I know exactly what you're talking about and that is so adorable! I've never seen a kitten that young doing it, so you should definitely get some footage to share. I have owned cats for most of my life and to tell the truth I'm not so sure what exactly the cats are saying when they do this. I have an old apple tree in my front yard with a couple of bird feeders and my cat does that all the time too, though he is a senior now. From what I can gather, your kitten is expressing that he is excited to see the animal activity. Perhaps he wishes to play with them or eat one. Either way share in your pets extreme joy and pleasure in watching the birds and squirrels.