Audubon chose Whatman paper for his originals. Manufactured in Kent, England, this was the finest paper available. James Whatman developed wove paper, and continued manufacturing it until his death in 1759. His son, James, then ran the business, but sold it to Thomas Hollingsworth in 1792. The Hollingsworth family continued making this paper until 1976.
This double elephant paper measures about 29 x 39 inches and has a watermark on the back - J WHATMAN - often followed by a date. Some sheets additionally have TURKEY MILL countermarked into the paper. This is thought to refer to the mills original purpose, grinding Turkey wheat from India.
George Washington signed State documents on Whatman paper, Napoleon wrote his will on Whatman paper while on the Island of St. Helena, and, yes, Queen Victoria used Whatman paper for her personal stationary.