Theory:

     It seemed to Princess September that her little bird had been away a long time. Something might have happened to him. What with hawks and with snares you never knew what trouble he might get into. Besides, he might forget her, or he might take a fancy to somebody else. That would be dreadful. She wished he were safely back again.
 
     Suddenly September heard a ‘tweet-tweet’ just behind her ear and she saw the little bird sitting on her shoulder. He had come in so quietly and alighted so softly that she had not heard him.

     “I wondered what on earth had become of you,” said the Princess.

     “I thought you’d wonder that,” said the little bird. “The fact is I very nearly didn’t come back tonight at all. My father-in-law was giving a party and they all wanted me to stay, but I thought you’d be anxious.”
Explanation:
 
Princess September was in a troubled mind due to her eight sisters' misguided direction. She looked forward to the arrival of her pet bird. The bird was absent for a very long period, according to her.
 
The Princess' anxiety for the welfare of the small bird peaked as time went by. She worried how his wellbeing might be challenged by the hazardous atmosphere. The likes of hawks and snares could cause the biggest damage.
 
Collage.jpg
Hawk and the snare
 
The Princess had to go through a tornado of emotions while waiting for the bird to arrive. The Princess was troubled by the notion of the bird choosing someone other than her. She shuddered at the thought of it. Her emotional frustration heightened her concern for his safety.
 
She sensed a 'tweet-tweet' noise beside her ear all of a sudden. The tiny bird was perched on her shoulder, content. The songbird had come in silently and leapt without leaving a trace. So she had not been alerted of the bird's arrival.
 
The Princess instantly expressed worry to the bird. She had inquired as to what had happened to him. The bird's attitude was calm in response. The bird informed the Princess that his father-in-law had not let him go quickly. The celebration at his father-in-law's took a little longer to wrap up. However, the bird had chosen to abandon the party as the Princess would be upset.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
SnaresA trap set to catch birds
2
AlightedTo land on something
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.