The whole subject of bird flight is very interesting but it is difficult to find much published information on the subject outside specialist scholarly journals. Apparently Lielenthal wrote about bird flight but I have not located his book. I once posted a question here asking whether any bird can aerobat or fly inverted but no-one knew the answer. As to birds flying IMC, I read in some tome (I think it was "Weather Flying" by some US bloke)that it is believed that certain species of migratory birds may be able to fly in cloud, but little is known of this, as with other aspects of bird nav techniques.