With high raises on the increase in a city like Chennai, the natural habitats of various birds & animals seems to be vanishing very quickly.
This leads to having concrete nests for these pigeons in the window sills of various tall buildings. I have always thought / seen these earlier only in Mumbai till a couple of years back, but nowadays chennai also is moving in the same direction.
P.s. : hope you could see the pigeon in the picture on the other side of the window glass !