Nestlings that have fallen out of the nest must either be returned to the nest, if obvious where it has come from, or rescued.
Fledglings (just out of the nest) will usually have parents close by, and should always be left alone unless in immediate danger. If you are worried about your cat finding it then pick it up and place itin a flowerpot, peg the flowerpot to a washing line. The parents can get to it to feed it but the cat can't.
All birds caught by cats will need an antibiotic to avoid the high danger of septicaemia and should be rescued. All injured birds should be rescued. Place in a box, bucket or similar and cover, making sure there is ventilation. They will settle down if put in a dark place. Then seek help.
If you come across a baby owl, these need rescuing if still at the fluffy stage, but if there are feathers coming through, if in no immediate danger and not injured, should be left alone as they will be able to climb back up the tree.