These are sketchbook pages from Disney artist and animal drawing teacher Bernard Garbutt. Beautiful, loose drawings from life. They give us an insight into the depth of research that was done in order to get a thorough understanding of ostriches' anatomy and motion. All this effort paid of when it came too animating these birds for Fantasia's Dance of the Hours.
This is what I've always loved about Disney's approach to animation. You learn as much as you possibly can about your subject, and then go to town with that knowledge in your animation.
These drawings go back as far as 1932, even though Fantasia wasn't released until 1940.