Of course its cruel. A bird is a symbol of freedom - the whole sky and earth are its playground - then someone imprisons it in a tiny box, 100,00000 times smaller than its natural environment. I don't believe it is if the bird was bred in captivity with no knowledge of the wild. What would it do if it's released into a nonative environment but die? Also, if it's a bird like a cockatiel, as long as you let it out, it's fine with being put in a cage. The only thing that's cruel is if you deprive it of food, water, or any other comfort it needs.