Paulson hypothesized that diurnal birds undergo selection for variation for its own sake: it's harder for prey to learn what you look like: sort of a reverse "search image." Some evidence supports this theory, polymorphism in hawks related to those with visually acute prey. Generally insectivorous and snail-eaters aren't polymorphic. Australian Goshawks not polymorphic where they don't feed on birds. And most polymorphism is ventral.
ON THE OTHER HAND: prey key on silhouettes not colors. And the rarer morph should have adaptive advantage and therefore should increase: no evidence for this. Owls often show polymorphism and they are nocturnal.
Jaegers are predatory. There are three species in the Arctic: Pomarine (feeds on lemmings), Parasitic (birds), and Long-tailed (insects) little polymorphism. The rare white morph in Iceland is more successful than the others, so why don't they increase? PERHAPS DARK AND LIGHT MORPHS GENETICALLY LINKED TO SOMETHING ELSE.
Screech Owls' gray form is most common everywhere but least common in N and S part of range. Red is most common in mid latitudes. Turns out that more red birds die in severe winters: in lab below 5` centigrade, red birds died while gray birds needed less food.
Eastern Screech Owl