Bird condos are good investments if the owner can get the grey chested swiflet to live there. Birds' nests made by them are clear and demand the best prices from Chinese buyers. Black chested swifts make nests that have grass mixed in and the brown chested swifts make nests that have mud mixed in, both have to be cleaned by hand with great effort. The bird droppings in the condos are also bought every three months to be used as natural fertilizer. The swifts eat fish and lots of insects so their droppings are rich in potasium and phosphate.