Bird wing simulation using Gazebo

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You're best bet would be to write your own custom plugin that "fakes" the physics of an Ornithopter.

For reference you might want to check out this paper:
http://www.cim.mcgill.ca/~dmeger/ICRA..., page 4
The authors used Gazebo plugins to implement empirical equations for a swimming robot's buoyancy, drag, and flipper thrust underwater.

Flexible surfaces can be approximated by using smaller rigid surfaces connected with joints.

Finally, Gazebo may not be the best option for your particular design. You may be better off looking for another simulator that's designed to handle soft-bodies / fluids.