How do birds remain waterproof?
It isn't the natural oils on bird feathers that make a bird
It was originally thought that a bird's ability to remain waterproof related to the oil-producing gland located on their back. While it's true that birds do coat their feathers with this oil as they are preening, it isn't what makes their feathers waterproof. The natural oil that birds distribute across their feathers as they preen acts as a conditioner. A bird's waterproofing actually relates to the position and alignment of their feathers. When you see a bird preening, you are witnessing the animal meticulously aligning each of its feathers so that they are perfectly interlocked.
The interlocking hooks and barbules on feathers is what makes a bird waterproof
Bird feathers are amazing when you examine them closely. Feathers have a series of hooks, barbs, and barbules, which allow feathers to remain tightly locked together. This interlocking of hooks and barbules is what actually provides an airtight seal allowing bird's skin to remain insulated from water and the elements. In addition to the hooks and barbules locking together, a bird's feathers lay flat against their body similarly to the shingles on your roof. Not unlike the shingles on your roof, if feathers are not in alignment, they are not waterproof. As you can imagine, this is why birds spend so much time preening and aligning their feathers-their survival depends on it.
Glencoe Science: New York Science, Grade 8
Mostly they will seek shelter under natural cover. The oil that birds have on their feathers is not a water-proofer, the waterproof nature of feathers is a function of their structure. Birds with oil glands surgically removed are still waterproof but suffer vitamin D defficiency. (Only a few storks and closely-related birds, such as turkey vultures, have any appreciable sense of smell).
The feather is not waterproof because of its construction. Birds spend a great deal of time grooming their feathers (preening). While they preen, they squeeze an oil gland at the base of their body, and spread this oil over their feathers. This is what causes the feathers to repel water. The feather construction, however, is amazing and a worthwhile study.