L. Frank Baum 
Policeman Bluejay [EPUB ebook] 

Two small children, Twinkle and Chubbins, are enchanted by a wicked ‘tuxix’ so that their bodies become those of birds. While so transformed, they can speak to all the birds and animals, and they learn a great deal about the wild creatures of the forest. Of course, this is no ordinary forest; it is a nature fairyland, and the Royal Necromancer of the bird Kingdom of Paradise informs them that they must each eat a tingle-berry as antidote to undo the evil enchantment. So they set out to find a tingle-berry bush and lose no time in resuming their natural forms.