39.6 Saturday, Jan. 5 Comparing Aerodynamic Efficiency in Birds and Bats Suggests Better Flight Performance in Birds MUIJRES, FT*; JOHANSSON, LC; BOWLIN, MS; WINTER, Y; HEDENSTRöM, A; Univ. of Washington, Seattle; Lund Univ., Sweden; Univ. of Michigan-Dearborn; Humboldt Univ, Berlin, Germany; Lund Univ., Sweden firstname.lastname@example.org
Has the independent evolution of powered flight in birds and bats, with the apparent convergence in size, shape and flight style, resulted in the same overall flight performance? Or do they differ due to morphological peculiarities, such as feathers and membranous wings? We test which of these scenarios fit to two measures of aerodynamic flight efficiency in two passerine bird species and two New World leaf-nosed bat species. Using time-resolved particle image velocimetry measurements of the wake of the animals flying in a wind tunnel, we derived the span efficiency, a metric for the efficiency of generating lift, and the lift-to-drag ratio, a metric for mechanical energetic flight efficiency. We show that the birds significantly outperform the bats in both metrics, and that the difference in performance is primarily caused by differences in body shape and wing upstroke function. The bats have less streamlined bodies than the birds, partly due to the presence of protruding ears used for echolocation in bats. During the upstroke, birds retract their wings and spread the wing feathers making the wing aerodynamically inactive, while the bats have a more complex upstroke motion where the membranous wing generates thrust and negative lift. Our findings suggest that, despite millions of years of evolution, bats may have not reached the same flight performance levels as birds, and that this could be due to conflicting selection pressures for echolocation and flight in bats. The results may help explain ecological differences between birds and bats, such as why birds typically fly faster, migrate more frequently and migrate longer distances than bats.