Do birds speak complex languages?
Not really. The songs themselves are complex, but there’s no actual meaning behind them other than “look at this wide range of sounds I can make”. To a potential mate, a complex song may be attractive because it takes time to build a wide repertoire, which implies a level of fitness which has allowed the individual male to survive.
Yeah. If they’re reared in isolation or with another species they’re unable to effectively communicate with their own kind. Their speech patterns can even vary regionally. It’s some pretty cool stuff