In Act II, Konstantin enters with a gun in one hand and a dead seagull in the other. The seagull represents at least two large and significant symbols. In addition to the symbols, the seagull has another function in that, though no gunshot is heard at this moment in Act II, it represents foreshadowing of the final moments of the play when an offstage gunshot actually is heard and Dorn announces in the play's final line: "You see, Konstantin has shot himself."
Since Konstantin has laid the dead bird at the feet of the young actress with whom he is in love, Nina, and said to her, "I shall kill myself in the same way soon," it can be assumed that Konstantin sees himself as doomed. He believes that his art is not appreciated and that the woman he loves does not respect him as an artist or love him as man. The seagull, in this case, represents the vulnerability of Konstantin to being "shot down" by critics of his work and the woman he loves.
But the seagull is also a symbol for the young woman at whose feet Konstantin lays it -- Nina. In Act I, she herself has likened her carefree and happy life on the lake to that of a seagull. But, when Trigorn, an older, famous writer, enters the scene in Act II, just after Konstantin exits, and sees the bird at Nina's feet, he says:
Got an idea. An idea for a short story: A young girl has lived in a house on the shore of a lake since her childhood, a young girl like you; she loves the lake like a seagull, and she's as free and happy as a seagull. But a man comes along, sees her, and just for the fun of it, destroys her like that seagull there.
And this is exactly what Trigorin does to Nina. When she returns in the final act of the play, hoping for a glimpse of the man who has destroyed her, she confesses this to Konstantin. So, both Nina and Konstantin are connected to the symbol of the seagull. They both attempt to fly free and attain their dreams, and both are shot down and destroyed.
For more on Nina, Konstantin and the seagull as symbol, please follow the links below.