When a bird opens its mouth, you’re most likely to hear a chirp. This is how birds communicate. They open their mouths and let out a series of chirps. Chirp chirp chirp!
The best place to experience the chirp chirping of birds is when you’re out in nature (な). You’re not going to hear birds chirping in a crowded city, so you need to get out into nature to fully experience the birds and their chirps.
Currently there are no similar looking kanji which exist in the WaniKani learning system