from the video description on YouTube:
This video celebrates the long musical trill of the American Toad, which Henry David Thoreau described as the "Dream of the Toad." Trills may last thirty seconds or more. To make their amazing sound, a male first pumps air into his midsection. Then he closes his nostrils and quickly inflates his large vocal sac by contracting his midsection. The trill is produced as powerful muscles in the midsection force air across vocal cords in the throat.