In the play, the seagull is a symbol for Nina. In the beginning, she is pure and innocent. She "flies" freely.
She makes the mistake of falling in love, at least she believes she is in love, with Trigorin. Before leaving the estate on his first visit, Trigorin shoots a seagull. He does it because he can just as he uses Nina and destroys her, because he can.
When she compares herself to a seagull at the end of the play, she jumps between saying she is a seagull and being an actress. Due to what has happened to her in Moscow and her affair with Trigorin and the loss of their child, plus her family's rejection, she is in a delicate mental state. She has been destroyed like the seagull Trigorin shot.
At the beginning of the play, the seagull is a pleasant symbol to Nina. It represents safety, warm memories, and freedom.
At the end, it is a negative symbol for her. It represents the destroyed state she is in, because of love.
In the expository portion of the play, it is a positive imagery which symbolizes freedom ang purity, but in the latter part, it turns negative.
Normally, seagulls are attracted to the sea, but Nina says she's attrackted to the lake. It subtely implies she's attrackted to a wrong place. that's how it becomes negative.