Lawrence Hargrave — Australian Scientist born on January 29, 1850, died on July 14, 1915

Lawrence Hargrave, MRAeS, was an engineer, explorer, astronomer, inventor and aeronautical pioneer... (wikipedia)

If you direct your attention to the position of a bird with regard to the wave surface, it will speedily be noticed to be nearly always on the rising side or face of the wave and moving apparently at right angles to the wave's course, but really diagonal to it.
The wings are moved several times by hand to charge the crank chamber with mixture, which flows on through the external pipe and inlet valve to the compression space and cylinder.
The closer the bird is to the surface of the water, the firmer and more inelastic is the uplift of the rising air. The bird appears to almost feel the surface with the tip of its weather wing.
Common sense steps in here and says: Separate the parts you want to be mobile from the parts you want to be inert. You have seen the result, and I know many have the skill to apply it.
The most ordinary conditions for observing sailing birds are then the wind and sea are both aft.