutive years, although the likelihood of taking a year off increased with the number of consecutive attempts. In any one season, about 19% of potentially breeding adults did not breed. Early breeders were more likely to succeed than late breeders and comprised 84% of breeding attempts of known timing. Successful birds in one season usually bred late in the following season, whereas failed attempts were usually followed by an early attempt. Non-breeding was more likely to occur after a successful outcome than a failed one, and after a year off 93% of birds bred early. It is incorrect to refer to the King Penguin as a biennial breeder.     

Energy expenditure and testosterone in free-living male yellow-pine chipmunks

The onset of mating in yellow-pine chipmunks (Tamias amoenus) follows emergence from a prolonged period of energy conservation during hibernation. Energy expenditures are greatly accelerated to meet the demands of the reproductive season. When emerging from hibernation, typical male chipmunks (breeders) have enlarged testes and a high level of plasma testosterone (T). However, certain males that do not participate in reproduction (nonbreeders) maintain small testes and low plasma T levels and emerge several weeks later than the breeders. The timing of the terminal arousal from hibernation and onset of mating are associated with increased plasma T levels. Experimental elevation of T levels in T. amoenus outside the mating season has been associated with a decrease in body mass, further suggesting an effect of T on energy balance. To test this hypothesis, we measured daily energy expenditure (DEE) in free-living, nonbreeding male chipmunks in the presence and absence of a T-implant. We also measured DEE in breeding males when endogenous T levels were high. DEE of the nonbreeders was not affected by our manipulation of plasma T, and the DEE of breeding males did not differ from that of nonbreeders. We conclude that energy expenditure on a daily basis in male yellow-pine chipmunks is not influenced by levels of T. However, on a seasonal basis, the earlier emergence from hibernation by breeding males, which appears to be influenced by T, represents an overall seasonal energy expenditure that exceeds that of nonbreeding males.     

The breeding biology and population dynamics of King Penguins Aptenodytes patagonica on the Crozet Islands

Among King Penguins Aptenodytes patagonica at Possession Island, one of the Crozet Islands, the length of the moult period, pre-laying period, incubating and brooding shifts were highly variable according to the year and to the stage of the breeding season. The moulting period was shorter in late breeders than in early breeders. Only half of the birds which successfully reared a chick bred the following cycle, but late in the season. Almost all these late breeders were unsuccessful. The reasons for the high variability in the breeding pattern observed in this species between years, as well as between colonies and between individuals are discussed. Breeding success was on average 30.6% and survival during the first year at sea could reach 50%. The survival of adult birds has increased during the past 10 years from 90.7% to 95.2% per annum. Despite an almost biennial breeding frequency and a very high rate of chick loss during the winter fast, the King Penguin population of Possession Island has doubled between 1966 and 1985 due to a high survival rate of adult and immature birds. The increase during the last decade in adult survival and in adult and chick condition suggests that the population increase could be the result of an improvement in food availability.     

Intraspecies variation in avian energy expenditure: correlates and constraints

Data on energy expenditure by 553 individuals of 28 species of small bird (10–150 g) are presented. All estimates of energy expenditure were obtained using the doubly-labelled water technique. Intraspecies variation in daily energy expenditure was found to be positively correlated with brood provisioning rates, percentage of time flying and the frequency of non-resting activity. Correlations were also shown with body-mass, body-size and several environmental factors. Published data on basal metabolic rates (BMR) sometimes differed substantially from estimates either made specifically as part of the studies considered here or calculated from allometric equations. For the purpose of interspecific comparisons, specific estimates of BMR are to be preferred. When expressed as a function of BMR, energy expenditures of free-living birds ranged from 1 + to 7 + times BMR with a mode at 3 +. Values of daily energy expenditure exceeding 4 times BMR were found in up to 48% of species and 30% of individuals, so that, contrary to earlier suggestions, 4 times BMR is not a universal upper limit to the sustained work rate of small birds. Observed upper limits tended to be higher in species with energy-expensive foraging habits. Energy expended by breeding birds is likely to involve a balance between the benefits a greater expenditure has for offspring production and any fitness penalty associated with the high level of energy expediture which nest provisioning involves.     

