Is long-distance bird flight equivalent to a high-energy fast? Body composition changes in freely migrating and captive fasting great knots

We studied changes in body composition in great knots, Calidris tenuirostris, before and after a migratory flight of 5,400 km from northwest Australia to eastern China. We also took premigratory birds into captivity and fasted them down to their equivalent arrival mass after migration to compare organ changes and nutrient use in a low-energy-turnover fast with a high-energy-turnover fast (migratory flight). Migrated birds were as economical as any fasting animal measured yet at conserving protein: their estimated relative protein contribution (RPC) to the energy used was 4.0%. Fasted birds had an estimated RPC of 6.8% and, consequently, a much lower lean mass and higher fat content for an equivalent body mass than migrated birds. Lean tissue was catabolized from most organs in both groups, except the brain. Furthermore, a principal components biplot showed that individuals were grouped primarily on the basis of overall organ fat or lean tissue content rather than by the size of specific organs. This indicates that organ changes during migratory flight are similar to those of a low-energy fast, although the length of the fast in this study probably accentuated organ reductions in some functional groups. Whether the metabolic characteristics of a flying migratory fast follow the three-phase model described in many inactive fasting animals is unclear. We have some evidence for skeletal fat being catabolized without phase 3 of a fast having been reached.     

Transition between moult and migration in a long-distance migratory passerine: organ flexibility in the African wintering area

Phenotypic flexibility of organs in migratory birds has been documented for a variety of species of different genera during the migratory period. However, very little is known about phenotypic mass changes of organs with respect to other events within the annual cycle. This seems particularly interesting when birds face different physiological challenges in quick succession. We investigated mass changes of 13 organs from garden warblers (Sylvia borin) during the transition from moult to migration. These long-distance migratory birds perform a complete moult within their wintering area just shortly before the onset of spring migration. Birds were sampled in three successive stages according to their moult status: group I consisted of birds with growing primary or secondary wing feathers, group II consisted of birds with completed wing moult but with still moulting body feathers, and group III consisted of birds that had completed wing moult and body moult. Size-corrected flight muscle, kidney mass, and pancreas mass differed significantly among the three groups. Flight muscle was heaviest in birds that were about to leave their wintering area (group III) compared with birds still in body moult (group II). Kidney and pancreas showed a pattern similar to each other, with the heaviest mass occurring in birds with moulting wing feathers (group I) and significantly reduced mass in birds that had completed wing moult (group II) or both wing and body moult (group III). Mass reductions of kidney and pancreas during the transition from moult to migration are considered to be related to the demands of moult, while increased flight muscle may be due to moult, migration, or both. Phenotypic mass changes of organs in birds occur during their migration, but they also occur during the transition between other phases of the annual cycle such as moult and migration and are not restricted to the flight muscle.     

Female butterflies modulate investment in reproduction and flight in response to monsoon‐driven migrations

Migratory species may display striking phenotypic plasticity during individual lifetimes. This may include differential investment in body parts and functions, differential resource use and allocation, and behavioural changes between migratory and non‐migratory phases. While migration‐related phenotypic changes are well‐reported, their underlying mechanisms are usually poorly understood. Here we compare individuals from migratory (reproductive diapause) and non‐migratory (reproductive) phases of closely related aposematic butterfly species to study how sexual dimorphism and migratory behaviour underlie significant morphological tradeoffs, and propose a plausible scenario to explain the migration‐related phenotypic plasticity observed in females of migratory species. We found that female butterflies invested significantly more in their abdominal mass compared to males irrespective of their migratory phase, and underwent a clear shift in their body mass allocation after the switch from the reproductive diapause phase to the reproductive phase. In reproductive phase, females invested much more in reproductive tissue. This switch occurred as a result of increased abdominal mass (i.e. reproductive tissue mass) without significant reduction in the thoracic mass (i.e. flight muscle mass). Migratory males, however, were not significantly different from non‐migratory males in terms of relative investment in flight and reproductive tissues. These patterns were consistent between migratory and non‐migratory aposematic species within and across clades. While migratory habits may influence the physiology and behaviour of both sexes, long‐distance migration affected female morphology much more markedly compared to that of males. These results show the sex‐specific nature of adaptations to migratory behaviour, and suggest that seemingly disparate life‐history traits such as aposematism and migration may have similar influences on the lifetime energetic investments of insects.     

