Always on the wing - Fluid dynamics, flight performance and flight behavior of common swifts

Abstract: The aerodynamics and kinematics behind the flight of animals are relatively unknown. Although animal flight has been studied for several hundreds of years, it is only in recent time that we have the technical abilities to study the mechanistic basis of animal flight. This thesis represents an attempt to widen the knowledge about animal flight by studying one of the most advanced flyers of the natural world, the common swift (Apus apus). Common swifts, or swifts for short, are aerial insectivores that spend almost their entire lifetime on the wing. In paper I, the aerodynamics and kinematics of a swift in flapping cruising flight were studied in a wind tunnel. The results showed that the rotational strength, or circulation, of the vortices that were shed into the wake behind the bird varied in a very smooth manner, which was different from wakes previously described for other birds. In paper II, the wake of a swift in flapping flight was studied over a range of flight speeds. The results showed that the wake of the swift in addition to the features found in Paper I, consisted of a pair of trailing vortices behind the tail and the wing base. The fact that vortices are shed at the wing base suggests that the two wings operate, to some extent, aerodynamically detached from each other. In paper III, the gliding flight of a swift was examined. The results showed that the bird generated a simple wake, consisting of a pair of trailing wingtip and tail vortices. The gliding efficiency of the swift was found to be relatively high compared to other birds. In paper IV, swifts were studied in free flight using tracking radar during spring migration, autumn migration and when the birds sleep on the wing. The objective was to compare the birds’ flight speeds to predictions from flight mechanical theory. The results showed that the birds changed their flight speed between behaviors less than predicted from theory. In paper V, the swifts were studied in display flights, often called ‘screaming parties’. During these events, the birds appear to reach high speeds. The results showed that the birds flew on average at flight speeds twice the speed on migration, suggesting that the birds are capable of high power output during short bursts of anaerobic muscle work. In paper VI, the swifts’ ability to compensate for wind drift during migration was studied using tracking radar. The results showed that the birds compensated to a large extent for the winds, both by changing their heading direction and by increasing their airspeed. Increasing airspeed had been theoretically described previously as a possible wind response, but this study was the first to show this empirically.