Seasonal pattern in age, sex and body condition of Barn Owls Tyto alba killed on motorways

The Barn Owl Tyto alba was the most common owl killed on motorways in northeastern France. The possible causes of this mortality and the age, sex and body condition of the road-killed birds in 1991–1994 have been investigated. The number of birds killed on roads was highest in the period from early autumn to late winter, i.e. during the non-breeding period, and showed a pattern similar to that of the temporal difference between sunset, which varies with day length, and peak of traffic, the occurrence of which is constant throughout the year. An autumnal mortality peak, concomitant with the post-fledging dispersal, was mainly of immature birds, especially females. A second mortality peak in late winter was composed mainly of mature birds, with an equal proportion of males and females. From autumn to winter, there was no significant change in body mass in the different age and sex categories of birds killed on roads, except for mature males which had a significantly lower body mass in winter. From early autumn to late winter, the mean body mass of immature owls killed on motorways did not differ significantly from that of captive immatures fed ad libitum. This suggests that the immature birds were in good body condition. In contrast, the body mass of road-killed mature females was significantly lower than that of captive mature females over the same time periods. In mature males in late winter, a drop in body mass in both road-killed and captive birds suggests an endogenous seasonal phenomenon. Except for mature females, Barn Owls killed on roads in 1991–1994 were in good body condition. This does not support the idea that only birds in poor body condition were killed. We conclude that the mortality of Barn Owls on motorways in autumn and winter was probably related to the concomitance between the peak of traffic and the onset of hunting activity and the large number and dispersal of immature individuals during the same period.     

The winter fattening model: a test at low latitude using the Clamorous Reed Warbler

Small birds at high latitudes accumulate fat during the day so that they can survive long and cold winter nights. The winter fattening model suggests that birds increase their minimum (morning) mass in cold weather, build up their mass during the day, and then rely on the energy reserves so accumulated until the morning. While data from mid and high latitudes support this model, little is known about the strategies of birds inhabiting lower latitudes (< 40°N). We use an 18-year data set to investigate whether the winter fattening model holds in a mid-latitude (32°N) population of the Clamorous Reed Warbler Acrocephalus stentoreus . We show that morning mass and fat score vary as predicted by the winter fattening model. In addition, adults appear to be better adapted to long and cold winter nights than first-year birds. A long-term trend of increased body mass may be attributable to character release following reduced interspecific competition.     

Does avian winter fat storage integrate temperature and resource conditions? A long‐term study

Theoretical models of short‐term avian behaviour suggest that small birds adaptively balance the ecological costs and benefits of winter fat to maximize survival probability. When low starvation risk eliminates benefit but not cost of fat, birds are leaner than when under high starvation risk. Most models focus on single factors affecting starvation risk and subsequent choice of adaptive body mass; however, in complex environments, more than one factor affects starvation risk. To test for multiple interacting factors affecting fat reserves, long‐term geographical data on winter fat in a ground‐feeding finch, the dark‐eyed junco Junco hyemalis were analyzed. Two measures of fat were used: (1) visible subcutaneous fat class, and (2) body mass residuals left after age, sex and wing length effects were factored out. Site means for fat measures were obtained from juncos visiting supplemental feeding sites in midwest and northwest North America. In backward elimination regression of fat class, the temperature‐snowfall interaction term and its constituent variables, proximate temperature (averaged over capture day and the preceding ten days) and snowfall (frequency over the same time interval) were significant explanators of variation. Snowfall frequency is considered to be a surrogate measure of resource deterioration. The interaction term, also found in backward regression of body mass residuals, showed that as temperature declined at low snowfall frequency, less fat was deposited than when temperature declined at high snowfall frequency. Thus, in a recently cold environment suggesting relatively high resource predictability, perceived starvation risk is low, and less costly fat is needed to reduce starvation risk compared to a cold and unpredictable resource environment. The analysis of mass residuals also yielded a significant effect of daylength, suggesting an underlying fattening programme independent of proximate environmental conditions. A longitudinal study of junco fat stores indicated that individual environmental responses contributed significantly to midwinter fat peaks. These results agree with predictions of a synergistic model of adaptive fat regulation in small birds by suggesting that a ground‐feeding bird may maximize winter survival probability by integrating multiple environmental factors affecting starvation risk.     

Seasonal body mass changes in Eurasian Golden Plovers Pluvialis apricaria staging in the Netherlands: decline in late autumn mass peak correlates with increase in raptor numbers

Eurasian Golden Plovers Pluvialis apricaria staging in the Netherlands during the non-breeding season show strikingly constant seasonal changes in body mass with a first mass peak in late November and December and a second peak in late April and May. Despite huge sample sizes, variations in this pattern over successive years in the 1990s and among age classes were minuscule. However, in contrast to the body mass levels at other times of the year, there was a marked decline in the winter peak mass of Golden Plovers from the 1970s/early 1980s to 1989–2000. The decrease, by an average of 29 g, was about half the extra mass previously stored in autumn. This additional mass is known to consist of fat and may be interpreted as an energy store − insurance − for sudden cold spells when a negative energy balance forces the birds to move south and stay in front of the frostline. As the rate of the mass increase in September–October showed no change from the 1980s to the 1990s, changes in food availability are unlikely to explain the long-term mass decline. Also, there were no differences in two factors known to influence energy expenditure and feeding rate, air temperature and rainfall. The one striking environmental change relevant to plovers was the steep increase in the occurrence of raptors in the northern Netherlands in the 1980s, notably Peregrine Falcons Falco peregrinus and Goshawks Accipter gentilis . We argue that the halving of the winter mass peak over a decade is consistent with the hypothesis that under increased risk of predation, birds lower their body mass in order to reduce individual vulnerability, a reduction that may be traded off against an increased risk of starvation.     

The effects of temperature, pH and water hardness on winter starvation of young-of-the-year smallmouth bass, Micropterus dolomieui Lacepede

Year-class strength in northern populations of smallmouth bass is strongly influenced by winter starvation of young-of-the-year. We examined starvation among young bass under both winter and summer light and temperature conditions. During starvation, body condition declines to a specific level and then the fish dies. Body condition at death is a well defined function of body size that remains relatively constant over a wide range of environmental conditions. Starvation rate varies systematically with body size, temperature, pH and water hardness. Available stored energy increases more rapidly with body size than starvation rate. Therefore, lifetime under starvation conditions tends to increase with increasing body size. The Q₁₀ for starvation rate over the temperature range 2.5-8° C is 2.2. Starvation rate increases as pH declines from 7.0-4.9: the rate at pH 4.9 is ∼ 1.25 times the rate at pH 7 Starvation rate decreases as Ca concentration increases from 1 mgl⁻¹ to 80mgl⁻¹: the rate at 80 mg Ca 1⁻¹ is ∼0.80 times the rate at 1 mgl⁻¹.     

Mass-dependent predation risk and lethal dolphin-porpoise interactions

In small birds, mass-dependent predation risk (MDPR) is known to make the trade-off between avoiding starvation and avoiding predation dependent on individual mass. This occurs because carrying increased fat reserves not only reduces starvation risk but also results in a higher predation risk due to reduced escape flight performance and/or the increased foraging exposure needed to maintain a higher body mass. In principle, the theory of MDPR could also apply to any animal capable of storing energy reserves to reduce starvation and whose escape performance decreases with increasing mass. We used a unique situation along certain parts of coastal Britain, where harbour porpoises (Phocoena phocoena) are pursued and killed but crucially not eaten by bottlenose dolphins (Tursiops truncatus), to investigate whether a MDPR effect can occur in non-avian species. We show that where high levels of dolphin 'predation' occur, porpoises carry significantly less energy reserves than would otherwise be expected and this equates to reducing by approximately 37% the length of time that a porpoise could survive without feeding. These results provide the first evidence that a mass-dependent starvation-predation risk trade-off may be a general ecological principle that can apply to widely different animal types rather than, as is currently thought, only to birds.     

Assessing the role of body mass and sex on apparent adult survival in polygynous passerines: a case study of cetti's warblers in central Portugal

