If I had to guess, I think that maybe the director chose to stray from the literary narrative, or add more symbolism to the owl's role in that scene than perhaps the author originally intended.
In my interpretation of the scene, I think that the owl represents Smiley, or at least, the watchdog element that was going on behind the scene. I've only seen the movie once, and while ornithology may be more my field of study, anyway, I just wanted to put this theory out there... I think that owls are not only wise, but that they also happen to play a more critical, complex role in the ecosystem.
Birds of prey, and any other birds that hunt mammals and smaller birds, actually end up governing or otherwise managing/"policing" animal communities. They do this by hunting on instinct, targeting specific individuals that stand out from the herd/flock, and it's their instincts as individual predators that cause lasting impacts on the fauna community as a whole. Maybe I'm just applying my bird nerd perspective onto the director's own intentions for the film, but that's how I see it, anyway...
I just think that the owl might represent Smiley. Just wanted to throw that out there.