Traditional aircraft design requires a tail to provide stable and controllable yaw and pitch. Birds do not, but adjusting wing shape independently. Inherently providing less drag, and therefore better efficiency.
Prandtl-D [Preliminary Research Aerodynamic Design to Lower Drag]
Albion Bowers
Prandtl-D1
Prandtl-D2
Prandtl-D3
Prandtl-D3c
NASA MIT (project from NASA and MIT explores shape-shifting aircraft further, but from a distinctly different direction. This new wing technology, made up of hundreds of identical pieces, is the foundation for aircraft with flexible wings that transform dynamically in flight to create the optimal shape for their moment-to-moment flight conditions.)
Manx, 1940's twin-engine tailless design of pusher configuration, partially-swept wings supported the vertical stabilisers of a twin tail, with elevons for pitch and roll control.
NASA - MIT flexible wing