Empirical evidence for differential organ reductions during trans-oceanic bird flight

Since the early 1960s it has been held that migrating birds deposit and use only fat as fuel during migratory flight, with the non-fat portion of the body remaining homeostatic. Recent evidence from field studies has shown large changes in organ sizes in fuelling birds, and theory on fuel use suggests protein may be a necessary fuel during flight. However, an absence of information on the body condition of migrants before and after a long flight has hampered understanding of the dynamics of organs during sustained flight. We studied body condition in a medium-sized shorebird, the great knot (Calidris tenuirostris), before and after a flight of 5400 km from Australia to China during northward migration. Not only did these birds show the expected large reduction in fat content after migration, there was also a decrease in lean tissue mass, with significant decreases in seven organs. The reduction in functional components is reflected in a lowering of the basal metabolic rate by 42% [corrected]. Recent flight models have tried to separate the 'flexible' part of the body from the constant portion. Our results suggest that apart from brains and lungs no organs are homeostatic during long-distance flight. Such organ reductions may be a crucial adaptation for long-distance flight in birds.     

Flexible remodeling of organ size during spring migration of the garden warbler (Sylvia borin)

The energetic demands of long-distance migratory birds change drastically, depending on the stage of their life cycle. Changing demands are reflected in the up and down regulation of adipose tissue and organ mass. This paper presents new data on organ size changes during different stages of spring migration of garden warblers (Sylvia borin). Phenotypic mass changes were quantified in 13 organs of birds caught in Tanzania, Ethiopia and Egypt. We also sampled birds after a simulated stopover in Egypt. Some organs increased in mass up to about 1.5-fold during migration from Tanzania to Ethiopia, while some remained unchanged or even decreased in mass. During flight across the Sahara, nearly all organ masses including heart and flight muscles were reduced. Exceptionally large reductions (approximately 50%) were observed for liver, bile, spleen, kidney and digestive tract organs. The only exceptions were the testes, which increased 4-fold in mass. During the simulated stopover in Egypt, a significant recovery was observed for kidney, liver, heart, proventriculus, and small intestine. The testes continued to increase in mass. Flexible remodeling of organ size in the course of spring migration thus comprises significant changes for all quantified organs, with a variety of organ-specific patterns. Individual organ patterns are differentially shaped by functional aspects according to the different organ requirements in the alternation of flight and stopover phases, energetics, future demands, and protein requirements. Anticipatory mechanisms account for the size change of the testes, and we suggest the same for the kidney and the gall bladder.     

Magnetic cues and time of season affect fuel deposition in migratory thrush nightingales (Luscinia luscinia)

Bird migration requires high energy expenditure, and long-distance migrants accumulate fat for use as fuel during stopovers throughout their journey. Recent studies have shown that long-distance migratory birds, besides accumulating fat for use as fuel, also show adaptive phenotypic flexibility in several organs during migration. The migratory routes of many songbirds include stretches of sea and desert where fuelling is not possible. Large fuel loads increase flight costs and predation risk, therefore extensive fuelling should occur only immediately prior to crossing inhospitable zones. However, despite their crucial importance for the survival of migratory birds, both strategic refuelling decisions and variation in phenotypic flexibility during migration are not well understood. First-year thrush nightingales (Luscinia luscinia) caught in the early phase of the onset of autumn migration in southeast Sweden and exposed to a magnetic treatment simulating a migratory flight to northern Egypt increased more in fuel load than control birds. By contrast, birds trapped during the late phase of the onset of autumn migration accumulated a high fuel load irrespective of magnetic treatment. Furthermore, early birds increased less in flight-muscle size than birds trapped later in autumn. We suggest that the relative importance of endogenous and environmental factors in individual birds is affected by the time of season and by geographical area. When approaching a barrier, environmental cues may act irrespective of the endogenous time programme.     