The effect of dietary amino acid composition on egg production in blue tits

Most studies on the interaction between food supply and reproduction in animals have assumed that energy is likely to be the factor limiting egg number and/or size. In this paper, we investigate whether dietary protein proximately constrains egg production in birds. We provisioned breeding blue tits with two food supplements that differed only in the concentration of five essential amino acids. Birds receiving a supplementary diet containing an amino acid balance close to that required for egg protein formation laid significantly larger clutches (18% greater) than control birds, whereas birds receiving an otherwise identical supplementary diet but without a favourable amino acid balance did not increase egg production. To our knowledge, this is the first demonstration that dietary amino acid composition may limit egg production in free-living birds.     

Floaters as intraspecific brood parasites in the grey starling Sturnus cineraceus

The breeding season of the grey starling Sturnus cineraceus is divided into two periods: the early and late breeding season. The birds that breed in each season are referred to as early and late breeders, respectively. In this study, the late breeders mainly consisted of new immigrants that did not breed in the early season. This suggested that these new immigrants were probably floaters in the early breeding season. Because intraspecific brood parasitism occurred frequently, it is possible that the parasites were floaters without nesting boxes. To check for the presence of floaters, two field experiments were conducted and floaters were captured with traps. With additional nesting boxes provided during the breeding season, all new boxes were quickly occupied by floaters from the period of incubation to hatching in the early breeding season, but were not occupied by floaters during the egg laying period of the late breeding season. The addition of boxes before the start of the breeding season significantly decreased the parasitic rate and number of parasitic eggs per nest. There was a positive correlation between the relative occupancy of nesting boxes and the parasitic rate. The removal of boxes again increased the parasitic rate. As for the capture of floaters with traps, the number of trapped birds per day was also related to the relative occupancy of nesting boxes. The floaters trapped between incubation and nesting periods of early breeders became the late breeders. Judging from these results, many floaters were present from the incubation to hatching periods of early breeders, and were probably intraspecific brood parasites.     

Consequences of a changing environment on the breeding phenology and reproductive success components in a long-distance migratory bird

Migratory birds have a narrow time window to breed, especially in the Arctic, where early nesting typically yields the highest reproductive success. We assessed temporal changes (1991–2015) in reproductive success components in relation to timing of breeding in greater snow geese (Chen caerulescens atlantica). This species breeds in the Canadian Arctic, a region that has experienced a strong warming trend. We tested the effect of laying or hatching date, year and their interaction on six reproductive components: Total clutch laid, nesting success, egg survival, hatching success, prefledging, and postfledging survival. Over 25 years, mean laying date changed little, even though it advanced 1.8 days in early breeders and was delayed 3 days in late breeders. Likewise, the number of eggs in nests initiated early in the season decreased by 0.6 egg, whereas in late nests it increased by 0.3 egg. Success of nests initiated early and late in the season was lower than nests initiated near the population mean, and consistently increased over time. The proportion of eggs surviving to partial predation and postfledging survival decreased with laying date but the pattern did not change over time. In contrast, prefledging survival was not affected by laying date initially but declined in nests initiated late in the season toward the end of the study. Overall, nests initiated close to the population mean showed little temporal change for most components of reproductive success and seem to be less affected by environmental change than nests initiated early and late in the season.     

THE EFFECT OF AGE ON THE BREEDING BIOLOGY OF THE KITTTWAKE RISSA TRIDACTYLA

1. From 1954 to 1956 inclusive, the biology of individual marked Kittiwakes was studied at North Shields, Northumberland.
2. It was concluded that older Kittiwakes reacted to the breeding stimulus earlier, more intensively and with greater success than younger breeding birds.
3. Birds with previous breeding experience returned to the colony before birds breeding for the first time and these before non-breeders.
4. Before breeding started, birds which had bred previously spent more of their time at the colony than those about to breed for the first time.
5. Birds breeding for at least the second time laid the first egg 7·5 days earlier than those breeding for the first time.
6. Breeding started one day later for every four days the return to the colony was postponed.
7. Older breeding birds showed greater nest-site tenacity, laid larger clutches and had greater breeding success than younger birds.
8. The chicks in broods of two (but not of one) increased in weight more rapidly where the parents had previous experience.
9. Breeding Kittiwakes showed strong colony tenacity, but 24% of the marked non-breeding birds were subsequently seen in other colonies.
10. Over half the birds retained the same mate as in the previous year.     

