When the Birds Fly South - Stanton A Coblentz

Dan Prescott, an American adventurer, discovers the hidden valley of Sobul in a mountainous region of Afghanistan, inhabited by a strange race of winged people known as the "Ibandru". He falls in love with one of them, Yasma, and they marry in a scene of general celebration. When fall comes, however, the Ibandru abandon their valley to fly south with the birds for the winter, taking Yasma with them. The importance of Stanley Coblentz's Lost World fantasy was confirmed by its reprinting as the 23rd volume in the prestigious Newcastle Forgotten Fantasy series in 1980. - Summary by Adapted from Wikipedia