Birds. Specifically the nightingale (a night bird) or a lark (a morning bird).
The morning (A Lark is a bird typically heard calling in the morning); after Romeo and Juliet's wedding night.
Juliet claims that Romeo hears the morning lark singing when she wants to convince him that it is not yet dawn and he does not need to leave.
Romeo spends his last night in Verona consummating his marriage with Juliet before he is banished. He then waits until the morning to flee to Mantua, where he will eventually learn of Juliet's apparent death.
Evaporation
Juliet sends a messenger to Romeo in the morning to find out if he still wants to marry her as they had discussed the night before. She is eager to know his intentions and is anxious to hear his response.
In Act 3, Scene 5 of "Romeo and Juliet," Shakespeare uses the lark as a symbol of the morning and the nightingale as a symbol of the night. The lark's song signals morning and Juliet's plea for Romeo to stay, while the nightingale's song signals night and Romeo's need to leave before dawn. These birds serve as a reminder of the urgency and limitations of Romeo and Juliet's love.
Early Monday afternoon. The nurse goes to meet Romeo at nine and they chat and then she returns and talks to Juliet, taking her sweet time about it. By the time Juliet got there it would be probably just after noon. It is mid-afternoon when Romeo has his run-in with Tybalt.
The nightingale, a bird that sings in the night and the lark, a bird that sings in the morning.
She tells him that the bird they hear is not a lark (a morning bird) but rather a nightingale (a night bird).
Juliet was given the sleeping potion on Tuesday, and was supposed to drink it on Wednesday night, but Lord Capulet changed the wedding date to Wednesday so Juliet drank it Tuesday night. Juliet woke up late Thursday night or early Friday morning and killed herself. So she was sleeping for about two days, forty-two hours to be exact.
Scene 4 of Romeo and Juliet takes place at dawn. At the beginning of the scene, Romeo and Juliet debate over whether the birds song they hear is a nightingale or a lark.. This means the night was just turning to morning.
Juliet thinks she hears the nightingale, which is a symbol of love and longing. Its song indicates that it is still night and Romeo can stay a little longer before he leaves.