I watched as Bobby Hawk visited the nest two times during my hour and a half visit to Washington Square Park today:
He brought what looked like building material to the nest:
It resembled asphalt roofing tile. I saw Bobby descend to roof tops minutes before but I couldn't see where he landed. It's a shame because it would have been great to see where he got the material from.
Bobby then landed on various perches around the park before visiting the nest again: