Stochastic winter weather events are predicted to increase in occurrence and amplitude at northern latitudes and organisms are expected to cope through phenotypic flexibility. Small avian species wintering in these environments show acclimatization where basal metabolic rate (BMR) and maximal thermogenic capacity (MSUM) are typically elevated. However, little is known on intra-seasonal variation in metabolic performance and on how population trends truly reflect individual flexibility. Here we report intra-seasonal variation in metabolic parameters measured at the population and individual levels in black-capped chickadees ( Poecile atricapillus ). Results confirmed that population patterns indeed reflect flexibility at the individual level. They showed the expected increase in BMR (6%) and MSUM (34%) in winter relative to summer but also, and most importantly, that these parameters changed differently through time. BMR began its seasonal increase in November, while MSUM had already achieved more than 20% of its inter-seasonal increase by October, and declined to its starting level by March, while MSUM remained high. Although both parameters co-vary on a yearly scale, this mismatch in the timing of variation in winter BMR and MSUM likely reflects different constraints acting on different physiological components and therefore suggests a lack of functional link between these parameters.

Small avian species wintering at northern latitudes typically show increases in basal metabolic rate (BMR) and maximal thermogenic capacity (Msum). Those are widely assumed to reflect changes in body composition, with enlargement of digestive and excretory organs resulting in elevated winter BMR and larger body muscles driving the increase in Msum. Using free-living black-capped chickadees (Poecile atricapillus) as our model species, we investigated seasonal changes in body composition and tested for relationships between mass variations of body organs and variability of both BMR and Msum. Our results confirmed the expected winter increase in mass of body muscles and cardiopulmonary organs (heart + lungs) and showed that 64% of the observed Msum variations throughout the year were explained by changes in these organs. In contrast, we found little support for an effect of the digestive organs (gizzard + intestines) on BMR seasonal changes. Instead, this variable was mainly influenced by variations in mass of body muscles and excretory organs (liver + kidney), explaining up to 35% of its variability.

International audienceDetermining the year-round distribution and behaviour of birds is necessary for a betterunderstanding of their ecology and foraging strategies. Petrels form an important componentof the high-latitude seabird assemblages in terms of species and individuals. The distributionand foraging ecology of three sympatric fulmarine petrels (Southern FulmarFulmarus glacialoides, Cape Petrel Daption capense and Snow Petrel Pagodroma nivea)were studied at Adelie Land, East Antarctica, by combining information from miniaturizedsaltwater immersion geolocators and stable isotopes from feathers. During thebreeding season at a large spatial scale (c. 200 km), the three species overlapped in theirforaging areas located in the vicinity of the colonies but were segregated by their dietand trophic level, as indicated by the different chick d15N values that increased in theorder Cape Petrel < Southern Fulmar < Snow Petrel. During the non-breeding season,the three fulmarines showed species-specific migration strategies along a wide latitudinalgradient. Snow Petrels largely remained in ice-associated Antarctic waters, Southern Fulmarstargeted primarily the sub-Antarctic zone and Cape Petrels migrated further north.Overall, birds spent less time in flight during the non-breeding period than during thebreeding season, with the highest percentage of time spent sitting on the water occurringduring the breeding season and at the beginning of the non-breeding period beforemigration. This activity pattern, together with the d13C values of most feathers, stronglysuggests that moult of the three fulmarine petrels occurred at that time in the very productivehigh Antarctic waters, where birds fed on a combination of crustaceans and fish.The study highlights different segregating mechanisms that allow the coexistence of closelyrelated species, specifically, prey partitioning during the breeding season and spatialsegregation at sea during the non-breeding season