THE BREEDING BIOLOGY OF THE BLACK-FACED DIOCH QUELEA QUELEA IN NIGERIA

A study of the breeding biology of Quelea quelea in Nigeria, and particularly at a large breeding colony near Lake Chad, showed that losses of eggs and young were extremely small. 95% of eggs laid hatched successfully, and 87% give rise to fledglings. Nestling deaths were density-dependent and apparently due to starvation.
The incubation period was 10 days or less. By day, eggs were heated to 34°- 37° C. by the sun; at night the females incubated. The nestlings were initially fed mainly on insects, their diet gradually changing to one of seeds—mostly of the grass Echinochloa pyramidalis . The deep body temperatures of young birds were determined. It is suggested that the nestlings left the nest after, on average, 11¹/₂ days to escape intolerable temperature conditions in the nest.
Fat reserves were accumulated by nestlings and fledglings, and were utilized when the young became independent. The adults put on fat during the incubation period and lost it during the time spent feeding nestlings.
It is concluded that the most common clutch-size of Q. quelea , which is everywhere three, corresponds to the largest number of young the parents can normally nourish. This conforms to Lack's theory on the significance of clutch-size, and gives no support to Skutch's opinion that the theory does not apply to tropical birds.     

THE MIGRATION PATTERNS OF QUELEA QUELEA IN AFRICA

Red-billed Queleas migrate, at the beginning of the wet season, away from their dry-season concentration areas and towards areas where rain started several weeks earlier. Considerable fat deposits are accumulated for this “early-rains migration”. The direction taken by the migrants, the distance they must fly, and the timing of the movement are dependent upon the timing of the rains and the way the rain-front moves. On the return “breeding migration” individuals in breeding condition stop to breed (in large aggregations) wherever they find conditions suitable for the founding of colonies. The location of the colonies can vary greatly from year to year. The migrations performed by several populations, in different parts of Africa, are predicted on the basis of general rainfall patterns, and the predictions tested against the facts available. There is evidence that individual females are able to produce a succession of broods in the same breeding season, at colonies which may be very far apart, and probably with different mates. The adaptive value of this “itinerant breeding” is discussed. Many other bird species, which breed at a particular phase of the wet-season/dry-season cycle, are expected to perform similar “itinerant breeding”.     

FEEDING ECOLOGY OF THE BLACK-FACED DIOCH QUELEA QUELEA IN NIGERIA

The results of a three-year study of the diet of Quelea quelea in the Lake Chad region of Nigeria indicate why this bird periodically does extensive damage to crops of dry-season guinea-corn.
The normal food of the birds in the early dry-season consists of small grass seeds collected off the ground. As the supply of these diminishes there is a gradual change-over to larger seeds.
In years when this change begins early, it is liable to coincide with the ripening of the cereal crop and severe damage may be caused.
At the beginning of the rains the dry-season food supply is lost owing to the simultaneous germination of the seeds over large areas.
The birds lay down fat reserves in preparation for this lean period.
After a short time the birds migrate south to regions where rain has been falling for some weeks.
In the short period spent in these parts, Queleas feed on ripening grass seeds—including those of cultivated millet.
There is evidence for considerable mortality during this time of food scarcity and it is suggested that the numbers of Queleas are limited by the food supply.
The significance of communal roosting is discussed. It is proposed that the phenomenon is related to food finding, the roost being considered as an "information centre" for the large area prospected by the birds constituting the roosting community. A method is outlined by which information on food availability within the feeding area may feasibly be transmitted within the community.     

Body condition changes of sand martins (Riparia riparia) during breeding, and a comparison with fledgling condition

The body condition of adult sand martins during the breeding season and a sample of fledglings was investigated by carcass analysis and by examination of live birds in the field. Fat scoring of live individuals was a reliable predictor of an individual's lipid content in most instances. Body mass changes during breeding were related to changes in the size of birds' lipid reserves, pectoral muscle mass, body water, and to development of the reproductive system. In both males and females, reserve lipid declined between the onset of breeding and nestling rearing. Pre-breeding males had significantly greater pectoral muscle masses than did nestling rearing females. The potential significance of protein in pectoral muscles as an energy reserve during breeding was small compared with lipid reserves. By comparing potential energy reserves in body lipid and protein with daily energy expenditure, it was calculated that neither incubating nor nestling rearing adults could survive a day of normal activity without feeding. The selective premium on adults optimizing their use of time and energy for self-maintenance behaviour during breeding is therefore likely to be great.     

