While rain is falling heavily, a bird usually keeps up a posture; it will keep its head backward, its body erects, and its beak is looking up for the rain.
Maintaining this stance will allow a bird to preserve more energy while also reducing the contact the rain makes with the body, causing raindrops to slide off the feathers. Birds cuddle each other to prevent rain and to stay warm. Some birds can still weather a drizzle, but birds have to seek cover and protection in the case of a storm.
In the case of small birds, a storm or heavy rain is a threat to them. However, their size can also be a blessing to them as it means they can seek cover from the rain in small places. Small birds can keep warm in small and cozy places; this helps them keep warm and not lose their body temperature.
However, if the rain persists for a long time, they have to look for food to keep their energy up. These little birds need to feed regularly to energize themselves. Big birds, however, because of their size, fare better during the rain than smaller birds. They may even decide to keep flying if there is no stormy weather. However, when the rains get heavy, it can disrupt their flying.
Rain can knock big birds off their routes when flying; the rain can also make their vision blurry. This can cause them to crash into buildings, trees, windows etc.