A national television audience witnessed a massive swarm in March during the World Baseball Classic semifinal between the Netherlands and the Dominican Republic, and similar invasions have occurred throughout the year.
Hundreds of gulls sometimes land on the field during play. They also defecate on fans and create cleanup headaches for staff.
Giants shortstop Brandon Crawford said batters are well aware of the birds.
"You're focused on the pitcher," he said. "But you definitely notice them."
Biologists said they don't know how the seagulls time their arrival for the end of games to feast on discarded hot dogs, popcorn and other food.
"They are incredibly intelligent animals," said biologist Russ Bradley, a seagull specialist. "Unless you want to build a dome, there's no easy answer."
The gulls disappeared late in the 2011 season when a red-tailed hawk nicknamed "Bruce Lee" began frequenting the park. But when the hawk disappeared, the gulls returned.
Federal law prohibits shooting the birds, and hiring a falconer to scare them away would cost $8,000 a game, said Jorge Costa, the Giants' manager of operations. The Giants are also concerned a falcon could gruesomely kill a seagull in front of families and a television audience, Costa said.
"It's an issue we are taking seriously," Costa said. "But it's delicate. You don't want to get to the point where you do anything that looks inhumane."