Yes, that is a bird's nest. Or rather a according the artist, a nest installation constructed of “the crowns of 23 silver birches, two straw bales, one bucket of spit, three bags of sand, twelve buckets of glue, and nineteen cans of polyurethane foam.”
For about a week now, Benjamin Verdonck, the Belgian artist, has been living in his nest outside the 13th floor in Rotterdam, Netherlands. His performance has been including dropping bird things to his audience like feathers, a giant egg, and "copious amounts of bird shit".
Bring an umbrella if you visit it!
Or bird shit free on Super Touch for more details.