http://kotsanas.com/gb/exh.php?exhibit=2001001

It was the first autonomous volatile machine of antiquity. It
consisted of a light but powerful shell that had the form of a pigeon and had
internally the bladder of a big animal. The aerodynamic pigeon was placed with
the opening of the bladder adapted to the utmost opening of a heated airtight
boiler (or a powerful piston air-pump). When the pressure of the steam or air
exceeded the mechanical resistance of the connection, the pigeon was launched
and continued its flight for some hundreds of metres with the help of the
compressed air force coming out of the bladder according to the principles of
aerodynamics.

Thanks to Ed Horton.