Dark 'eyeliner' feathers of peregrine falcons act as sun shields to improve the birds' hunting ability, a new scientific study suggests. Scientists have long speculated that falcons' eye markings improve their ability to target fast-moving prey, like pigeons and doves, in bright sunlight. Now research suggests these markings have evolved according to the climate; the sunnier the bird's habitat, the larger and darker are the tell-tale dark 'sun-shade' feathers.