I don't agree with the conjectured reason why Geoffrey of Monmouth chose "Merlin" over "merdin" as proposed in this name's listing in the main database, because Myrddin in Welsh is pronounced, "meer-thin", not "mair-din". Quite possibly Monmouth chose the "L" spelling because "merle" finds its roots in "melaina", Greek for "dark, black". Or maybe he just made a mistake in pronunciation. Welsh is easy to mess-up.
A Merlin is the name of a small breed of European falcon, which gets its name from the Anglo-French word "merilun", derived from the Old French "esmerillon", which is in Modern French "émerillon", which finds its roots in the Frankish "smiril".Merlin the Magician, from Arthurian legend, is probably from the Old French form of the Welsh proper name "Myrddhin", which is probably from the Celtic "Moridunon", meaning "of the sea-hill," ("mori-", meaning "sea" and "dunom", meaning "hill".)