This beautiful and unique collection, which represents _The Birds of America_ in the bud, illustrates the development of Audubon's art from about 1800 or a little later to 1821,[158] and clearly shows that the fuller mastery which he attained after the latter date was manifested in no small degree at a much earlier period. His drawings of the Wood Thrush (1806), the Whippoorwill and Kingfisher (1810), the Carolina Parrot (1811), and the Nighthawk (1812), though detached and less ambitious as pictures, for truth of line and delicacy of finish would compare favorably with the best of his later work. After 1820 his ability had so far outstripped his ambition that there was needed only the stimulus of a powerful motive and a well defined plan to bring his powers into full fruition at once. A little later, when he began to revise, enrich and standardize all of his previous work, he used the brush and water colors more freely than ever before. Hundreds of his earlier studies were cast aside; many, to be sure, were hastily drawn in pastel, crayon and pencil, and had not time failed him at the end, nothing of his earlier American period would have remained in the final product. Nearly all of these rejected drawings bear serial numbers, which from the lack of sequence now observed, show that they were subject to constant change and that their total number must have been great. All bear the scientific and common names in French or English or both, and many are signed with the artist's initials or name; besides giving the place and date, in some cases the weights and measurements of his subjects are added, with detailed sketches of foot, bill, or eggs.[159]