A sonic shield could safely drive birds away from farm fields and tall buildings.Farmers aren’t much better than scarecrows when it comes to protecting harvests from pesky birds:Avian interlopers cost the United States roughly $1.9 billion each year in crop losses and damage control. Now researchers have landed on a potential solution that uses speakersto send out a directional buzzing noise — a sonic shield — to keep feathered pests away from farm fields as well as tall buildings.