Why didn't Blu learn to speak human?
- Bird's can pronounce human words so he can't use being anatomically different as an excuse.
- They can pronounce them, but they don't understand them (with extremely rare exceptions like certain African Greys). They just treat human words like odd-sounding flock calls. That Blu can't understand human talk is actually a bit of Fridge Brilliance. Besides all that - Blu and Linda understand each other just fine, human talk or none.
Why didn't Tulio threaten Linda with legal action in order to get Blu?
- It's understandable why he wouldn't want to, being a nice guy and all, but it can't be legal for Linda to own such a rare bird. Plus, you would think she would need some sort of license to own Blu, which, given that Blu literally fell off a truck she probably doesn't have.
- You answered your own question - It would be out-of-character for him to do so.
- There's also a huge pile of practical and ethical problems with taking birds that bond tightly to their owners (like macaws) and throwing them bodily into unknown situations - the stress alone can kill them. Blu was raised by Linda from hatchlinghood. He'd have to be gradually acclimated to free-living, which actually happens over the course of the movie.