Symbolism, Imagery, Allegory
When Tom, Sophia, and Master Blifil are all kids together, Tom gives Sophia a little bird to raise. She calls the bird "Tommy" and attaches him to her wrist by a string. Master Blifil tricks Sophia into lending him little Tommy, at which point Mr. Blifil releases the bird into the air. The bird flies away and, sadly, gets killed by a hawk.
Sophia's love for Tommy-the-bird obviously implies something about her feelings for Tommy-the-boy. But even beyond the obviously lovey-dovey (bird pun!) symbolism of this bird, we also think his fate has a deeper meeting. In a few short books, Tom is going to tie himself emotionally to Sophia in the same way that little Tommy-the-bird was attached to Sophia's wrist by a string. And then that link is going to be broken by the cruel, manipulative intervention of Master Blifil, just as Master Blifil stole Tommy-the-bird from his owner, Sophia.
Like Tommy-the-bird, Tom is going to be set free to fly a difficult road. So it seems like Tommy-the-bird's death by hawk is a bad omen for our hero. Luckily, Tom's fate is very different from little Tommy's. But the twists and turns of Sophia's bird's life strongly foreshadow the early relationship between Sophia and Tom, before Tom is forced to leave Somerset to find his own fortune.