Adult survival, an important fitness component, is usually 1) lower in lighter individuals due to their reduced ability to survive winter conditions compared to heavier ones, especially in resident species at northern temperate latitudes and 2) lower in females compared with males due to higher reproductive costs incurred by females. In this paper, a capture–mark–recapture dataset of 649 cetti's warblers Cettia cetti ringed seasonally at two wetlands in central Portugal over an 11‐yr period (2000–2010) was modelled in a multi‐state framework to examine the influence of these individual covariates on apparent adult survival, while controlling for the presence of transient individuals in our study area. The probability of change in mass state (ψ_{Light→Heavy}, ψ_{Heavy→Light}) during the annual cycle was also estimated. Overall, birds survived better during spring–summer (breeding/moulting periods) compared with autumn–winter, but there was no effect of body mass on apparent adult survival probability. The modelling detected a significant interaction between sex and season, in which resident females survived better than resident males in spring–summer (?_RF= 0.857 ± 0.117 and ?_RM= 0.698 ± 0.181) while the opposite pattern was found in autumn–winter (?_RM= 0.440 ± 0.086 and ?_RF= 0.339 ± 0.084). In addition, cetti's warblers had a tendency to lose mass in spring–summer (ψ_{Heavy → Light}= 0.560 ± 0.063) and to regain mass in autumn–winter (ψ_{Light→Heavy}= 0.701 ± 0.069). This pattern of body mass change during the annual cycle may reflect energetic costs to reproduction and moulting, and/or a response to increased starvation risk during winter. High body mass, however, did not increase adult survival in this population presumably due to the relatively mild winter weather prevailing in central Portugal. Survival estimates are more likely to be explained by important ecological and behavioural differences between the two sexes in polygynous passerines. Our results highlight that studies aiming to identify the main factors shaping survival and individual fitness in polygynous species should be conducted during different phases of their annual cycle.     

Sex, diet and feeding method-related differences in body condition in the Oystercatcher Haematopus ostralegus

Measurements were made of Oystercatcher corpses to find the most reliable measure of body condition in live birds. Total body weight was as accurate as muscle thickness in assessing the weight of the pectoral muscles, a major store of protein reserves. Total body weight was also a good indicator of fat reserves. Wing‐length was the most reliable measure of body size. Body condition in live birds was measured as an individual's residual weight derived from a general linear model of (loge) body weight against age, month, (loge) wing‐length and time since capture. On the Exe estuary in autumn, there was no significant difference in body condition between same‐age Oystercatchers of different diets and feeding methods. In the winter months, however, body condition differed significantly between mussel feeders using different feeding methods, and between mussel feeders and birds that fed on worms and clams. Differences between diet/feeding methods were not the same for each age group. Birds with the highest mean body condition index in adults were mussel hammerers, in immatures mussel stabbers and in juveniles, worm/clam feeders. No significant difference in body condition was found between the sexes in any age group in autumn. In winter, when birds with the same diet/feeding method were considered, females were significantly heavier than males. However, when birds of all diets and feeding methods were combined, weight differences between the sexes were not apparent. We examine reasons why birds with certain diets and feeding methods may be heavier than others in winter, and why females are sometimes heavier than males. We conclude that, in most cases, it is due to differences between birds in their ability to achieve a common target weight. However, we conclude that adult females try to achieve a higher target weight than males, probably as a premigratory and prebreeding strategy. In the light of this, we consider the consequences for adult females, in terms of reproductive success and survival, of specializing in diets and feeding methods associated with lower body weights in winter.     

Rate of energy depletion and overwintering mortality of juvenile walleye pollock in cold water

The winter energy deficit and mortality of juvenile walleye pollock at extremely cold temperature were examined by field observations and laboratory experiments. In the Doto area, along the northern coast of Japan, juvenile walleye pollock resided on the continental shelf despite extremely cold temperatures (mean 0·4° C) during the latter half of winter (March to April). Measurements of the rate of energy depletion (equivalent to the routine metabolic rate) revealed that juvenile walleye pollock consumed 37% less energy at 0·5° C than at 2·0° C, suggesting an energetic benefit of residence in cold water (<1·0° C) over the shelf during winter. Prior to the starvation experiments, temperatures and ration level in the holding tanks were adjusted to create two different body condition groups of fish. Under the thermal condition of the latter half of winter (0·5° C), fish with a mean condition factor of 0·6 and 0·5 suffered 19·1 and 74·5% mortality, respectively, at the end of the experiments (after 56 days). The residual analysis of total body energy demonstrated that the cause of mortality was mainly associated with the depletion of energy reserves. When a logistic regression model for mortality derived from the experiments was applied to wild fish collected in March, the estimated overwintering mortality in 2004 and 2005 was 25·4 and <2·3%, respectively, assuming no feeding during the winter. Considering that juvenile walleye pollock feed during winter as shown in previous studies, intense overwintering mortality induced by energy depletion is improbable during the latter half of winter in the Doto area.     