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.     

Variation in body condition of breeding Savi’s Warblers Locustella luscinioides: the reproductive stress and flight adaptation hypothesis revisited

Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce wing-loading during nestling feeding) or result from physiological stress. To test which might be more important in determining mass loss in breeding Savi’s Warblers (Locustella luscinioides), we used a new approach in which the variation in four indices of body condition was described: weight, fat score, muscle score and lean weight (i.e. excluding fat and muscle). We expected weight variations to be adaptive if they involved changes in fat and lean weight, whereas physiological stress should influence the muscle score to a greater extent. As in other species, females showed a greater variation in weight, and carried more fat, than males during the breeding cycle. During incubation, females had greater weight and fat score than males. The weight remained constant and lean weight declined in both sexes, whereas females increased in muscle, which probably reflects the regression of the reproductive organs. During the nestling stage, both sexes declined significantly in all four indices of condition, showing evidence of physiological stress. However, the greater decline in weight in females than in males is consistent with the flight-adaptation hypothesis, as are the cyclic changes in lean weight associated with the various nesting attempts. The fact that both sexes declined significantly in weight, muscle and lean weight with an increasing number of nesting attempts, but not in fat, which was recovered after each nestling period, also indicates that both reproductive stress and adaptive changes occur during breeding. When the whole breeding season was considered, females showed a greater decline in muscle than males, which we interpret to be evidence for a greater reproductive stress in females. We suggest that the small breast muscle size and depleted protein reserves at the end of the breeding period might influence future survival through impaired flight ability and a compromised post-breeding moult.     

The effect of female body condition on egg laying in Lesser black-backed gulls Larus fuscus

Female Lesser black-backed gulls, culled from a moorland nesting site in Lancashire for public health reasons, were examined to compare their body condition with the number and quality of eggs that they would lay. The clutch size and stage of the laying cycle of each bird were determined by examining their ovaries. Fat condition was obtained by extraction of the whole carcass with chloroform and an index of protein condition was obtained from the lean dry weight of the flight muscles. Oviduct eggs were analysed for the amounts of fat and protein present in the yolk and albumen.
Lesser black-backed gulls are not fully indeterminate layers. Most females develop only enough enlarged ova to produce the normal clutch of three eggs, with some birds able to lay an additional one or two replacement eggs if necessary. The female's protein reserve, but not her fat reserve, declined during egg formation and was correlated with the total number of eggs she could potentially have laid. Similarly egg quality, as measured by yolk weight and protein content, was correlated with female condition but not her fat reserves. Albumen weight was not correlated with body condition.
It is shown that egg production is a demanding process in Lesser black-backed gulls and breeding success is influenced by the amount of the female's protein reserve both through its effect on egg quality and on her ability to lay replacement eggs if necessary.     

The rôle of fat and protein during breeding in the White-bellied swiftlet (Collocalia esculents)

The breeding biology of the White-bellied swiftlet was studied in Malaysia. Birds were captured every month, and more intensively during the breeding season. To investigate the role of fat and protein reserves in the annual cycle, carcasses were fat extracted after separating the pectoral muscles. The stage of the laying cycle was investigated by histological examination of the ovaries.
Fat and protein reserves were correlated with the body size of the birds. In non-laying females and males, both reserves were correlated with date and feeding conditions determined by the weather. In egg-laying birds, protein reserves were again correlated with feeding conditions, but fat reserves were negatively correlated with the day of the laying cycle. Assessment of the daily foraging abilities of the birds showed that, for egg-layers, proteins were likely to be in greatest supply and fats in least supply. It is suggested that a fat reserve serves as an insurance against poor feeding conditions for laying birds.     

