The tree dream at the beginning of the novella signifies the radical change in luck and fate that Santiago Nasar will experience. After a late night of celebrations for the wedding of Angela Vicario and Bayardo San Roman, Santiago awakes from a happy dream where he was moving "through a grove of timber trees where a gentle drizzle was falling.” Feeling content and free of worry, he looks forward to the day and the arrival of the bishop’s ship. The previous week, he had dreamt that he was flying though “trees without bumping into anything.”
Up to this point in his life, Santiago has met few obstacles (the trees), except for his father’s death. Instead, he has freely enjoyed many pleasures (the gentle drizzle) like chasing women and deflowering virgins. In fact, Santiago may have been the man who took Angela's virginity.
Upon awakening, however, he feels like he is "completely spattered with bird shit." His life will take a drastic turn as foreshadowed by the title Chronicle of a Death Foretold. The couple’s wedding night’s abruptly changes from celebratory joy to rejection and vengeance when Bayardo discovers that Angela is not a virgin. He sends her home in shame. Her twin brothers Pedro and Pablo force her to confess the identity of the man who defiled her—Santiago—and then pursue him to murder him. Santiago is no longer a carefree bachelor who does what he pleases (like sexually assault women, especially those younger and in lower social positions), but a hunted man. He is no longer like a bird of prey but the prey itself.
Ironically, being pooped on by a bird is supposed to bring a person good luck. In Santiago’s case, however, his feeling that he was doused with bird poop signifies his downturn in luck and foreshadows his death.