Changes in supercooling with body size, sex, and season in the long-lived milliped Polyzonium germanicum (Diplopoda, Polyzonidae)

The supercooling points (SCPs) of a field population of Polyzonium germanicum were measured at different stages in the life cycle. There was no post-freezing survival under the experimental conditions used. The mean SCP was -26.2 ° C for eggs and increased to -18.2 ° C for juveniles of stadium III experiencing their first winter. From stadium V onwards, SCP varied considerably among individuals, ranging from -22.7 to -4.7 ° C. SCP was shown to increase significantly with body size in both sexes. Seasonal changes also occurred in males, SCP being lower in autumn-winter than in spring-summer. Measurements from a sample starved in the laboratory showed that the winter decrease could result from starvation affecting a proportion of males in the field. There was no relationship between the SCP and relative water content of individuals. After starvation in the laboratory, however, there was a relationship between the SCP and relative fat content of males. The results show that freezing is not a significant threat to the field population studied, with the possible exception of the largest overwintering females.     

Starvation Resistance in a Stenothermal Species from the Indian Subcontinent: Mechanistic Basis of Clinal Variation

Parallel clines for starvation resistance and lipid content are well documented among drosophilids on the Indian subcontinent. However, the mechanistic basis of these clines has not been investigated so far. Here, we investigate the utilization of lipids during starvation as a function of duration of stress in D. ananassae. We found higher lipid content responsible for high starvation resistance at lower latitudes. Lipids were utilized during starvation only; not during any other climatic stresses like desiccation or thermal stresses. We also found a cline for consumption of total body lipids; as more content (out of total amount of lipids) was utilized by flies at lower latitudes and lesser at higher latitudes. But, there was no latitudinal cline for threshold lipid amount in the case of females while for males there was a positive cline. Lastly, parallel clines have evolved under contrasting climatic conditions i.e. drier and colder northern localities have flies with lower lipid and reduced starvation resistance while hot and humid localities favor flies with higher lipid levels and greater starvation tolerance. Thus, the evolution of clines associated with starvation and lipid content might have resulted due to specific ecological conditions i.e. humidity gradient on the Indian subcontinent.     

A tradeoff between perceived predation risk and energy conservation revealed by an immune challenge experiment

Energy is typically a limiting factor for animals during boreal winters, when low temperatures increase the cost of thermoregulation at the same times as short day‐lengths and snow cover constrain foraging opportunities. Under these circumstances animals use a suite of behavioural and physiological adaptations to avoid overnight starvation. However, it is poorly understood how such strategies are affected by increased energy demands from other physiological systems. Thus, we used free‐ranging blue tits, Cyanistes caeruleus, to test if competing demands for energy (here induced by a non‐inflammatory, antibody‐mediated immune challenge) would affect nocturnal body temperature (a predictor of energy expenditure in small animals) and energy‐saving nest box roosting behaviour. We also assessed if the immune challenge incurred long‐term survival costs. We found no evidence that body temperature regulation differed between immune‐challenged and saline‐injected birds. Nor did the immune challenge reduce survival to the next breeding season. However, old (second winter or older) immune‐challenged birds continued roosting in nest boxes to a larger extent at the peak immune response, despite increased perceived predation risk induced by the preceding capture and immunization. In contrast, old control birds were less prone to roost in nest boxes after capture and saline injection. This difference was less pronounced in young (first winter) birds. We interpret the increased risk‐taking behaviour in immune‐challenged birds as a consequence of a higher need for exploiting the thermal benefits of nest box roosting to reduce energy loss. This suggests that resource deficiency might be a stronger predictor of overnight survival than the threat of nocturnal predation in this system. As such, our study provides insights into the classic tradeoff between starvation and predation risk, in suggesting that priority is given to minimizing the risk of starvation in situations where both starvation and predation risks increase during cold winter nights.     

Comparative energetics and physiology of parental care in smallmouth bass Micropterus dolomieu across a latitudinal gradient

The energetic and physiological status of parental smallmouth bass Micropterus dolomieu was investigated across the majority of their latitudinal range at the onset and near the end of care. Variables such as tissue lipid stores, plasma indicators of nutritional status and chronic stress and white muscle were used to define energetic and physiological status. Results showed that northern males (48° N) were larger and heavier than mid-northern (44° N) and southern (36° N) latitude males. For a given body size, northern males had greater whole-body lipid across the parental care period and tended to feed more (based on gut contents) than mid-northern and southern latitude conspecifics. Indicators of nutritional status were also highest in northern males. Conversely, the southern males exhibited the greatest capacity for biosynthesis across the entire parental care period as indicated by the highest level of nucleoside diphosphate kinase activities. Collectively, these finding suggest that the energetic costs and physiological consequences of care vary across latitudes, providing some of the first mechanistic evidence of how environmental conditions can influence both the ecological and physiological costs of reproduction for wild animals during parental care. The data also suggest that lake-specific processes that can vary independently of latitude may be important, necessitating additional research on fish reproductive physiology across landscapes.     

