Can you ID this baby?

1. Its baby photo looks nothing like its adult photo.
2. Full grown, they are medium sized (like crow size).
3. They have long legs.
4. It nests in cavities, dense trees, or abandoned buildings.
5. This bird is a predator.

Clearly, this bird has an appearance only its mother could love, but boy is it pretty when it is full grown. What is it?