"An unconventional use of the early French flageolet was to train captive song-birds to sing. The most plausible explanation for why a flageolet was used rather than, for example, a recorder, was that at the very high pitches that were required, having the lower three holes alternating between the front and back of the instrument made it easier for bird breeders to play with less dexterous fingers.
By the 18th Century, some extremely small and very high pitched “bird flageolets” were made for these purposes. Whilst they tended to be made in a variety of different keys (no doubt because they were mostly played unaccompanied), the largest were, perhaps, the size of a garklein recorder (i.e. with a lowest note the C or D two octaves above middle C) and many were even smaller, pitched in F or G at least two-and-a-half octaves above middle C. Often, these flageolets were quite finely made, with very slender pipes made out of ivory. It seems likely that they were marketed to wealthy song-bird owners (as well as breeders). The most notable work was called the Bird Fancyer’s Delight, published by John Walsh and William Hill; like the Pleasant Companion it includes pieces in tablature as well as conventional notation." [Source: The Pleasant Companion : The Flageolet Site]