Step 1: Poetic image.
The porter's heart is compared to a bird — trembling, fluttering, and emotional.
Step 2: Checking options.
(A) crow: Incorrect.
(B) rabbit: Incorrect, rabbits don't twitter.
(C) squirrel: Incorrect.
(D) bird: Correct, "twitter" belongs to birds.
Step 3: Conclusion.
Correct answer is (D) bird.