迁徙鸟类中途停歇期的生理生态学研究

大多数候鸟的迁徙活动由迁徙飞行和中途停歇两个部分组成。在迁徙过程中,鸟类要多次交替经历消耗能量的飞行阶段和积累能量的中途停歇阶段。从鸟类在中途停歇时期的能量积累速度、体重变化模式以及迁徙飞行中的禁食或食物限制、食物种类的改变、中途停歇的能量快速积累过程对消化器官的影响等方面,对目前迁徙鸟类的生理生态学研究成果进行回顾,并提出有待解决的问题及今后的研究方向。     

Intra‐organ flexibility in the eared grebe Podiceps nigricollis stomach: a spandrel in the belly

Adjustments in body composition over the annual cycle have been documented in many organs and muscle groups. Here we consider the nature and significance of intra‐organ variation in the eared grebe Podiceps nigricollis stomach, a large and variable organ that can weigh > 30 g when birds are staging, drop to 8–11 g before setting off, or to as little as 6.6 g after a several‐day migration. Weight reduction in association with migration is conventionally regarded as an adaptation to reduce wing loading and flight costs. This interpretation applies to the premigratory reduction of the entire stomach. However, it does not fit the differential in‐flight reduction of the proventriculus, because grebes require a large proventriculus to initiate digestion, and its smaller size when they need to rebuild the entire stomach and resume feeding quickly is opposite that expected in a functional context. We view the reduction of the proventriculus as a non‐adaptive response, or spandrel, stemming from its intrinsically higher turnover rate. Starving birds, like migrants, also need to digest food quickly. In their case, the proventriculus is maintained as body weight declines and the gizzard is sacrificed. Mechanisms by which individual organisms achieve different responses to similar challenges, including starvation, merit further investigation.     

Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Birds during migration must satisfy the high energy and nutrient demands associated with repeated, intensive flight while often experiencing unpredictable variation in food supply and food quality. Solutions to such different challenges may often be physiologically incompatible. For example, increased food intake and gut size are primarily responsible for satisfying the high energy and nutrient demands associated with migration in birds. However, short-term fasting or food restriction during flight may cause partial atrophy of the gut that may limit utilization of ingested food energy and nutrients. We review the evidence available on the effects of long- and short-term changes in food quality and quantity on digestive performance in migratory birds, and the importance of digestive constraints in limiting the tempo of migration in birds. Another important physiological consequence of feeding in birds is the effect of diet on body composition dynamics during migration. Recent evidence suggests that birds utilize and replenish both protein and fat reserves during migration, and diet quality influences the rate of replenishment of both these reserves. We conclude that diet and phenotypic flexibility in both body composition and the digestive system of migratory birds are important in allowing birds to successfully overcome the often-conflicting physiological challenges of migration.     

Carry‐over effects and compensation: late arrival on non‐breeding grounds affects wing moult but not plumage or schedules of departing bar‐tailed godwits Limosa lapponica baueri

In the annual cycle of migratory birds, temporal and energetic constraints can lead to carry‐over effects, in which performance in one life history stage affects later stages. Bar‐tailed godwits Limosa lapponica baueri, which achieve remarkably high pre‐migratory fuel loads, undertake the longest non‐stop migratory flights yet recorded, and breed during brief high‐latitude summers, may be particularly vulnerable to persistent effects of disruptions to their rigidly‐timed annual routines. Using three years of non‐breeding data in New Zealand, we asked how arrival timing after a non‐stop flight from Alaska (>11 000 km) affected an individual godwit's performance in subsequent flight feather moult, contour feather moults, and migratory departure. Late arrival led to later wing moult, but godwits partially compensated for delayed moult initiation by increasing moult rate and decreasing the total duration of moult. Delays in arrival and wing moult up to 34–37 d had no apparent effect on an individual's migratory departure or extent of breeding plumage at departure, both of which were extraordinarily consistent between years. Thus, ‘errors’ in timing early in the non‐breeding season were essentially corrected in New Zealand prior to spring migration. Variation in migration timing also had no apparent effect on an individual's likelihood of returning the following season. The bar‐tailed godwits’ rigid maintenance of plumage and spring migration schedules, coupled with high annual survival, imply a surprising degree of flexibility to address unforeseen circumstances in the annual cycle.     

