What do Robins Hear

Pleasant end of the afternoon. I went to the North Central Gate at 3:30. (One tree down across the trail south needs a saw.) From the herpetology array I went west. This section could still use some additional clearing of Verbesina frostweed and Symphoricarpos deerbush from summer's growth.
In the center of the Woods there were a couple of white-tailed deer, a doe and yearling. I waved to them and sang. They stared for a while, frozen in place, and then nervously twitched their tails and began to feed again.. partly ready to dismiss me as the harmless local nut.
There are small patches of green Stellaria and Glechoma (?) Gill over the.. in the Woods west of the trail south of the East Pond. The patches are discreet, the size of a bathtub or maybe 2-3 times that. Interesting to return with new spring canopy and see if extra light (beneath dead trees?) explained the location of these spots of green.. or water .. or?
   I took the trail along the west side of the Wash and stopped to watch the robins. There were only 30-40 in sight at any one time but the air was filled with the calls of a hundred or more.
I wonder what they hear. Is it all noise to them.. or do they pick out particular messages from particular individuals? I sat and listened for a quarter of an hour. Then abruptly (in < 2 seconds) 90% of the calls stopped. I looked up to the sky and saw a short burly owl taking flight from the wash, heading west across the Woods accompanied by a troupe of crows.  Another minute gone by and all was returning back to normal.
  I was sitting across the Wash from the oldest and largest Albizia mimosa, I had cut a year or more ago and started thinking about invasive plants. The mimosa stump now has a cluster of two-meter high stump sprouts.. wilting after the cold freeze Saturday night. Down in the Wash walking north to the Bur Oak bridge, I was struck by the still green and fresh curtain of Clematis virgin's bower growing there up into the sub-canopy branches. On the sides of the Wash north there are many stems of Lonicera maacki asian amur bush honeysuckle. I need to cut them and watch to see if they resprout.
Returning up the hill along the Northen Rim trail I spotted the two Ailanthus Tree-of-Heaven meter high saplings that remain, after we pulled up 30+ in the same area. I am inclined to leave them and watch their development.