Feathers by day, membranes by night - Aerodynamic performance in bird and bat flight

Abstract: The efficiency and performance of a flying animal is directly related to the aerodynamics around its body and flapping wings. Here, I have developed methods for quantifying the wake dynamics around a flying animal. The results are used to estimate the aerodynamic performance of flapping flight. Using these methods, I have studied flight of the Pied Flycatcher (Ficedula hypoleuca), the Pallas’ Long-tongued bat (Glossophaga soricina) and the Lesser Long-nosed Bat (Leptonycteris yerbabuenae). In paper I, the aerodynamics close to the wing surface of slow flying G. soricina bats was studied, showing that bats use a Leading Edge Vortex (LEV) to enhance lift with up to 40% of the total. LEVs are known to be used by insects, but here I have shown that also larger vertebrates can use LEVs. In paper II, the aerodynamics close to the wing surface of a slow flying Pied Flycatcher was studied. This results showed that Pied Flycatchers generate LEVs with similar strength as in G. soricina, but the LEV structure is significantly different from that of bats and insects. In paper III, a new high-speed stereoscopic Particle Image Velocimetry (PIV) system for studying animal flight was introduced. Using this system, the wakes of the two bat species were captured, and new methods for visualizing and analyzing wake data were introduced. In paper IV, the wake dynamics and aerodynamic performance of flapping flight for the two bat species was studied. Although the wake dynamics for the two species was similar, maximum aerodynamic performance was achieved at a significantly higher speed for the highly mobile and migratory L. yerbabuenae than for the non-migratory G. soricina. In paper V, I introduced an actuator disk model for analyzing time-resolved PIV data of flapping flight. Analysis of the wake data for the two bat species showed that the model can be used to compare flight efficiency of different animal species. In paper VI, the wake dynamics in flycatchers was studied. The results showed that the wake of slow flying flycatchers is more similar to that of fast flying passerines than to that of hummingbirds, and that flycatchers are probably aerodynamically more efficient than hummingbirds. In paper VII, the wake dynamics and aerodynamic performance for the three studied species was compared. This showed that birds outperform bats in aerodynamic efficiency, which could be ascribed to differences in aerodynamic function of the body and of the wing upstroke, and which were proposed to be a result of differences in phylogenetic constraints between birds and bats.