From the OED:
“1. A man concerned with birds; an ornithologist.
1697 Lond. Gaz. No. 3269/4 At Black Joe’s, the German Bird-man..canary-birds. 1729 M. BROWNE Pisc. Eclog. VIII. (1773) 119 The Fisher on the green-sea-deep, And Birdsman in the osier copse. 1796 MORSE Amer. Geog. II. 17 The birdmen or climbers..bringing away the birds and their eggs. 1844 C. WATERTON Ess. Nat. Hist. (ed. 3) p. lxviii, These birdmen outwardly had the appearance of Italian banditti, but it was all outside and nothing more. 1969 Observer 16 Feb. 7/5 The Smithsonian staff itself is..envious of the birdmen for getting so much money. Those involved in the ornithological expedition have had to receive military clearance.
1909 Daily Chron. 27 Oct. 4/4, I shall say: ‘I saw the first bird-men in England, my dears.’ 1917 ‘CONTACT’ Airman’s Outings 244 Even intrepid birdmen (war correspondentese for flying officers) tire of trying to be offensive on a patrol. 1957 J. BRAINE Room at Top iv. 44 You were both intrepid birdmen, weren’t you?”
Famous bird-men include: