Synonyms for Loon:

n.

crank, crazy, dipper, eccentric, grebe, kook, loony, lunatic, nut, screwball, weirdie, weirdo, wise, dingbat, ding-a-ling. madman, madwoman, oddball, oddity, space cadet, headcase, nutjob. animal (noun)
diver.
bird (noun)
albatross, bald eagle, barn owl, bird, bittern, blackbird, blue jay, bluebill, bluebird, booby, bullfinch, buzzard, Canada Goose, canary, cardinal, cassowary, chickadee, cockatoo, condor, coot, cormorant, crane, crow, cuckoo, curlew, dodo, dove, duck, eagle, egret, emu, falcon, finch, flamingo, goldfinch, goose, grackle, grosbeak, grouse, gull, harrier, hawk, heron, horned owl, hummingbird, ibis, jackdaw, jay, kingfisher, lark, lovebird, macaw, magpie, mallard, mockingbird, mourning dove, myna, nightingale, nuthatch, oriole, osprey, ostrich, owl, parakeet, parrot, partridge, peacock, pelican, penguin, peregrine, pheasant, pigeon, plover, puffin, quetzal, raven, redbird, redwing, ringtail, road runner, robin, rook, sandpiper, sea gull, sheldrake, snipe, songbird, sparrow, spoonbill, squab, starling, stork, swan, swift, teal, tern, thrush, turkey, turtledove, vulture, waxwing, weaver, woodpecker, wren, yellowbird, crested jay.
crackpot (noun)
crazy, eccentric, lunatic, nut.
fool (noun)
blockhead, bonehead, bungler, clod, cretin, dimwit, dolt, dullard, dumbbell, dummy, dunce, fool, half-wit, ignoramus, imbecile, moron, nincompoop, nitwit, simpleton, crackpot, chuckle-head, knuckle-head, lame-brain, meathead.
loon (noun)
addle-head, birdbrain, diver.

Other synonyms:

screwball, weirdie, weirdo, dingbat, ding-a-ling. kook. loony. lunatic. crank, crazy, nut. Other relevant words:
addle-head, birdbrain, crank, crazy, diver, eccentric, kook, loony, lunatic, madman, nut, screwball, weirdie, weirdo, dingbat, ding-a-ling, nutjob.

Usage examples for loon

  1. She knew me for the merry loon who had entertained her guests one night a fortnight since, when on my way from Pesaro to Rome. – The Shame of Motley by Raphael Sabatini
  2. " Ninth Baron of Dimbledon," added Forbes, " and as crazy as a loon – The Voice in the Fog by Harold MacGrath
  3. She's crazy, crazy as a loon – The Land of Long Ago by Eliza Calvert Hall