Swallows' Paradise

Nakamora used to wonder what the building is for. He noticed not just one, but several. He noticed not just in the east coast but also in the west coast. To take it for a place to store grains (rice and corn as well, may be) to replace the old construct of inverted long slender bucket may not be quite correct. The building is not that tall. They are located not necessarily anywhere near paddy field. Thus it cannot be that place for storing anything, especially food item. Birds can easily enter it and finish them all.
The building is about 3-4 story tall with several small, slightly less than A4 paper size openings near the top floors walls. Other than those openings there are no other windows. The wall is plastered with cement and the owner did not bother to either paint it or smoothen the surface of the wall.
It was only a few months back that Nakamora learned that indeed the building is for birds to enter. Only now Nakamora has the opportunity to write about it. Not just any birds. It is for swallows to nest. Birds nests fetch good price as they are used as medicine.
It is nice to know. But at the same time a flash of thought on how that could alter the lifestyle of swallows in the future, or succeeding generations of swallow played in Nakamora's head. Perhaps future generation of swallows would no longer be interested to nest elsewhere, except in that kind of building... or could they lose the ability to nest except in buildings such as those swallows' paradise...