Final answer:
Romeo claims to hear the lark because it signifies the arrival of morning, which means he must leave Juliet to avoid being caught.
Step-by-step explanation:
When Romeo says he hears the lark in Romeo and Juliet, it is because it is morning, and the lark's song signifies that dawn has arrived. As a result, Romeo must leave to avoid being discovered by the Capulet household, as he is now secretly married to Juliet and their families are enemies. The fact that the lark sings at dawn is used by Shakespeare to create urgency, a reason for Romeo's departure, signifying the fleeting time the lovers have together. In contrast to the lark, which sings with the coming of day, the nightingale sings at night; any reference to the nightingale within the play signifies nighttime and safety for the two young lovers to be together.