Varying energetic costs of Brent Geese along a continuum from aquatic to agricultural habitats: the importance of habitat-specific energy expenditure

We investigated how habitat-specific differences in behavioural patterns affected Brent Goose energetics along a feeding continuum from natural aquatic to inland agricultural habitats. Time-budgets showed that geese using salt-marshes and inland habitats spent more time flying, being aggressive and alert than birds feeding in aquatic areas, and also spent much less time roosting. Frequency of disturbance was found to be higher in terrestrial habitats compared to aquatic habitats. These stress-related behavioural differences between habitats highlight the vulnerability of the species associated with adapting to different food sources. Combining time-budgets with activity-specific BMR-multiplicators showed that activity-based metabolic rates ranged from 1.7 to 2.7 x BMR within habitats exploited by Brent Geese, and emphasized that aquatic areas represent the energetically least expensive foraging habitat for these birds. This is largely the result of habitat-specific variation in time spent flying. These findings underline the importance of measuring habitat-specific behaviour and disturbance when studying avian energetics, and demonstrate the risk of uncritically using allometric relationships between body mass and energy expenditure in energetic studies and impact assessments across different habitats.