Trichomonas gallinae in wintering Common Wood Pigeons Columba palumbus in Spain

We studied the prevalence and potential subclinical effects of infestation by Trichomonas gallinae in 91 hunter-harvested Common Wood Pigeons Columba palumbus from northern ( n  = 30) and southern ( n  = 61) Spain during the winter period. All animals were measured, sexed, aged, necropsied and their organs were weighed. Infestation with T. gallinae was diagnosed using three different methods: direct inspection for the presence of lesions, direct microscopic observation and culture. Of the sampled birds, 34.2% were positive for the presence of T. gallinae . Prevalence was significantly higher in adult Wood Pigeons than in the juvenile group, and prevalence was significantly lower in birds sampled from the north. No significant differences in prevalence were found between males and females. Culture was significantly more sensitive than the other methods of diagnosis. Parasitized birds were in poorer body condition, as revealed by their lower body mass and fat reserves. No significant variation could be detected in heart or spleen weight between parasitized and healthy birds. However, juvenile Wood Pigeons in which T. gallinae was detected had a significantly larger bursa of Fabricius. Variations in the prevalence of T. gallinae in Wood Pigeons could be related to migration as well as increased exposure through shared feed and water where these are artificially provided. We also discuss the potential effect of T. gallinae on body condition and the eventual risk for endangered predators through increased exposure to infected prey.     

Femur-Marrow Fat of White-Tailed Deer Fawns Killed by Wolves

Abstract: I present marrow fat (MF) data from a large sample of white-tailed deer fawns killed by wolves and a sample of fawns that died by accident in a single area, and I use these data to explore the extent that poor nutritional condition may have predisposed fawns to wolf predation. Percent MF of 110 5-10-month-old white-tailed deer (Odocoileus virginianus) fawns killed by wolves (Canis lupus) from November through April 1984-2002 in northeastern Minnesota, USA, was lower than MF for 23 fawns killed by accidents in the same area and period. The MF of both male and female wolf-killed fawns decreased over winter. The MF of male fawns decreased as a snow-depth index increased, but MF of females showed little relationship to the snow-depth index and was higher than that of males. Poor nutritional condition is one factor that predisposes deer fawns to wolf predation during winter and spring. This information expands our knowledge of wolf-prey relations by documenting that, even with younger prey animals that might be thought vulnerable because of youth alone, poor nutritional condition also is an important factor predisposing them to wolf predation.     

Breeding and movements of goshawks in boreal forests in Sweden

Goshawks were studied in their boreal forest breeding habitat in central Sweden from 1977 to 1982. Nests were regularly spaced, with a nearest neighbour distance of 6.3 km. corresponding to a density of about 3 pairs 100 km⁻². Of initiated breedings. 68% were successful. There was no difference in breeding performance between years. Average brood size was 2.7. Sex ratio among first captures of adults was even, but males were recaptured more often than females. Nearly all juveniles left the area in winter. Among radio-tagged adults 20% of the males, and 50% of the females moved out of the area. When moving, both adults and juveniles moved into agricultural areas, the juveniles generally covering longer distances. From autumn to spring, body condition of females improved, whereas that of the males deteriorated. It is argued that the males' stronger tendency to stay in winter is due to the advantage of possessing a territory in early spring. Females, on the other hand, can move to areas with as good food conditions as possible.     

Age- and sex-dependent spring arrival dates of Eastern Kingbirds

ABSTRACT.   Among migratory passerines, the first birds to arrive on the breeding grounds are usually older males. Early arrival by older birds may be driven by experience, age-dependent changes in body condition, age-dependent access to resources during the nonbreeding period, or latitudinal segregation by age. Males may arrive earlier than females (protandry) because males maintain better condition due to greater access to resources during the winter, or because selection favors early arriving males that acquire the best territories or experience enhanced mating opportunities. During a 4-yr study (2004–2007) in Oregon, we found that older Eastern Kingbirds ( Tyrannus tyrannus ), regardless of sex, arrived nearly a week before younger birds and that males arrived about 5 d before females. Age- and sex-dependent arrival dates do not appear to be related to differences in body condition, social dominance in winter, or latitudinal segregation, and protandry is unrelated to the ability of early-arriving males to acquire high-quality territories. Instead, we propose that young birds have less to gain from early arrival because of their probable inability to displace experienced birds from prime territories and that protandry evolved due to enhanced mating opportunities for early arriving males that arise from the high rates of extra-pair matings in our population of Eastern Kingbirds.     

