The bird is a symbol of the individual soul.A bird is a usually a symbol of some soul power when it is not the soul itself. Birds often indicate either mind-powers or soul-powers.
The soul and life are two different powers. The soul is a spark of the Divine Spirit which supports the individual nature; mind, life, body are the instruments for the manifestation of the nature.
Every soul is not evolved and active; nor is every soul turned directly to the Divine before practising yoga. For a long time it seeks the Divine through men and things much more than directly.
The soul in itself contains all possible strength, but most of it is held behind the veil and it is what comes forward in the nature that makes the difference. In some people the psychic element is strong and in others weak; in some people the mind is the strongest part and governs, in others the vital is the strongest part and leads or drives. But by sadhana the psychic being can be more and more brought forward till it is dominant and governs the rest. If it were already governing, then the struggles and difficulties of the mind and vital would not at all be severe; for each man in the light of the psychic would see and feel the truth and more and more follow it.
As so he grew into his larger self,
Humanity framed his movements less and less,
A greater being saw a greater world.
A fearless will for knowledge dared to erase
The lines of safety reason draws that bar
Mind's soar, soul's dive into the Infinite.