What is the secret to navigating the waters off Punta Margarita?

Captain Kirk shows Tully that the secret to navigating the shallow waters off the island of Punta Margarita is to shoot at the birds. Years before, a captain had placed wooden decoys of popular shorebirds, like herons and egrets, along the channel. Kirk shoots at the birds, causing the real birds to fly off and leaving the decoys to show the way through the channel.