Do birds fly in fog? very carefully.
Sillert asked that question. With thick fog in my garden, the birds are reluctant to fly. Those species that fly in flocks do a lot of squawking to each other, fly in more circles, and closer together when viz is poor. Of course they only have VFR, and a sense of the earth's magnetic fields. The Red Kites that frequent the Chilterns and now points west, are solitary in flight as a rule.
They certainly keep an eye on other air traffic, as we have shared thermals on many occasions.