How can a bird wings and human arm look and function so differently when the same basic tissue that construct them?

Birds Are Made To Fly.Their Wings Are Bigger In Size Than Their Body.Birds Are Of Light Weight And Due To Continuation Of Flapping Of The Wings The Earth's Gravitational Pull To Doesn't Effect Them Too Much,Whereas Human Body Is Heavier Than The Birds,So The Gravitational Pull Pull's Us Down Hence The Humans Can't Fly In The Air