Variable energetic responses to clutch size manipulations in white-throated dippers Cinclus cinclus

We used the doubly-labelled water technique to measure daily energy expenditure (DEE) of a free-living uniparental incubator, the white-throated dipper Cinclus cinclus , in Scotland. DEE was 205±8 (s.e.m.) kJ d⁻¹ for 17 females incubating their natural clutch sizes, equivalent to 3.2±0.1×basal metabolic rate (BMR). To investigate the influence of clutch size on the energy budget, we measured the DEE of 14 females with clutches increased or reduced by a single egg. Birds with reduced clutch sizes had an energy expenditure with a mean and variance that did not differ from those of birds with unmanipulated clutches. Enlarging the clutch led to an increase in energy expenditure to over 4×BMR for some individuals but not for others, resulting in greater variance in energy expenditure for birds with enlarged clutches. Individual variation in energy expenditure could not be fully explained by environmental conditions, by patterns of behaviour or clutch size. Incubating females received a maximum of only 4 kJ d⁻¹ (2% of DEE) from provisioning by the male, and mobilised up to 6 kJ d⁻¹ (3% of DEE) from reserves. Females spent 2.9±0.2 h (n=20) away from the nest each day, so a foraging rate of 95 kJ h⁻¹ was required during incubation recesses to balance DEE. This 'required foraging rate' is double previous estimates of the maximum rates of energy acquisition for birds of this size. We suggest that the greater likelihood of a raised energy expenditure associated with larger clutches, combined with the difficulties in maintaining energy supplies, may constitute a constraint on avian clutch size.     

Energy expenditure during egg laying is equal for early and late breeding free-living female great tits

In many bird populations, variation in the timing of reproduction exists but it is not obvious how this variation is maintained as timing has substantial fitness consequences. Daily energy expenditure (DEE) during the egg laying period increases with decreasing temperatures and thus perhaps only females that can produce eggs at low energetic cost will lay early in the season, at low temperatures. We tested whether late laying females have a higher daily energy expenditure during egg laying than early laying females in 43 great tits (Parus major), by comparing on the same day the DEE of early females late in their laying sequence with DEE of late females early in their egg laying sequence. We also validated the assumption that there are no within female differences in DEE within the egg laying sequence. We found a negative effect of temperature and a positive effect of female body mass on DEE but no evidence for differences in DEE between early and late laying females. However, costs incurred during egg laying may have carry-over effects later in the breeding cycle and if such carry-over effects differ for early and late laying females this could contribute to the maintenance of phenotypic variation in laying dates.     

Metabolic costs of egg production in the European starling (Sturnus vulgaris)

The energy cost of egg production in passerine birds has typically been estimated to be 45%-60% of basal metabolic rate (BMR), but this is based on theoretical models using data on energy content of eggs and reproductive tissue; there are still very few empirical data on egg production costs. In this study, we directly measured resting metabolic rate (RMR) in egg-laying female European starlings (Sturnus vulgaris) over 3 yr. We compared these data with RMR of nonbreeding and chick-rearing birds and with estimated energy expenditure generated from a typical energy content model by using empirically derived data from body composition analysis for this species. We found marked variation in RMR between years and between reproductive stages, which complicates comparisons among breeding stages for the assessment of relative egg production costs. On the basis of this method, RMR during egg laying varied from +74% to -13% of nonbreeding RMR and from +20% to -7% of chick-rearing RMR. We therefore used an alternate approach: measuring changes in RMR through the complete cycle of follicle development and ovulation. The increase in RMR from the beginning of prelaying to the six-follicle stage (before first ovulation) when birds have a complete developing follicle hierarchy was 22.4%. This value is still much lower than that estimated from our energy content model. We discuss conceptual problems associated with the theoretical energy content approach but also suggest, on the basis of earlier work done in our lab, that the measured increase in RMR might still underestimate the actual cost of egg production if birds reallocate energy between different physiological systems.     

