Theory:

      And next day when she awoke she cried out good morning in a cheerful voice. She got no answer. She jumped out of bed and ran to the cage. She gave a startled cry, for there the little bird lay, at the bottom, on his side, with his eyes closed, and he looked as if he were dead. She opened the door and putting her hand in lifted him out. She gave a sob of relief, for she felt that his little heart was beating still.
Explanation:
 
When September awakened the following day, she greeted the bird with a joyful good morning. She was taken aback when she received no response. She sprung from her bed and raced to the cage.
 
September was struck by what she saw. She gave a shocking shriek. The bird, laying on its side on the surface, closed its eyes. He looked as though he had been dead.
 
She unlocked the cage and took the bird out. She could still sense the bird's heartbeat. As a consequence, she let out a relieved sob since he was still alive.
Reference:
National Council of Educational Research and Training (2008). It so happened. Princess September - Somerset Maugham (pp. 34 - 44). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.