Is round cage bad for birds? Round cage or rectangular cage?

My avian vet told me that a rectangular or square cage is better than a round cage for exactly that reason - birds like to have corners to retreat to.

I can attest to the fact that birds do get scared of random things even after adapting to their environments. Loud noises from outside, me accidentally bumping into his cage, an unfamiliar object in the room, etc. My bird even freaked out a couple of weeks ago when my husband changed the month on the calendar. The picture for March was a blue jay, and my husband brought it over to show me because I love blue jays. Buzzy had been sitting on my shoulder at the time, and when he saw the picture, he panicked and tried to fly away. It's that whole "prey animal flight response" defense mechanism. Anyway, when something scares him and he's in his cage at the time, he does like to hang out near one of the back corners.