Today I spent 25 minutes around 2 pm watching the UNO tern colony from an adjacent building.
28 adults were sitting- apparently incubating. Some of these locations are different from where I mapped them sitting last week, and may be new nests (re-nests after failures).
Young were more in evidence- 5 or so visible, all still downy, including one large enough to be running large distances over the roof (which I did while I watched). Still weeks away from being capable of flying off, however. Another four adults were up close to metal fixtures, holding their bodies in posture that suggested they were sheltering young beneath.
Two adults had chicks shading on the north side of the same metal fixture. One of these adults took a stab at the other's young, resulting in aerial pursuit of the aggressor by the assailed chick's parent. I wonder what prompted the stab? The aggressor moved a fair distance to reach the young bird before pecking it- it was a quick, deliberate aggressive attack rather than a get-away-from-me action. The pecked chick was a bit larger than the two belonging to the aggressor- perhaps it had been somehow been interfering with them (?).
One mass alarm event happened, raising many birds off the roof- when a Mississippi Kite came over fairly low.