Wind assistance: A requirement for migration of shorebirds?

Auk, The, Jul 1997 by W, Robert, D, Tony, Anne, Mary

ROBERT W BUTLER,1,2,6 TONY D. WILLIAMS,2 NILS WARNOCK,2,3,7 AND MARY ANNE BISHOP4,5

ABSTRACT.-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 Lindstrom 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 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 migration. 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, Lindstrom 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, Lindstrom and Alerstam 1992, Holmgren et al. 1993, Lyons and Haig 1995, Skagen and Knopf 1994, Iverson et al. 1996). Although there have been several studies of energy costs of flight (e.g. Masman and Klaassen 1987, Castro and Myers 1988, Pennycuick 1989), much less attention has been focused on the importance of variation in rates of energy utilization during long-distance migratory flight.

Two factors that might affect energy costs of flight, and therefore the energy or fat-deposition rates required for migration, are wind speed and direction. Wind has long been recognized as an important variable to migrating birds (Parslow 1969; Able 1973; Alerstam 1979, 1990b; Richardson 1978, 1990; Elkins 1988; Piersma et al. 1990; Dau 1992; Piersma and van de Sant 1992; Marks and Redmond 1994). With a constant air speed, wind will affect ground speed and thus flight duration (time spent in migration). Conversely, if birds adjust air speed to maintain ground speed in variable winds, then wind speed will affect power output (energy cost of migration; Richardson 1990). In general, following winds (i.e. tailwinds) should minimize the energetic cost of migration. Most studies have mainly considered the effect of wind conditions on the number of birds taking off or departing from a site each day, or the number of birds aloft (Piersma et al. 1990, Gauthreaux 1991, Hall et al. 1992, Tulp et al. 1994). The relative importance of wind-assisted flight in determining energy reserves required by species that make long-distance, nonstop flights has been estimated (Stoddard et al. 1983, Piersma and Jukema 1990). Several authors suggested that favorable tailwinds are essential for large-bodied shorebirds to complete long, nonstop flights (Bar-tailed Godwit [Limosa lapponica], Piersma and Jukema 1990; Great Knot [Calidris tenuirostris], Tulp et al.1994; Bristle-thighed Curlew [Numenius tahitiensis], Marks and Redmond 1994). However, the requirement of winds for small-bodied shorebirds that make relatively short flights has not been assessed, and the relative importance of wind-assisted flight has been largely unquantified in all studies (Piersma et al. 1990, Holmgren et al. 1993). As Richardson (1990) pointed out, the interaction between wind conditions and the physiological readiness to migrate (i.e. fat status and fat-deposition rate) is poorly understood. It is important to consider these issues over large geographical scales because a bird's decision to depart or stay is a complex interaction of the position of the individual along the migration route, the time remaining before the commencement of the breeding season, the body condition of the individual, and the frequency and duration of favorable winds (Piersma and Jukema 1990). For an individual to minimize its use of time and energy (Alerstam and Lindstrom 1990), it should match its energy levels with the frequency and duration of favorable winds. However, the frequency (but not duration) of favorable winds for migration has low predictability (Richardson 1979, 1990). In this situation, an individual should maintain its energy reserves at a high level (vs. a minimum level; cf. Alerstam and Lindstrom 1990) so that it can depart as soon as winds become favorable, fly for the entire duration of favorable winds, and arrive at the next stopover site prepared for rapid departure at the arrival of the next favorable wind event.