This paper documents the daily activities and feeding rates of Barnacle Geese Branta leucopsis through the autumn and winter in relation to changes in biomass of their food supply. Energy intake and expenditure were estimated and energy surplus or deficit for each month calculated. A net surplus was achieved in the months of October (+ 235 kJ d-1) and November (+ 247 kJ d-1) when most food was available. Geese suffered a net deficit in December (225 kJ d-1) and January (113 kJ d-1) when biomass and food quality were lowest. In February, when grass began to grow again, the birds achieved a surplus once again (+ 167 kJ d-'). Body mass changes through the winter were modelled using observed intake rates and estimated energy expenditure. Body mass predictions were consistent with observed mass and body condition changes. It is argued that mass loss in midwinter in this species is enforced by the limited feeding opportunity and declining food supply rather than an adaptive response to reduce predation risk. Geese gain substantial body reserves in autumn prior to this predictable mass loss. As soon as daylength and food biomass allow, lost reserves are replaced and mass increases rapidly in preparation for migration and breeding.

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