Partial migration in birds: tests of three hypotheses in a tropical lekking frugivore

1. Partially migratory species provide opportunities to understand which ecological factors cause some animals to migrate when others remain resident year round. Partial migration in birds has been explained by the dominance, arrival-time, and body-size hypotheses. 2. Testing these hypotheses has proven difficult due to the similarities of the predictions they make in temperate-breeding long-distance migrants. In tropical altitudinal migrants, however, these hypotheses make different predictions regarding the sex, age, and condition of migrants and residents. 3. Among white-ruffed manakins in Costa Rica, young birds were not more likely to migrate (as predicted by the dominance hypothesis), nor were females more likely to migrate (as predicted by the arrival-time hypothesis). All condition-related variables interacted with sex, together explaining much of the variation in migratory behaviour. 4. I re-articulate the body-size hypothesis in the context of tropical altitudinal bird migration, focusing explicitly on how limited foraging opportunities and differences in individual condition affect fasting ability during torrential rains. Despite ample food, the smallest birds or those stressed by parasites or moult may risk starvation at breeding elevations due to a reduction in foraging time. These results highlight how intrinsic and extrinsic factors may interact to produce observed patterns of within- and among-species variation in migratory behaviour.     

Energy reserves stored by migrating Gray‐cheeked Thrushes Catharus minimus at a spring stopover site in northern Colombia are sufficient for a long‐distance flight to North America

Stopover sites used to accumulate the energy that fuels migration, especially those used prior to crossing ecological barriers, are regarded as critically important for the survival of Nearctic?Neotropical migratory birds. To assess whether South American stopover sites are used to store the energy required to cross the Caribbean Sea and the Gulf of Mexico to North America by a Neotropical migratory landbird, we studied Gray‐cheeked Thrushes in northern Colombia through constant effort mist‐netting during spring migration in 2010 and 2011. We combined stopover duration estimates and models of body mass change based on recaptures to estimate departure body mass and potential flight range from our study site. We recaptured 62 birds, the majority of which gained mass. Models indicated significant differences in rates of mass gain between years and age groups and with arrival date. Estimated total stopover durations varied between 15.4 (2010) and 12.5 days (2011). Predicted departure mass ranged between 41.3 and 44.9 g, and potential flight range was estimated at between 2727 and 4270 km. Gray‐cheeked Thrushes therefore departed our study site with sufficient energy reserves to cross the Caribbean Sea and the Gulf of Mexico (2550 km). As the first demonstration that birds departing from South American stopover sites can reach North America without refuelling, this has important implications for stopover site protection. Strategic conservation measures in the Sierra Nevada de Santa Marta could protect habitats in which up to 40% of the energy required to complete spring migration is stored by a Neotropical migratory land bird.     

Body mass variations in disturbed mallards Anas platyrhynchos fit to the mass‐dependent starvation‐predation risk trade‐off

For passerines the starvation‐predation risk theory predicts that birds should decrease their body mass to improve escape flight performance, when predation pressure increases. To investigate whether this theory may apply to large birds, which manage body reserves differently from small passerines, we experimentally increased the predation risk in mallards Anas platyrhynchos. Two groups were disturbed at different frequencies during experimental sessions lasting one week, while a control group was left undisturbed. We found that body mass loss and final wing loading were similar in both disturbed groups and significantly differed from the control group. Food intake in disturbed groups was reduced up to day four of the disturbance session and was lower than in the control group. Altogether our results suggest that disturbed mallards may adjust their body mass to reach a more favorable wing loading, supposedly to improve escape flight performance. Nevertheless, body mass loss in our mallards was double than what has been observed in passerines. This greater mass decrease might be explained by different strategies concerning energy storage. Furthermore, in large birds the predation component of the starvation‐predation trade‐off might be of greater importance. Hence, the observed relevance of this trade‐off over a large size range suggests that the starvation‐predation risk theory is of major ecological significance for many animal species.     

