Answer:
it only sings at night
Step-by-step explanation:
Juliet wants the bird to be the nightingale instead of the lark because the nightingale only sings at night, while the lark sings in the morning. If it's the nightingale they heard, that means Romeo can stay with her a bit longer, but if it's the lark, then Romeo has to leave - her room, and Verona.