Fat reserves of migrating passerines at arrival on the breeding grounds in Swedish Lapland

Mist-net capture data taken during 6 years (1988–1990 and 1992–1994) of field work were used to describe the arrival sequences and fat loads of nine species of migratory passerines which breed in a near-Arctic environment in Swedish Lapland. Long-distance migrants arrived with significantly larger mean fat reserves than did medium- and short-distance migrants. Long-distance migrants carried fat loads at arrival which corresponded to potential remaining flight distances between 242 and 500 km. When females and males arrived on the breeding grounds simultaneously, females carried significantly larger energy reserves than did males in seven out of nine species. In contrast, when the sexes showed a significant difference in median arrival date (two out of nine species), there was no difference in mean fat load carried into the breeding area. A relationship was found between the migratory habits and foraging ecology of each species and the amount of fat reserves at arrival, suggesting that species-specific migratory distances and feeding habits determine the amount of fat that is needed during the transition period between migration and onset of breeding. The short growing season in the study area restricts the time available for breeding and moult, and large energy reserves at arrival may speed up the breeding schedule to counteract possible time constraints. Overloading at the last stopover site during spring migration may be an adaptation allowing birds to cope with a restricted time frame for breeding and moult at high latitudes.     

Producing extra eggs does not deplete macronutrient reserves in European Starlings Sturnus vulgaris

Recent studies have suggested that the size of avian clutches may be determined in part by the costs of egg production, which are thought to involve the depletion of endogenous nutrient reserves. I tested the hypothesis that producing extra eggs would deplete protein and lipid reserves in a passerine species, the European Starling Sturnus vulgaris . The first three eggs of clutches were removed on the days they were laid to induce females to lay extra eggs. Females responded to this manipulation by laying, on average, 1.3 eggs more than controls. However, based on the distribution of clutch sizes, it appears that some females responded to the removal of three eggs by producing more than one extra egg, whereas others responded by laying no extra eggs or by deserting. In unmanipulated females, the lean dry mass of the flight muscles and total body lipid were found to decline throughout breeding, an observation which is often interpreted as evidence that egg production depletes endogenous reserves. However, females experimentally induced to produce an extra egg did not have reduced stores of protein or lipid at clutch completion. No effect was observed in starlings even though this study had substantial (80%) power to detect a change in protein reserves of the same magnitude as that observed in a previous study of gulls. The decrease in flight muscle mass that has frequently been observed during breeding in passerines may not be due simply to mobilization of endogenous protein reserves.     

Body mass variation in breeding Florida Scrub‐Jays

ABSTRACT Many birds lose mass when feeding dependent young and multiple hypotheses have been proposed to explain this loss. The reproductive‐stress hypothesis suggests that mass loss results from an energy deficit. The flight‐efficiency hypothesis suggests that breeders lose mass in advance of feeding young to save energy during flight. The reserve‐mobilization hypothesis suggests that female breeders accumulate energy reserves during egg production and incubation and mobilize those reserves to meet their own energy needs after eggs hatch. Finally, birds may lose mass due to gonadal regression. From 1999 to 2001, we attracted Florida Scrub‐Jays (Aphelocoma coerulescens), sedentary cooperative breeders, to a portable electronic balance. Our objective was to determine which hypotheses might best explain mass variation during breeding. Both male and female Florida Scrub‐Jays lost mass during the period of nestling care (males, 3.2%; females, 6.5%), but not when feeding fledglings, despite this being the period of peak effort. Such results are consistent with both the flight‐efficiency and reserve‐mobilization (females only) hypotheses. We also found a significant negative influence of brood size on mass change in males, providing support for the reproductive‐stress hypothesis, and we conclude that, for males, both the flight‐efficiency and reproductive‐stress hypotheses apply. For female scrub‐jays, our results were consistent with the flight‐efficiency and energy‐reserve mobilization hypotheses, both of which view mass loss as beneficial.     