Different responses to cold weather in two pied flycatcher populations

We compared how breeding parameters differ according to prevailing weather conditions between a marginal, subarctic (69°N) and temperate (61°N) population of the pied flycatcher Ficedula hypoleuca, a small migratory insectivorous passerine. We predicted that the effects of weather on breeding performance (clutch size, hatching success, nestling growth, fledging success) would be greater at northern latitudes, where the weather conditions are more extreme and unpredictable. We found that the breeding parameters, except clutch size, were not, however, inferior in the north. Northern birds, unlike the southern ones, responded to colder conditions by laying smaller clutches and maintaining a larger energy reserve (indicated by higher female body mass and higher levels of subcutaneous fat). If a cold spell occurred during the nestling period, southern flycatchers had 5–10% lower fledging success than the northern ones. Our results indicate that in the north, the breeding individuals coped with cold and variable weather better than the individuals in the southern population. This could be adaptive, because at high latitudes there is a higher probability of cold weather at the time of breeding.     

Demographic variation in timing and intensity of feather molt in migratory Fork‐tailed Flycatchers (Tyrannus s. savana)

Understanding the annual cycle of migratory birds is imperative for evaluating the evolution of life‐history strategies and developing effective conservation strategies. Yet, we still know little about the annual cycle of migratory birds that breed at south‐temperate latitudes of South America. We aged, sexed, and determined the progression and intensity of body, remige, and rectrix molt of migratory Fork‐tailed Flycatchers (Tyrannus s. savana) at breeding sites in southern South America and at wintering sites in northern South America. Molt of both body and flight feathers occurred primarily during the winter. In early winter, a similar proportion of young and adult flycatchers molted remiges and rectrices, but remige molt intensity (number of remiges molting) was greater and primary molt progression (mean primary feather molting) more advanced in adults. In late winter, remige molt intensity and primary molt progression did not differ between age groups. We found no difference between males and females either in the proportion of individuals molting in winter or in the intensity or progress of remige molt. Our results suggest that the nominate subspecies of Fork‐tailed Flycatcher undergoes one complete, annual molt on the wintering grounds, and represents the first comprehensive evaluation of molt timing of a migratory New World flycatcher that overwinters in the tropics. Given that breeding, molt, and migration represent three key events in the annual cycle of migratory birds, knowledge of the timing of these events is the first step toward understanding the possible tradeoffs migratory birds face throughout the year.     

Cold tolerance,and not earlier arrival on breeding grounds,explains why males winter further north in an Arctic‐breeding songbird

Sex biases in distributions of migratory birds during the non‐breeding season are widespread; however, the proximate mechanisms contributing to broad‐scale sex‐ratio variation are not well understood. We analyzed a long‐term winter‐banding dataset in combination with spring migration data from individuals tracked by using geolocators to test three hypotheses for observed variation in sex‐ratios in wintering flocks of snow buntings Plectrophenax nivalis. We quantified relevant weather conditions in winter (temperature, snowfall and snow depth) at each banding site each year and measured body size and condition (fat scores) of individual birds (n > 5500). We also directly measured spring migration distance for 17 individuals by using light‐level geolocators. If the distribution pattern of birds in winter is related to interactions between individual body size and thermoregulation, then larger bodied birds (males) should be found in colder sites (body size hypothesis). Males may also winter closer to breeding grounds to reduce migration distance for early arrival at breeding sites (arrival timing hypothesis). Finally, males may be socially dominant over females, and thus exclude females from high‐quality wintering sites (social dominance hypothesis). We found support for the body size hypothesis, in that colder and snowier weather predicted both larger body size and higher proportions of males banded. Direct tracking revealed that males did not winter significantly closer to their breeding site, despite being slightly further north on average than females from the same breeding population. We found some evidence for social dominance, in that females tended to carry more fat than males, potentially indicating lower habitat quality for females. Global climatic warming may reduce temperature constraints on females and smaller‐bodied males, resulting in broad‐scale changes in distributional patterns. Whether this has repercussions for individual fitness, and therefore population demography, is an important area of future research.