My personal favourite's probably the European Blackbird (Turdus Merula) but there are excellent birds to listen to worldwide.
Some birds can sing two different notes at once? They produce their songs from their SYRINXes, V shaped fork in where their bronchus splits into the two pipes called bronchi which lead to their lungs. The muscles of each bronchus can produce different notes. Some can even sing one note whilst modulating the other.
What's more I'm sure that a lot of bird song is built on recognisably consonant scale forms, I think a musical ear is something common to birds and humans alike, perhaps this is only to expected given that for a particular length of resonating material particular harmonic overtones are more natural anyway.
If I couldn't laugh at myself how could I laugh at someone less ridiculous?