Elliott Ladd Coues (/ˈkaʊz/; September 9, 1842 – December 25, 1899) was an American army surgeon, historian, ornithologist, and author.

He served as a medical cadet in Washington in 1862–1863, and in 1864 was appointed assistant-surgeon in the regular army,[3] and assigned to Fort Whipple, Arizona.

While there was not yet any legal provision for divorce under its laws, the 1st Arizona State Legislature granted Coues an annulment of his marriage to Sarah A.

[4][5] His marriage to Jeannie Augusta McKenney ended in divorce in 1886,[6] and he married the widow, Mary Emily Bates in October 1887.

[7] In 1872, he published his Key to North American Birds, which, revised and rewritten in 1884 and 1901, did much to promote the systematic study of ornithology in America.

[8] His work was instrumental in establishing the currently accepted standards of trinomial nomenclature – the taxonomic classification of subspecies – in ornithology, and ultimately the whole of zoology.

[3] He was a careful bibliographer and in his work on the Birds of the Colorado Valley, he included a special section on swallows and attempted to resolve whether they migrated in winter or hibernated under lakes as was believed at the time: I have never seen anything of the sort, nor have I ever known one who had seen it; consequently, I know nothing of the case but what I have read about it.

In addition to ornithology he did valuable work in mammalogy; his book Fur-Bearing Animals (1877) being distinguished by the accuracy and completeness of its description of species, several of which were already becoming rare.

[14] He felt the inadequacy of formal orthodox science in dealing with the deeper problems of human life and destiny.