Flapping flight has evolved on at least four occasions (probably many 
more), i.e. in birds, bats, Pterosaurs (extinct) and insects. The size 
ranges from sub-mm (insects) to about 5 m wing span in birds and 
Pterosaurs. Even if organismal evolution is a tinkering process with 
material at hand, physical laws dictate what is feasible and required. 
Hence, the wing design is very different between birds and bats, while 
the overall shape and performance are quite similar. Studies in a 
low-turbulence wind tunnel crafted for animal flight studies have 
revealed new insights into bird and bat aerodynamics. In this talk I 
review recent studies on birds and bats using digital particle image 
velocimetry (DPIV). Overall wake topology varies with forward flight 
speed, but the change is rather smooth and does not indicate the use of 
discrete ~gaits~ as hypothesized by some researchers. Quantitative 
measurements of wakes suggest that unsteady high-lift mechanisms are 
involved at slow speeds, and in bats the mechanism is a leading edge 
vortex.