With all the early missions with animals in space, the dogs and chimpanzees, the thing was that we had no idea what the effect of zero gravity would be on a human body. There is no way to simulate that on the ground - except for a few seconds at a time. Would a human being even survive for minutes in zero g? How could we know?

Even today we get surprises in zero g experiments. So they tried it with animals first. Imagine how complex spaceflight would be if humans died on exposure to, say, ten minutes of zero g!

Some animals behave in strange ways in zero g. For instance snakes don’t recognize their own bodies.

They also did experiments for brief moments of weightlessness in early versions of the “Vomit comet”. Here they are testing pigeons in microgravity

Full documentary

Here's What Happens When A Pigeon Tries To Fly In Zero Gravity