Low cost of shifting flight altitude in migrating great snipes

Many birds make migratory flights lasting several days. Migrants do not only repeatedly make small-scale shifts in altitude; some of them also climb and sink several thousand metres in a diel cycle. This is a seemingly energetically costly behaviour, adding to an already high energetic cost of flight. Here, we describe the climbing and sinking behaviour of migratory great snipes, Gallinago media, and discuss whether their diel altitude cycle could represent an additional energy cost to their migration. Making use of tracking data from multisensor data loggers, we quantified altitude variation and compared climb and sink rates from non-stop migratory flights lasting up to 4 days (84 h). In general, great snipes climb and sink at comparatively slow rates, averaging 0.21 m s-1 and -0.22 m s-1, respectively, almost invariably within ±0.7 m s-1, and hardly ever close to a theoretical maximum climb rate of 1.8 m s-1. Great snipes do not climb faster at night than in the day and climb and sink rates are similarly low for all long non-stop flights. The additional cost of shifting flight altitude for great snipes is apparently negligible compared with the cost of forward flight (<1%). This is largely explained by the birds flapping during a slow descent, thereby turning most of the potential energy built up during previous climbs into aerodynamic work. We conclude that the overall cost of shifting flight is probably low for most migratory birds, given that climb and sink rates are low.