 Audubon also authored the companion book "Ornithological Biographies".About 1820, around the age of 35, Audubon declared his intention to paint every bird in North America.In his bird art, he mainly forsook oil paint, the medium of serious artists of the day, in favour of watercolours and pastel crayons (and occasionally pencil, charcoal, chalk, gouache, and pen and ink).