After a couple of establishing shots showing Tippi Hedren walking through San Francisco’s Union Square in The Birds, the actress walks behind a large poster advertising the city (and clueing in anybody who still doesn’t know where the scene is set). The poster serves another purpose, however: it hides a cut.
Before Tippi Hedren walks behind the poster (above), we’re there in San Francisco. By the time she emerges (below), we’ve moved to a studio set, including the exterior of the pet shop.
It’s nicely done, but the lighting is a dead giveaway. You can’t really see it in these stills, but when you watch the scene, look at the shadows on the ground: they’re hardly visible in the location shooting in natural light, but on set, the actors cast multiple, well defined shadows.