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)
A windcap flageolet after Prudent Noblet
A flageolet after an anonymous eighteenth century instrument