I really like the bird metaphor a lot. It's sweet and is precisely the sort of comparison/explanation she'll understand and enjoy.

Are you and her mom making sure you're breaking down the syllables of troublesome words and practicing them with her? That extra reinforcement will help what she's working on in speech therapy, and if you incorporate that, say, as part of the process of your reading together, she'll just interpret that as quality time with you guys, which she'll love on lots of levels. I had some trouble with saying words correctly when I was small. It's a very common problem and one they usually grow out of. There were some speech pathologists at my school who helped a time or two, and then my dad, who's a linguistics professor, decided he could do a better job and he apparently did because the problem resolved itself.