Arthur Hay, 9th Marquess of Tweeddale

Colonel Arthur Hay, 9th Marquess of Tweeddale (9 November 1824 – 29 December 1878), known before 1862 as Lord Arthur Hay and between 1862 and 1876 as Viscount Walden, was a Scottish soldier and ornithologist. He was born at Yester, Gifford, East Lothian. He served as a soldier in India and the Crimea. He succeeded his father to the marquessate in 1876. He died at Chislehurst, and was succeeded by his brother.

Hay purchased a lieutenantcy in the Grenadier Guards in 1841. He purchased a captaincy in 1846 and was promoted lieutenant-colonel without purchase in 1854 and Colonel in 1860. In 1866 he transferred to the 17th Lancers.

He was president of the Zoological Society of London from 16 January 1868. His ornithological works were published privately in 1881 by his nephew, Captain Robert George Wardlaw-Ramsay, with a memoir by Dr W. H. Russell, and the attribution Walden is used in taxonomic listings.

He had a private collection of birds, insects, reptiles and mammals. He employed Carl Bock to travel to Maritime Southeast Asia and collect specimens. Tweeddale described about 40 species collected by Bock for the first time.