FOOD EXPLOITATION PATTERNS BY BRENT GEESE BRANTA-BERNICLA DURING SPRING STAGING

The exploitation pattern of a salt marsh community by Brent Geese in spring is discussed. The two main vegetation types on the salt marsh, one dominated by Puccinellia maritima and the other characterized by the presence of Plantago maritima and Triglochin maritima, are grazed very intensively, so that the geese have to rely on freshly produced plant material. The both vegetation types differ in the frequency of grazing by Brent Geese: Puccinellia dominated areas are visited every 1 or 2 days, while the Plantago areas are visited only once every 4 to 8 days. An explanation for this temporal visitation pattern is sought for by examining the growth patterns of the food plants. A high frequency of visits to Puccinellia areas is in favour of the structure of the vegetation. However, individual Puccinellia shoots need a considerable time (about 20 days) to grow after defoliation in order to offer geese a bite of nearly 2 blades. This urges the geese to graze only a small portion of the shoots present and to scan the sward for the largest available tillers. Grazing frequency and intensity (portion of tillers grazed) are adjusted in such a way to allow the shoots to grow sufficiently. Plantago plants are much more restricted in occurrence than Puccinellia, while on the other hand the larger leaves of the former species offer a higher intake rate to the geese. The timing of visits by goose flocks every 4 to 8 days in the course of the spring to Plantago areas leads to a maximization of the harvested plant material. Social factors must be invoked in the search for mechanisms underlying the timing of visits to the successive sites utilized by the flock in an orderly grazing sequence