Skylark optimal flight speeds for flying nowhere and somewhere

Flight is energetically very costly. For birds the mechanical power in relation to airspeed is characterized by a U-shaped function. From this function one can derive optimal flight speeds associated with minimum power (V-mp), minimum cost of transport (V-mr) and minimum overall time of migration (V-mt). Since flight is energetically so costly, aerial displays and song flight can potentially serve as signals reliably indicating the individual quality or resource potential of the signaller. In order to mazimize the amount of song flight produced, one expects V-mp during song flight, while during migration one should rather expect V-mr or V-mt. The authors compared flight speeds of skylarks (Alauda arvensis) during song flight and migration flight, respectively. In this species predicted V-mp=5.5m/s, V-mr=10.5m/s, and V-mt=12.7m/s. The preferred airspeed during song flight did not differ significantly from the predicted V-mp, while airspeed during migration was significantly higher than V-mr and V-mt, indicating that flight speed is a flexible trait that birds adjust to different situations. Why the skylarks speed up so much on migration is still unclear, but it may be that due to the shape of the predicted power curve, variation in cost of transport at high speeds is relatively small.