The effects of dietary macronutrients on flight ability,energetics, and fuel metabolism of yellow‐rumped warblers Setophaga coronata

The catabolism of protein from organs and muscles during migratory flight is necessary to produce glucose, key metabolic intermediates, and water, but may have negative effects on flight range and refueling at stopovers. We tested the hypothesis, suggested by previous studies, that birds that eat high‐protein insect diets use more protein for fuel in flight than those that eat high‐carbohydrate fruits. First, we fed migratory yellow‐rumped warblers synthetic fruit or mixed insect/fruit diets, and measured metabolic rates and fuel mixture under basal conditions and during exercise in a hop/hover wheel respirometer. Birds eating the fruit diet had greater plasma triglyceride and non‐esterified fatty acid concentrations, and the higher protein mixed diet increased plasma uric acid only during feeding. Diet did not affect metabolic rates or the fuel mixture under resting or exercise conditions. We then fed yellow‐rumped warblers synthetic diets that differed only in the relative proportion of carbohydrate and protein (60:15 versus 15:60 as % dry mass) and tested them in wind tunnel flights lasting up to six hours. Birds fed the high carbohydrate diet became heavier and fatter than when fed the high protein diet. Plasma uric acid concentration was increased and plasma phospholipid concentration was decreased by the high protein diet in the pre‐flight state (after a 3 h fast), but diet only affected plasma phospholipids during flight (lower in high protein birds). Neither diet nor amount of body fat affected the rate of loss of lean mass or fat during flight. Inter‐individual or seasonal differences in diet do not appear to influence the amount of protein catabolized during endurance flight. However, birds fed the high carbohydrate diet had greater voluntary flight duration, independent of body fatness, suggesting that there may be other performance benefits of high carbohydrate diets for migratory birds.     

Phenotypic flexibility of a southern African duck Alopochen aegyptiaca during moult: do northern hemisphere paradigms apply?

Phenotypic flexibility during moult has never been explored in austral nomadic ducks. We investigated whether the body condition, organ (pectoral muscle, gizzard, liver and heart) mass and flight‐feather growth Egyptian geese Alopochen aegyptiaca in southern Africa show phenotypic flexibility over their 53‐day period of flightless moult. Changes in body mass and condition were examined in Egyptian geese caught at Barberspan and Strandfontein in South Africa. Mean daily change in primary feather length was calculated for moulting geese and birds were dissected for pectoral muscle and internal organ assessment. Mean body mass and condition varied significantly during moult. Body mass and condition started to decrease soon after flight feathers were dropped and continued to do so until the new feathers were at least two‐thirds grown, after which birds started to regain body mass and condition. Non‐moulting geese had large pectoral muscles, accounting for at least 26% of total body mass. Once moult started, pectoral muscle mass decreased and continued to do so until the flight feathers were at least one‐third grown, after which pectoral muscle mass started to increase. The regeneration of pectoral muscles during moult started before birds started to gain overall body mass. Gizzard mass started to increase soon after the onset of moult, reaching a maximum when the flight feathers were two‐thirds grown, after which gizzard mass again decreased. Liver mass increased significantly as moult progressed, but heart mass remained constant throughout moult. Flight feather growth was initially rapid, but slowed towards the completion of moult. Our results show that Egyptian geese exhibit a significant level of phenotypic flexibility when they moult. We interpret the phenotypic changes that we observed as an adaptive strategy to minimize the duration of the flightless period. Moulting Egyptian geese in South Africa undergo more substantial phenotypic changes than those reported for ducks in the northern hemisphere.     

Wing size but not wing shape is related to migratory behavior in a soaring bird

Both wing size and wing shape affect the flight abilities of birds. Intra and inter‐specific studies have revealed a pattern where high aspect ratio and low wing loading favour migratory behaviour. This, however, have not been studied in soaring migrants. We assessed the relationship between the wing size and shape and the characteristics of the migratory habits of the turkey vulture Cathartes aura, an obligate soaring migrant. We compared wing size and shape with migration strategy among three fully migratory, one partially migratory and one non‐migratory (resident) population distributed across the American continent. We calculated the aspect ratio and wing loading using wing tracings to characterize the wing morphology. We used satellite‐tracking data from the migratory populations to calculate distance, duration, speed and altitude during migration. Wing loading, but not aspect ratio, differed among the populations, segregating the resident population from the completely migratory ones. Unlike what has been reported in species using flapping flight during migration, the migratory flight parameters of turkey vultures were not related to the aspect ratio. By contrast, wing loading was related to most flight parameters. Birds with lower wing loading flew farther, faster, and higher during their longer journeys. Our results suggest that wing morphology in this soaring species enables lower‐cost flight, through low wing‐loading, and that differences in the relative sizes of wings may increase extra savings during migration. The possibility that wing shape is influenced by foraging as well as migratory flight is discussed. We conclude that flight efficiency may be improved through different morphological adaptations in birds with different flight mechanisms.     

