Magical mystery doorways! I thought. Before someone pointed out that these are birdhouses, specifically for swifts.
In recent years people in Can Gio (75kms from Ho Chi Minh city) started farming swift birds for their nest and egg. Swift nest made from the bird’s saliva is a highly prized delicacy in South East Asia.
Unlike poultry, you have to seduce the birds and lure them to this “den” by maintaining the right temperature, sound and particularly by spraying chemical pheromones all over the building. The popular brand on the market is Close2You Aroma. Great name.