Puzzling Robin Behaviors

My advisor, Eagle Eyes and I have noticed a few puzzling robin behaviors over the past few weeks:

• Robin parents vigorously defend an empty nest, giving their alarm call even after we have taken the young out of the nest and are processing them on the ground. They dive bomb us as we climb back into the tree to return the young to their nest. Why defend an empty nest? I think they do this because if they can drive off the predator even after the nestlings have been removed, there is a chance that some of the nestlings will survive unscathed and that they can then raise these nestlings successfully on the ground or in a nearby shrub if they are only a few days from fledging. I think they dive bomb us when we try to return the young to the nest because they are trying to get us to give up, release the young, and go away. Little do they know that is exactly what we have in mind too!
  

• Robins treat us as predators when we are nest searching, which is strange because people do not usually bother robins. I've written about this before in my post The Word Of The Day Is Surreptitious. I can't imagine that there are very many robins that have been shot at or otherwise harmed by people, especially in such a metropolitan area, so why are they so wary? I guess it always pays to be on the lookout for a large predator that is tracking your movements, especially when you have a nest in the area. But I don't think that is the whole story. I think robins are high strung birds. Some species seem to be more naturally "stressy," while others seem to be chill no matter what. Robins are definitely difficult to handle, kicking and struggling continuously when you have them in the hand; my advisor says they are "spazzes." So are wood thrushes, a close relative of the robin. Other birds that stress easily when being handled are chickadees and titmice. I also noticed that different species had different personalities as a species, not as individuals, when I was doing wildlife rehabilitation in CA. I don't know if there is any evolutionary advantage to being a spazzy, high strung species, but whatever the reason for this trait, robins fall into this group and I think that is the main reason they are so suspicious of us when we are observing them.

• Robins do not generally seem to recognize my advisor, Eagle Eyes and I as individuals even when we return to their nest site repeatedly. However, at our site in Baltimore there are three pairs along Oriole stadium that do seem to recognize us. Why would these three pairs recognize us but not others? The only thing that I can think of is that these robins are in a relatively secluded area despite being in a big city, and that they do not encounter a lot of people near their nests so that we stand out to them because we are a rarity.