Energetics of reproduction in female and male greater snow geese

Arctic-nesting geese are classified as capital breeders (i.e., birds that rely largely on endogenous reserves to meet the high nutrient requirement of clutch formation) as opposed to income breeders (those that rely directly on ingested food). However, some evidence has suggested that energy reserves of greater snow geese (Chen caerulescens atlantica) are not sufficient to meet these costs. We tested whether greater snow geese were capital breeders and examined the consequences on their reproductive strategy. We collected 48 females and 47 males from arrival at the breeding colony on Bylot Island (NWT) to the postlaying stage, and determined fat and protein content in somatic and reproductive tissues. Rapid follicular growth was initiated only after arrival on the breeding ground. Somatic fat of females tended to increase during the prelaying and laying stages at the same time that most of the fat was deposited in developing follicles. Decrease in somatic fat in postlaying females was mostly related to its use for meeting metabolic requirements during early incubation. Hence, almost all fat invested in the clutch came from food intake, not endogenous reserves. Somatic protein was maintained during prelaying but decreased during laying, suggesting that some protein reserves were deposited in the eggs. There was no relationship between somatic fat and the number of developing follicles (incubating females excluded) but a relationship was found with somatic protein. Clutch size was not related to body size. In males, somatic fat, which was similar to females at arrival, was almost completely depleted by the postlaying stage. Male somatic protein remained stable. Male somatic reserves were not related to the somatic reserves, clutch size or nutrient investment in reproduction of their mate. We conclude that, in female greater snow geese, little fat reserves (if any) were used for egg formation though some protein reserves were used, whereas males relied heavily on their fat reserves during the same period. This is probably a consequence of the long and costly migration between the last staging area and the breeding grounds.     

BREEDING OF THE ADÉLIE PENGUIN PYGOSCELIS ADELIAE AT CAPE BIRD

Observations were made during four seasons (1967–68, 1968–69, 1969–70, 1970–71) on the breeding of Adelie Penguins at Cape Bird, Ross Island, Antarctica. Breeding data from individuals were related to date of return, laying date, clutch-size, nest location, and change of mate. Some females consistently laid near the mean date of laying, while others were consistently early or late layers. Laying date may be under direct genetic control, or may reflect feeding ability. The mean clutch-size was smaller in peripheral compared to central nests, and smallest of all in isolated nests. Clutches laid late in the season were smaller than those laid near the peak date, and small, late clutches were laid in peripheral rather than central nests. These differences may reflect age and/or feeding ability. Penguins that are better able to find food will return earlier, obtain central sites, and have larger clutches than those with a lesser ability. The two main causes of egg and chick losses were predation and parental failure. Losses were highest in single-egg clutches, at isolated and peripheral nests, and among eggs laid late in the season. These results may be partly related to the age and experience of the penguins. However, regardless of age, peripheral nesting and late laying were always disadvantageous. The sex ratio of adults in the colonies was 117♂:100♀. This may be explained by the higher mortality of females. Some males could not find partners, but females that did not breed had probably been unable to obtain sufficient food for gonad development. The return of penguins, incidence of non-breeding, adult mortality, clutch-size, and breeding success at Cape Bird were all markedly different in 1968–69 compared to the other three seasons studied. This season was marked by the persistence of sea-ice along the northwestern shores of Ross Island. The low reproductive output in 1968–69 was thought to result from a shortage of food for egg-laying and incubation.     

Haematocrit values of breeding Red-billed queleas Quelea quelea (Aves: Ploceidae) in relation to body condition and thymus activity

In a Red-billed quelea colony in north-eastern Nigeria that was later abandoned as a result of the poor nutritional condition of the adults, laying females showed low haematocrit values compared with normal pre-breeding haematocrit levels but nestbuilding males did not. In females the low haematocrits were correlated with extremely low levels of reserve lipid and labile protein but males in equally poor condition did not have such low haematocrits. This difference must result in some way from the additional nutritional burden for females of mobilizing sufficient protein to form eggs. Earlier observations of massive red blood cell production in the thymus of queleas during incubation suggest that low haematocrits, indicating anaemia, may be a normal occurrence in queleas during breeding, and were not peculiar to this abnormal abandoned colony.     

