The Traditional Yoga Association

In this book Swami Ambikananda explores the essence of the asanas and their relationship with the flow of vitality through the tattvas (elements) housed in the chakras.

From Healing Yoga on Kapotasana (Dove Posture):

“The bird imagery helps us in opening and balancing... Like us, birds are bound to the earth's atmosphere by gravity — and yet they alight on earth and take off from it again... Our practice as Yogis and Yoginis is to see the divine in the material. We have to have knowledge of the tattvas but to see them as ‘isms’ of the divine. Then, like birds, we are not bound to matter - we are free to soar.”