Wind assistance: A requirement for migration of shorebirds?

-We investigated the importance of wind-assisted flight for northward (spring) migration by Western Sandpipers (Calidris mauri) along the Pacific Coast of North America. Using current models of energy costs of flight and recent data on the phenology of migration, we estimated the energy (fat) requirements for migration in calm winds and with wind-assisted flight for different rates of fat deposition: (1) a variable rate, assuming that birds deposit the minimum amount of fat required to reach 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 migration 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 amounts of fat (up to 330% of observed values) to maintain body mass above 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 Alaska 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 complete migration with fat-deposition rates as low as 0.4 g/day, similar to values reported for this size bird from field studies. Our results extend the conclusion of the importance of winds for large, long-distance migrants to a small, short-distance migrant. We suggest that the migratory decisions of birds are more strongly influenced by the frequency and duration of winds aloft, i.e. by events during the flight phase, than by events during the stopover phase of migration, such as fat-deposition rate, that have been the focus of much recent migration theory. Received 3 September 1996, accepted 14 February 1997. THEORETICAL APPROACHES TO BIRD MIGRATION have focused on the rate at which energy reserves are obtained or replenished during migratory stopovers, i.e. the rate of fat deposition (Alerstam and Lindstr6m 1990, Alerstam 1991, Gudmundsson et al. 1991). Individual birds are assumed to adopt one of two different strategies during migration, either (1) minimizing the time spent on migration (i.e. migrating as fast as possible), or (2) minimizing energy expenditure during migration (i.e. keeping flight costs low by storing only as much fat as 6 E-mail: rob.butler@ec.gc.ca 7Present address: Forest and Rangeland Ecosystem Science Center, U. S. Geological Survey, 3200 SW Jefferson Way, Corvallis, Oregon 97331, USA. is needed to reach the next stopover site; Alerstam and Lindstrom 1990). Both hypotheses predict that rates of fat deposition will determine speed of migration and, therefore, that events during the stopover phase of migration are fundamental in determining successful migra ion. Many birds store large quantities of energy in the form of fat, and to a lesser extent protein, before and during migration to power flights between stopover sites (Helms and Drury 1960, Biebach 1985, Blem 1990, Klaassen et al. 1990, Lindstr6m and Piersma 1992, Berthold 1996). Despite this, few studies have found a significant relationship between fat reserves and the time spent at stopover sites (Post and Browne 1976, Lank 1983, Dunn et al. 1988, Lindstr6m and Alerstam 1992, Holmgren et al.