Surfing on a green wave - how plant growth drives spring migration in the Barnacle Goose Branta leucopsis

The nutritional quality of forage plants varies in space and time. This variation is presumed to drive the annual migration of herbivore species which follow peaks in the availability of high quality forage between sites. The green-wave hypothesis predicts that during spring migration to northern breeding sites, geese and other herbivorous waterfowl travel along a climatic gradient, taking advantage of the flush of spring growth of forage plants at each stopover site along the gradient. Here, we explore a basic assumption of the green wave hypothesis which states that there are successive waves of forage availability along the East-Atlantic Flyway from temperate to arctic sites, as spring advances. We use one of the migration routes of the Barnacle Goose Branta leucopsis as a model to compare data on food quality and quantity of forage plants with the timing of migration along its migratory corridor. We collected data on forage biomass and quality at three saltmarsh sites along the traditional migration route of the Barnacle Goose: a temperate staging site in the Wadden Sea, a Baltic stopover site and a Russian sub-arctic breeding site. In all areas forage biomass increased in spring, while the nutritional quality peaked early in the season and declined with increasing biomass. We combined data on forage biomass and nutritional quality in one measure, the nutrient biomass. For all sites, nutrient biomass showed a peak in early spring. We used observations on goose migration to examine whether the geese utilise these peaks in nutrient biomass, as is predicted by the green wave hypothesis. Our data show that the geese utilise the Wadden Sea staging site and the Baltic stopover site at the moments of peak nutrient biomass. At the Russian breeding site, geese arrive prior to the flush of spring growth of forage plants and profit from the peak in nutrient biomass when the goslings hatch and adult birds start moulting. We conclude that spring nutrient biomass is a key factor driving the timing of the annual northern migration of avian herbivore