How do birds remain waterproof?

It isn't the natural oils on bird feathers that make a bird waterproof.

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.

Sources:
[related links]
Glencoe Science: New York Science, Grade 8