How can birds comunícate to each other? Do they understand us when we talk?

Question: How can birds comunícate to each other? Do they understand us when we talk?
- Keywords:
  bird,
  communication
Asked by @Mari to Carrie on 8 Mar 2017.
- Carrie Ijichi answered on 8 Mar 2017:
  
  Birds use song and other vocalisations, their body language, and colours and shapes of their feathers to send information. So, a peacock when he shows up his gorgeous big feathers is communicating how strong her is (because he can carry them) and how healthy he is (because they’re glossy and colourful). Then the females can decide is he’s the one for them!
  
  Most birds don’t understand us when we talk, as far as we know, but some special birds that grow up with humans seem to be able to. Check out Alex the African grey parrot as an example – he’s very famous. The only things is, we’re not sure how happy Alex was being raised like a human so maybe it wasn’t a good idea.

Question: How can birds comunícate to each other? Do they understand us when we talk?