Habitat switching by dark-bellied brent geese Branta b. bernicla (L.) in relation to food depletion.

Seasonal changes in the distribution and feeding behaviour of dark-bellied brent geese Branta b. bernicla (L.) and the biomass of their food plants were studied in three successive winters on the Norfolk coast. The data was used, in conjunction with published information, to show how depletion, productivity and mortality of food plants drive the pattern of habitat switching in this species. It is then possible to explain the habitat shifts observed over the last 35 years and predict future changes. On arrival, geese fed first on algal beds and then on salt marsh, grass and arable fields before returning to feed entirely on the salt marsh in spring. The biomass of green algae, and subsequently the salt marsh vegetation, declined during the autumn and this could be attributed to depletion through goose grazing and natural mortality. As depletion occurred the geese fed more intensively, for a greater percentage of time and with an increasing pace rate, the net result, however, was a declining intake rate (as measured by defaecation rate). The algal biomass at which the geese switched from the algal beds to salt marsh was consistent between years, with heavy storm-induced loss of algae in one year resulting in an earlier switch. That the timing of habitat switches may be explained by depletion of food plants was further supported by historical data: the number of brent geese wintering at the site has increased dramatically over the last 30-35 years and the time of switching from algal beds to salt marsh and from salt marsh to salt marsh and fields has become progressively earlier, as expected from the increased depletion. The expected further increase in brent goose numbers will increase the rate of depletion of intertidal vegetation so that the switches between habitats will be more rapid and the geese will move inland earlier and remain inland longer. The expected increase in the brent goose population will thus result in a disproportionate increase in the levels of conflict between brent geese and agriculture.