Birds spend most of their time caring for their feathers; they use their bill to sort through their feathers, cleaning off parasites and dust; they smooth and align the small interlocking barbules that act like tiny zippers to hold the feathers together neatly. This personal grooming keeps their feathers well maintained and helps to keep water from reaching the skin.
You may also see birds reaching around to their rump with their bill, often with their tail fanned as they stretch around.
There is a special preening oil gland at the base of the tail in most bird species (called the uropygial gland). Birds wipe a waxy oil from this gland onto their bill and crown and apply it to the rest of the feathers. The preening oil makes the plumage shiny. Well cared for feathers repel water by their fine structure - more so than any properties of the preening oil itself.
So during a light rain shower birds generally stay out, finding food and living their lives. But if the rain is too harsh, or accompanied by winds, then the birds need to seek shelter.
What can we do to help birds during periods of bad weather?
Most wild birds are not that strong fliers, as they can be in danger of flying into objects such as power lines, tree branches in strong winds. They could also be hit by twigs or leaves blown by the wind.
During storms birds can hide in bushes and dense trees, they may be able to find calmer areas on the quiet side of a wood, protected from some of the winds.
Why do we not see birds flying in the rain?
Certain birds can fly in heavy rain such as geese, ducks, swans and gulls.
During storms, though they use a lot more energy to fly and it does become harder to find food and to refuel. So flying when the weather is stormy is not that advantageous so they generally find a place to sit out the storm. They do not migrate in heavy rain either unless they get caught by surprise.
They just keep doing what they do best.