A Bird in the Hand is one of Aesop's tales.
The tale[]
A nightingale was roosting in a tall oak tree, singing as they always do. A hawk saw her, and as he had nothing to eat, swooped down and stole her away. She worked to free herself from the claws of death by beseeching him to let her go. She was too small, she said, to make a meal for a hawk; if he were hungry, he had better hunt some greater bird. But the hawk’s answer was: ‘I would be mad to let slip the food I have in my claws to go after something which is not yet in sight.’
§ It is the same with man. It is mindless to let the hope of greater winnings draw thee away from what thou hast in thy grasp.