Changes in Calcium Reserves in Breeding Lesser Snow Geese (Chen caerulescens caerulescens)

Calcium and fat reserves of the femur medullary bone were examined in sexually mature lesser snow geese (Chen caerulescens caerulescens) collected during the 1974–1975 season. In females, femur calcium and fat levels increased by 80 and 30%, respectively, during the spring migration, much of the increase taking place while the birds staged in southwestern Manitoba and North Dakota prior to their departure for the breeding area. In males. femur calcium levels showed no seasonal change but femur fat increased in a manner similar to that found in the females, although the increase was not as great (17%). In the females, femur fat content fell by 40% during egg-laying whereas in males a decrease in femur lipid was not evident until incubation was well underway. Femur calcium levels in females declined during egg production and early incubation, showing a 56% decrease over spring migratory levels, indicating that dietary calcium intake was limited during the nesting period. However, the low femur calcium levels in birds collected during the spring were not significantly different from those of wintering birds, suggesting that no calcium deficiencies were apparent. Plasma calcium levels in males remained relatively constant throughout the year, although there was some elevation in May. Plasma calcium levels in the females increased almost threefold during egg laying and returned to pre-laying levels during incubation. Medullary bone was evident only in reproducing females and appeared during spring migration, concomitant with increased femur weight, fat and calcium content. Medullary bone degradation commenced during the first week of incubation and no medullary bone was in evidence by molt. Calcium reserves of medullary bone accounted for only 17.2% of the calcium required for eggshell production, suggesting that. at least during the laying period, the female must depend on some exogenous source, perhaps from grit or brackish water.     

Relationships between body composition, body size and alternative reproductive tactics in a lekking sandpiper, the Ruff (Philomachus pugnax)

1. The Ruff is a lekking sandpiper in which males have two genetically determined alternative mating strategies: 'satellite' or 'independent'. Satellite males are non-territorial, following groups of females on and off leks. Independent males attempt to establish territories on leks and can be either 'resident' territory holders, or non-territorial 'marginals'. The time budgets of the three behavioural tactics (resident, marginal and satellite) differ notably in time allocated to foraging, aggression and display activity. These differences have led to the suggestion that the benefits of size and energy stores may vary with mating tactic.
2. In this paper in vivo estimates of body composition (fat, fat-free mass) for breeding male Ruffs using total body electrical conductivity (TOBEC) are presented.
3. Satellite males have significantly shorter tarsi and wings than independent males.
4. After correcting for size, independent males are significantly heavier and fatter than satellites, and marginals in particular are heavier and fatter than both residents and satellites.
5. Estimates of energy expenditure during flight suggest that satellites may maintain reduced energy reserves to minimize flight costs, while the larger fat stores of independent males are consistent with the benefits of endurance as a territory holder.     

SEASONAL CHANGES IN BODY-WEIGHT OF OYSTERCATCHERS HAEMATOPUS OSTRALEGUS

The body-weights of Oystercatchers Haematopus ostralegus wintering in Morecambe Bay, north-west England, showed marked seasonal changes between late summer and late winter, with considerable differences apparent between adult and immature birds. An attempt is made to relate these changes to recorded seasonal variations in prey biomass and to the annual cycles of breeding, moult and migration of the Oystercatcher. The mean weight of females invariably exceeded the mean weight of males in samples collected on the same dates, regardless of age. Adults returned from northern breeding areas in very lean condition, with mean weights ranging from 526 g in males to 540 g in females. Mean weight then increased progressively, due mainly to fat deposition, to a peak in March (up to 662 g in males and 675 g in females) around the time of their main departures for breeding. Heaviest birds then exceeded 800 g. Birds migrating to Iceland in spring would need to be of above average weight in March to make the shortest crossing (850 km, 13 h), via Scotland, while Oystercatchers of 700 g and over could probably make a direct flight (1500 km, 25 h) from Morecambe Bay in favourable weather. Breeding weights of British Oystercatchers were similar to those of post-breeders returning to Morecambe Bay in late August. The mean weights of first-year Oystercatchers arriving in August were very low, 449 g in males and 478 g in females. Their weights, and those of second- and third-year immatures, then rose rapidly in autumn, with some fat deposition, and reached mean values ranging between 551 g (males) and 597 g (females) by November-December. Mean weight then fell by 10–17% from December to March returning close to or below the September levels, whereas adults gained a further 6% during these winter months. Summer and autumn weight gains, and the major moult of adults and older immatures, occurred when the biomass of their two staple mollusc preys, Mytilus edulis (mussel) and Cardium edule (cockle), was maximal. Winter loss in mean weight of immatures corresponded with declining prey biomass, suggesting either that they were less efficient than adults in coping with deteriorating winter food supplies, or that they had no need to accumulate further (premigratory) fat reserves. The autumnal increases in mean weight of immatures are interpreted as an adaptation for withstanding adverse feeding conditions in winter. The Oystercatcher appears to be the only wader species in Britain in which adults increase, rather than lose, weight during the winter. This may be a consequence of an early breeding season, but it may be regarded also as a measure of the success Oystercatchers have achieved by specializing on a difficult but plentiful prey source.     

