Douglas Spalding
If we realise that birds are descendants of dinosaurs, and (say) cats are mammals, the we can understand the effects of their births. A bird comes out of the shell ready-made. After an initial feeding that's it as far as the parents are concerned. On the other hand, cats have quite a long period when the mother looks after them. They are initially almost helpless. The role of play is very much another mammalian learning device. On the other hand, birds do not learn to fly. They fly when their anatomy is developed.