Theory:

     "There’s no joke,” said September, “but some of mamma’s cats are prowling about tonight, and I think you’re much safer in there.”
 
     “Well, just for this once I don’t mind,” said the little bird, “so long as you let me out in the morning.”

     He ate a very good supper and then began to sing. But in the middle of his song he stopped.
 
     “I don’t know what is the matter with me,” he said, “but I don’t feel like singing tonight.”
 
     “Very well,” said September, “go to sleep instead.”
Explanation:
 
September defended her capture of the bird by pointing to the kingdom's sneaking cats. As a result, she considered the cage to be the safest bet.
 
With the prospect of being released the next morning, the bird consented to remain in the cage. The bird even stated that he didn't mind for that particular occasion only.
 
The bird then feasted itself on a delicious meal. He then divulged into one of his typical lullaby sessions. He unexpectedly stopped midway of his wonderful vocal performances.
 
The bird was puzzled about what had happened to him. He told the Princess that he was not in the mood to sing. September, on the other hand, had no complaints. She suggested that the bird sleep.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
ProwlingMoving slowly
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.