Answer: Birds pick up oil on their beaks, by rubbing against the uropygial or preen gland near the tail, and then rub it over their feathers. This makes the water unable to break through the oil.
Feathers repel water because birds pick up oil on their beaks and when they fluff their feathers later the oil is transferred onto the feathers. As water cannot penetrate through the oil coating, the feather is waterprof.
