Meeting Abstract
Optimal migration theory posits that migratory birds should migrate at the lowest elevation at which they can find wind conditions profitable for migratory flight. This follows from the fact that climbing flight is more energetically expensive than horizontal flight and thus something to be avoided unless necessary. Additionally, high altitudes present other hurdles as well, such as reduced partial pressure of oxygen and extremely cold temperatures. As a result, even species such as Bar-headed Geese crossing the Himalayan Mountains attempt to minimize the altitude at which they migrate. Using GPS transmitters that record an individual’s altitude, flight speed, and flapping rate, we tracked the migrations of Black-tailed Godwits breeding in The Netherlands and wintering in sub-Saharan Africa. We found that godwits migrate at unexpectedly high altitudes (>5500 m) for long portions of their migratory flights and appear to do so irrespective of the profitability of the winds at lower altitudes or the elevation of the land below them. We then couple our measures of in-flight performance with analyses of the blood chemistry enabling such high-altitude flights. Our results challenge long-held theories about both the capabilities of low-elevation species to perform energetically costly activities at high elevations and the optimality of minimizing climbing flight during migration. As a result, it may be necessary to reassess when, where, and how high we expect birds to migrate.
