See the hole in the back of the bird's tongue? It is called a glottis. It is the opening of their airway (trachea).
All birds have a glottis, you just can't see it very easily in most birds. The glottis is the opening of the pathway directly to their lungs. This is why you should never drip water into a bird's open mouth - the water flows over the tongue and goes right into the glottis, through the trachea and sits in the bird's lungs (this is called "aspiration".)
Unlike humans, birds cannot cough water out of their lungs, so the water just sits in their lungs and they slowly suffocate with each breath. In addition they can get a bacterial infection like pneumonia. This is why you should never drip water into a bird's open mouth. It takes a trained avian rehabilitator to properly hydrate a bird without causing more damage.
If you find a bird, keep it contained and dark and quiet. If it's a baby, keep it very warm and get it to a licensed professional immediately. It's not only the law, but the right thing to do if you want it to have the best chance of survival.