Oh wait! You don't have to imagine:
Elijah Wood already looked like a sad owl throughout most of the LOTR films, so this was probably just the next logical step.
I like how the owls wear rusty old helmets in order to look evil. Because animals wearing tiny versions of human-clothes always scares the crap out of people, and certainly is not adorable in the slightest.
More weird things: this owl epic was directed by Zack Snyder, the macho bad-ass who gave us the film adaptations of Watchmen and 300. His films were already half-animated before, but they were far less cuddly.
Legend of the Guardians was also animated by an Australian company (who did that other "cute-but-sad birds" film Happy Feet), so we naturally get the voices of Aussie-all-stars Hugo Weaving and Claire from Lost.