WIND ASSISTANCE - A REQUIREMENT FOR MIGRATION OF SHOREBIRDS

We investigated the importance of wind-assisted night for northward (s pring) migration by Western Sandpipers (Calidris mauri) along the Paci fic Coast of North America. Using current models of energy costs of fl ight and recent data on the phenology of migration, we estimated the e nergy (fat) requirements for migration in calm winds and with wind-ass isted flight for different rates of fat deposition: (1) a variable rat e, assuming that birds deposit the minimum amount of fat required to r each the next stopover site; (2) a constant maximum rate of 1.0 g/day; and (3) a lower constant rate of 0.4 g/day. We tested these models by comparing conservative estimates of predicted body mass along the mig ration route with empirical data on body mass of Western Sandpipers at different stopover sites and upon arrival at the breeding grounds. In calm conditions, birds would have to deposit unrealistically high amo unts of fat (up to 330% of observed values) to maintain body mass abov e absolute lean mass values. Fat-deposition rates of 1.0 g/day and 0.4 g/day, in calm conditions, resulted in a steady decline in body mass along the migration route with predicted body masses on arrival in Ala ska of only 60% (13.6 g) and 26% (5.9 g) of average lean mass (22.7 g) . Conversely, birds migrating with wind assistance would be able to co mplete migration with fat-deposition rates as low as 0.4 g/day, simila r to values reported for this size bird from field studies. Our result s extend the conclusion of the importance of winds for large, long-dis tance migrants to a small, short-distance migrant. We suggest that the migratory decisions of birds are more strongly influenced by the freq uency and duration of winds aloft, i.e. by events during the flight ph ase, than by events during the stopover phase of migration, such as fa t-deposition rate, that have been the focus of much recent migration t heory. Received 3 September 1996, accepted 14 February 1997.