Best squirrels trade a long life for an early reproduction

Age at primiparity plays a crucial role in population dynamics and life-history evolution. Long-term data on female North American red squirrels were analysed to study the fitness consequences of delaying first reproduction. Early breeders were born earlier, had a higher breeding success and achieved a higher lifetime reproductive success than females who delayed their first reproduction, which suggests a higher quality of early breeders. However, early breeders had similar mass when tagged, and similar number of food caches available at one year of age as late breeders. Nevertheless, we found evidence of survival costs of early primiparity. Early breeders had a lower survival between one and two years of age than late breeders and a lower lifespan. Our study points out that two reproductive tactics co-occurred in this population: a tactic based on early maturity at the cost of a lower survival versus a tactic based on delayed maturity and long lifespan. High quality individuals express the most profitable tactic by breeding early whereas low quality individuals do the best of a bad job by delaying their first reproduction.     

The reliance on distant resources for egg formation in high Arctic breeding barnacle geese Branta leucopsis

Breeding in the high Arctic is time constrained and animals should therefore start with their annual reproduction as early as possible. To allow for such early reproduction in migratory birds, females arrive at the breeding grounds either with body stores or they try to rapidly develop their eggs after arrival using local resources. Svalbard breeding barnacle geese Branta leucopsis have to fly non‐stop for about 1100 km from their last continental staging site to the archipelago making the transport of body stores costly. However, environmental conditions at the breeding grounds are highly unpredictable favouring residual body stores allowing for egg production after arrival on the breeding grounds. We estimated the reliance on southern continental resources, i.e. body stores for egg formation, in barnacle geese using stable isotope ratios in the geese's forage along the flyway and in their eggs. Females adopted mixed breeding strategies by using southern resources as well as local resources to varying extents for egg formation. Southern capital in lipid‐free yolk averaged 41% (range: 23–65%), early laid eggs containing more southern capital than eggs laid late in the season. Yolk lipids and albumen did not vary over time and averaged a southern capital proportion of 54% (range: 32–73%) and 47% (range: 25–88%), respectively. Our findings indicate that female geese vary the use of southern resources when synthesising their eggs and this allocation also varies among egg tissues. Their mixed and flexible use of distant and local resources potentially allows for adaptive adjustments to environmental conditions encountered at the archipelago just before breeding.     

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.     

FACTORS AFFECTING BREEDING OF RAZORBILLS ALCA TORDA ON SKOKHOLM

A study of the breeding biology of the Razorbill was carried out on Skokholm (South Wales) during 1971-73. Birds ringed or colour ringed before the study began provided additional information upon the effects of age on breeding. Mean laying date was delayed in 1972, compared with 1971; the effect is attributed chiefly to stormy weather which upset colony attendance. Eggs were also smaller in 1972. A seasonal decline in egg size (volume) was noted in all three years, attributed mainly to the later laying of young birds. Egg size increased with age, at least up to the fifteenth year. Eggs lost totalled 30% of those laid; 73% of this total was due to predation by Herring Gulls and of Jackdaws. Most losses (45%) occurred during the first 10 days after laying. Of lost eggs, 25% were replaced, usually 14 days after the loss of the original; only eggs laid and lost early in the season could be replaced. Only 7% of the chicks which hatched failed to fledge. Most (62.5%) chick losses occurred in the first week of nestling life, when chick weight was related to egg size. Afterwards, both growth rate and fledging weight were independent of egg size. The chicks fledging early in the season were heavier than later chicks. Failure to fledge was mainly due to a breakdown in behaviour between parent and young, rather than to predation. Breeding success was highest for birds breeding early in the season, most of which were older, more experienced breeders. These laid early enough to replace an egg if it was lost; they produced large eggs, and their chicks were therefore both heavier than average during the critical first 7–10 days of life, and fledged at a high weight. Thus experience accumulated with age, and the ability to lay early in the season are important for successful breeding in the Razorbill.