Tawny owls make two principal sounds. The first is a sharp contact call, written kewick or ke-wick. The second is the drawn-out hoot, a first note followed by a pause and then a longer quavering phrase. Females give the kewick more often and males the hoot, though both sexes can produce both.
When a pair calls back and forth across a territory the two sounds interleave, and a listener hears one continuous twit-twoo. The phrase entered English through literature and stayed, which is why so many people expect a single owl to deliver it.
Colour morphs are the species' other quiet complication. Tawny owls occur in brown and grey forms, and long-term work in Finland has linked morph frequency to winter conditions, with the brown morph faring worse in snowy winters and becoming commoner as winters have warmed. That is one of the better-documented cases of a climate signal in a plumage trait.