Theory:

     "Thank you for nothing,” said September. “I have a pet bird which sings the most charming songs to me and I don’t know what on earth I should do with a green and yellow parrot.”

     “Well, my dear,” they said, “it’s absurd to talk of your bird when the little fellow flies in and out just as he likes.” They looked round the room and raised their eyebrows.
 
     “Do you mind our asking where your bird is now?” they said.
 
     “He has gone to pay a visit to his father-in-law,” said Princess September.

     “And what makes you think he’ll come back?” asked the Princesses.

     “He always does come back,” said September.

     “Well, my dear,” said the eight Princesses, “if you’ll take our advice you won’t run any risks like that. If he comes back, and mind you, if he does you’ll be lucky, pop him into the cage and keep him there. That’s the only way you can be sure of him.”

     “But I like to have him fly about the room,” said the young Princess September.

     “Safety first,” said her sisters ominously.

     They got up and walked out of the room, shaking their heads, and they left September very uneasy.
Explanation:
 
Princess September politely refused their offer when she realized she didn't need a new pet. She went on to add that she wouldn't know what to do with a green and yellow parrot. She even mentioned that she had a pet bird that sang her the most beautiful songs.
 
Disgusted by September's response, the princesses pointed out the small bird's tendency to fly in and out as he pleased. They thought September's fondness for the bird was pointless. They raised their brows and looked around the room to emphasize the bird's wayward behaviour.
 
They then inquired about the bird's whereabouts, to which Princess September responded that he had gone to see his father-in-law.
 
In an attempt to establish doubt in September's mind, the princesses slyly inquired whether there was any evidence of the bird returning. September replied in the affirmative.
 
The selfish princesses seized the opportunity to advise against the bird's certain return. They even suggested that if he returned, she put him in the cage for good.
 
Princess September stated that she enjoyed seeing him swoop around the room. The princesses responded by threateningly emphasising the need for safety and exited the room, leaving September disturbed.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
PopTo push, put, or thrust suddenly or briefly
2
AbsurdStupid and unreasonable
3
OmniouslyIn a way that suggests that something unpleasant is likely to happen
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.