BREEDING NUMBERS AND REPRODUCTIVE RATE OF EIDERS AT THE SANDS OF FORVIE NATIONAL NATURE RESERVE, SCOTLAND

The total population of Eiders at the Sands of Forvie National Nature Reserve, Aberdeenshire, increased from about 3000 birds during 1961–63 to about 4800 birds in 1964–70. At the same time the estimated number of breeding pairs increased from about 1200 in 1961–63, to about 1800 in 1965–68 and to 2000 in 1970. The mean size of completed clutches was 4.4; the number of eggs in a clutch showed a significant decline during the season from an average of 4.7 to 3.4 in most years. The overall hatching success was 63%, but there was a decline with season as a result of increased losses to predators. In 1964 and 1969 laying was retarded, mean clutch-size reduced, and the number of birds attempting to lay dropped by half, due to inclement weather just at the onset of laying. In 1969 the eggs produced were smaller than in other years, and hatching success was lower. The female incubates without feeding, and is required to store sufficient energy, in the form of fat, to last her through the 26 days in addition to producing her clutch of eggs. An hypothesis is advanced which relates the feeding efficiency of the paired female immediately prior to laying to her egg production and incubating pattern, and which offers an explanation of the decline in clutch-size and hatching success evident during the season. In most years less than 5% of hatchlings survived to fledge; high survival was recorded in 1963, 1968 and 1970. Following the high production in 1963 the population increased in 1964 and the breeding stock in 1965 (females start to breed at two years old). The corresponding changes, following good breeding in 1968, were not so clearly detected because of unusually high adult mortality from oil pollution at the winter grounds in 1968 and the ‘late’ breeding year in 1969. The breeding stock in 1970, however, was higher than in any other year. This population continues to grow apparently as a result of its own reproductive output checked only by low productivity in most years and occasional high adult mortality.     

SEXUAL AND SEASONAL, VARIATION IN CONDITION AND SURVIVAL OF SWEDISH GOSHAWKS ACCIPITER GENTILIS

Winglength was the most powerful intersex discriminator, giving 99.5% separation by itself and 100% combined with bodyweight or keel length. Females but not males, had longer wings and were heavier as adults than as juveniles. Pectoral fat and muscle indices explained 81–91% of variation in total fat and 93–96% of variation in total lean dry weight for male and female hawks but bodyweight change explained only 56–60% of variation in the fat index compared with 75–76% for pectoral lean dry weight. The best size index, keel length, explained only 6.4% of bodyweight variation in males and 11.2% in females. Winglength was very weakly correlated with keel length. Taking size difference into account, females were heavier with higher wingloading than males but had relatively lighter flight muscles and therefore a lower power availability relative to their flight requirements than in males. On the other hand, females had greater maximum fat stores (17% of bodyweight) than males (14%) and could have survived longer than males without food. These differences would be adaptive for the different role of each sex during breeding. The weight of first capture of live-trapped adults and juveniles increased between September and December, possibly because moulting (in adults) or hunting inexperience (in juveniles) kept weights low in autumn but also perhaps because the benefit of having large reserves for surviving, relative to the cost of decreased agility in flight because of increased wingloading, was greatest in mid-winter. Shot hawks weighed more than the live-trapped birds but showed the same trends, as did retrapped individuals. Females but not males, tended to lose weight in January before a further increase in the spring. Records of hawks retaken more than two months after first capture showed that between October and January survival in females but not males, was enhanced by high bodyweight. Since female weights were also significantly lower in southern Sweden than in central and northern areas, it is suggested that changing agriculture or other factors in the south may have reduced prey for females more than for males.     

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.