Differences in migratory pattern and habitat choice between social classes of the Brent Goose Branta bernicla bernicla.

Age ratio (% juveniles) counts of Brent Goose flocks provide a crude index of their social composition (families versus subadults and failed breeders). In 1975/76, the possible impact of habitat and season was studied in four areas along the Dutch Wadden Sea. Sightings of birds colour-ringed in England facilitated conclusions on turnover and site-fidelity. Numbers were small in autumn and residence time was short. The proportion of juveniles decreased from 50% in October to 25% in midwinter. A less favourable energy balance of the smaller juveniles compared to adults is probably the ultimate factor in this differential migration of families. The impact of seasonal feeding conditions in the potential staging areas is discussed. Peak numbers were found in spring. Except during a short period of mass migration, age ratios were then lower in populations dependent on intertidal habitats than in those feeding inland. In normal years, salt marshes are the principal habitat in April-May. In the extremely dry spring of 1976, inland grasslands were favoured, particularly by families. Results suggest that groups of families initiate feeding flights and are more exploratory within a field. The literature has shown families to be dominant over non-breeders. It is therefore postulated that competition plays a part in the distribution of Brent over the available resources.

Lambeck R.D, ARDEA 78 (3): 426-440.