The Caladan Frigate Bird warrants at least one more study before I figure out how the Atreides would build an ornithopter.
The frigate bird’s whole body has evolved for aerodynamic efficiency. Even their size helps with their ability to fly; larger wings work better because, while the wing is bigger, the air has not scaled up with it — there is effectively more air to provide lift.
The dual tails also prevent vortices from forming from the bird’s body. It’s a beautiful inspiration for a society that prides itself on flight.