Skytongue is one of the three languages of the Bravelands.
Skytongue is spoken by all species of birds, even flightless birds. There are some exceptions like Bats speaking Grasstongue and Skytongue.
The Great Parent can speak all languages, thanks to the Great Spirit, including Skytounge letting him/her speak to the flocks of birds who live in Bravelands.
Sometimes Vultures (more specifically Windrider) can briefly speak to other animals who do not speak Skytounge if they drink from a special pool in the mountains that contains The Great Spirit.