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

I would say that size is more important than shape, and a rectangular cage would provide more space. A small bird needs sufficient space to fly around in, and it is preferable to have more than one small bird in a cage, so it has company. A large parrot can spend some time on its own in a cage but would also need to be allowed out to exercise and spread his wings.

A rectangular cage might be best for small species of birds, but if you house them in a round cage you could provide a 'safety corner' with some twigs or a nesting box for them to use as a retreat.