I was helping my father put up new boards for the roof of my parent's house today. The old one was, well, old and rotten. It was the last remains covering the gutter on the north side of the house, we've already replaced the boards on the other sides of the house (got quite a thrill out of the top ones on a wobbly aluminium scaffold 8 meters above ground).
Anyways, as we've done about half of the boards and reach the far end of the house, a female blackbird starts examining the wooden box around the gutter carefully at the other end, apparently interesting in using it for a new home. A few moments later the male blackbird, maybe its mate, does the same while we stand there, hammering boards in a few meters away.
Leave things alone a few minutes and life around you starts occupying the good spots. Nature or perhaps rather evolution is wasteful with life, but certainly not with space.
I sometimes wonder how fast a city would disappear if all humans were all suddenly extinct by a new plague-like disease.