The bird flageolet is a tiny instrument used during the eighteenth and early nineteenth centuries for teaching cage birds to sing popular tunes in order to increase their market value.
This one is tuned in D at 435 Hz. Because of its very shrill sound quality it is difficult to use in ensemble music. The picture below shows how small it is compared to a fountain pen.
In this picture you can see it beside a "beak" flageolet in G.
You can listen to this instrument here
Tunes for the skylark and nightingale from The Bird Fancyer's Delight (anon. ca 1730)
This bird flageolet can be heard in A Prelude for the Birds by Henry Purcell,
played by l'Ensemble La Rêveuse, with Sébastien Marq (the second part).
.