What are the ways to keep the birds away from airfields?
The FAA has an FAQ on bird strikes and other wildlife hazards, which includes a (broken) link to their Wildlife Hazard Management at Airports - A Manual for Airport Personnel document.

Chapter 9 is called "WILDLIFE CONTROL STRATEGIES AND TECHNIQUES AT AIRPORTS" and includes the following techniques:


Change flight schedules
Modify the birds' habitat (food, cover, water)
Physically exclude the animals (not so easy with birds)
Repel the birds with vehicles, chemicals, sound, visual objects (flags, scarecrows, fake dead animals, lasers)
Repel the birds with trained falcons or dogs
Shoot nonlethal projectiles at the birds
Trap, remove and release the birds at a different location
Kill the birds with poison or other means


The document summarizes the various techniques like this:


  Habitat modifications to minimize food, cover, and water and physical
  barriers to exclude wildlife are the foundations of wildlife hazard
  management programs for airports. In addition, an integrated array of
  repellent techniques is necessary to disrupt normal behavior and to
  stress hazardous wildlife that attempt to use the airport. These
  repellent techniques must be used judiciously and backed by real
  threats to minimize habituation. To this end, lethal control of
  selected individuals of common species is sometimes necessary to
  reinforce repellent actions.