Birds flying by--usually looking for nectar--will settle for fruit if it's available. Since it's highly unfeasible to sit outside waving your arms and yelling at birds all day as they land in your fruit trees, a little more ingenuity is required to keep them away. Keeping them out requires a little effort, but the results are well worth it.
Utilize something shiny and reflective to keep birds away. Hang old used CDs from string or tie glitter tape around the branches. Aluminum pie pans can work as well. Birds don't care for the sparkles from the light reflecting off these items and it may keep them away.
Cover small fruit trees in netting to make it harder for the birds to get to the fruit. Drape the netting over the tree and tie it together at the bottom to keep the birds out.
Place plastic snakes in your tree and around the base of it. Birds suddenly aren't as hungry when they have to brave a snake to get to the fruit. Fake owls can work as well--just make sure you move the snakes and owls around frequently so the birds don't catch on.
Create an alternative feeding place for the birds, but make sure that you don't do it near your fruit trees. Set a birdbath or bird feeder where they can get to it.
Cut up an orange and toss it in your yard away from the fruit tree. Birds are less likely to damage the fruit on your tree if there's other food easily accessible.