Theory:

     The man stared closely at Mr Purcell, as though just now aware of his presence. He said, “I want something in a cage.”

     “Something in a cage?” Mr Purcell was a bit confused, “You mean—some sort of pet?”

     “I mean what I said,” snapped the man. “Something in a cage. Something that is small.”

     “I see,” hastened the storekeeper, not at all certain that he did. His eyes narrowed gravely and he pursed his lips. “Now let me think. A white rat, perhaps? I have some very nice white rats.”

     “No,” said the man. “Not rats. Something with wings. Something that flies.”

     “A bird!” exclaimed Mr Purcell.
Explanation:

After hearing Mr Purcell’s voice, the man looked closely at him. He stared at him because he was aware of the storekeeper's presence at that moment. When he entered the shop, he looked at the pets and was utterly unaware of Mr Purcell's presence.

When the storekeeper realised the customer required "Something in a Cage", he became confused and inquired if he meant any pets. Later, the stranger said, in an angry tone, “I mean what I said,” implying that the man only required what he asked and nothing more than that. In addition, the man indicated that he needed something in a cage, which looked small.
 
When the storekeeper heard that, Mr Purcell hurriedly answered, “I see.” He didn't understand what the man was trying to say, and he didn't like the behaviour of the strange man. As a result, Mr Purcell narrowed his eyes gravely and pursued his lips, thinking what the man required. Then Mr Purcell thought for a while and said that he had some lovely white rats. When the strange man heard that, he promptly replied, “No.” Also, he stated that he did not want any rats; what he wanted was something with wings that could fly. Mr Purcell later realised what the man was trying to say. At once, the storekeeper exclaimed, “A bird!”
 
rat-198351_1920.jpg
A white rat in a cage
 
Meanings of the difficult words:
 
S.No
Words 
Meanings 
1
ConfuseOf a person unable to think clearly; bewildered
2
SnapSomething said in an angry way
3
HastenTo make something happen sooner or more quickly
Reference:
National Council of Educational Research and Training (2007). Supplementary. I Want Something in a Cage - L E Greeve (36-42). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.