Light enough to travel or wise enough to stay? Brain size evolution and migratory behavior in birds

Brain size relative to body size is smaller in migratory than in nonmigratory birds. Two mutually nonexclusive hypotheses had been proposed to explain this association. On the one hand, the “energetic trade‐off hypothesis” claims that migratory species were selected to have smaller brains because of the interplay between neural tissue volume and migratory flight. On the other hand, the “behavioral flexibility hypothesis” argues that resident species are selected to have higher cognitive capacities, and therefore larger brains, to enable survival in harsh winters, or to deal with environmental seasonality. Here, I test the validity and setting of these two hypotheses using 1466 globally distributed bird species. First, I show that the negative association between migration distance and relative brain size is very robust across species and phylogeny. Second, I provide strong support for the energetic trade‐off hypothesis, by showing the validity of the trade‐off among long‐distance migratory species alone. Third, using resident and short‐distance migratory species, I demonstrate that environmental harshness is associated with enlarged relative brain size, therefore arguably better cognition. My study provides the strongest comparative support to date for both the energetic trade‐off and the behavioral flexibility hypotheses, and highlights that both mechanisms contribute to brain size evolution, but on different ends of the migratory spectrum.     

Mixed patterns of morphological adaptation to insularity in an aerial displaying bird,the Common Snipe Gallinago gallinago

On islands, colonizing birds may evolve behavioural and morphological adaptations to the new environment, often resulting in changes in body size and reduction or even total loss of flight. These island populations have therefore been used to test hypotheses related to adaptations for flight. However, in certain species in which flight is used not only in foraging and migration but also in mating displays, disentangling the effects of natural and social selection is difficult. Thus, sedentary populations of species that perform aerial displays (such as the Common Snipe Gallinago gallinago that breed in the Azores archipelago) may offer an opportunity to separate the effects of natural and social selection on morphology. If insular Common Snipe respond to the characteristic ecological context of oceanic islands, we expect them to differ from migratory conspecifics in body size and by having relatively smaller and more rounded wings. On the other hand, if social selection exerts a more powerful force over the morphology of this species, we expect that sedentary and migratory birds will not differ in flight‐related characters. We tested these hypotheses by comparing morphological characters measured on live Common Snipe captured in the Azores during the breeding season with those measured on migratory specimens hunted during autumn/winter in mainland Portugal. Sedentary Azorean birds were smaller and had relatively shorter tails but did not show the tendency for insular birds to possess more rounded wings as described in other taxa, including in the Azores. Bergman's rule might explain the difference in body size and shorter tails may be responsible for behavioural differences between populations. The lack of difference in wing shape might be explained by the need of the Common Snipe to perform aerial displays during courtship, suggesting an effect of social selection on the migratory strategy of this species.     

Navigating north: how body mass and winds shape avian flight behaviours across a North American migratory flyway

The migratory patterns of birds have been the focus of ecologists for millennia. What behavioural traits underlie these remarkably consistent movements? Addressing this question is central to advancing our understanding of migratory flight strategies and requires the integration of information across levels of biological organisation, e.g. species to communities. Here, we combine species‐specific observations from the eBird citizen‐science database with observations aggregated from weather surveillance radars during spring migration in central North America. Our results confirm a core prediction of migration theory at an unprecedented national scale: body mass predicts variation in flight strategies across latitudes, with larger‐bodied species flying faster and compensating more for wind drift. We also find evidence that migrants travelling northward earlier in the spring increasingly compensate for wind drift at higher latitudes. This integration of information across biological scales provides new insight into patterns and determinants of broad‐scale flight strategies of migratory birds.     

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.