The only thing I can think of is that maybe the robins have learned that the chickadees aren't intimidated by them, but the bluebirds are. In my experience, bluebirds are very gentle and don't put up much of a fight. It's funny.....I've often wondered how various animals can be right next to certain other animals (even larger ones), and not be intimidated, but they might be scared off by other things. I guess that's part of animal instinct we might not understand.

I wouldn't worry about the blue birds, they are tougher than you think. I have several pairs inhabiting my landscape. They are frequent users of the suet cakes and shelled sunflowers. They seem quite capable of holding their own. But, yes there is a pecking order among the species. The red headed woodpecker is the undisputed king, even the blue jays wont mess with him !

Last year, I had a bluebird pair raise their young in one of my houses. It wasn't long after they vacated the house before a chickadee and another bluebird squabbled over the same house. I witnessed the bluebird grabbing the chickadee by the head and pulling it out of the birdhouse !

I have been trying to keep Red-breasted Grosbeaks in my yard this Spring instead of disappearing like they did last Spring, so when a very small Hairy or Downy woodpecker attacked one of my male RBGs on a feeder I nearly passed out. The RBG never came back! But the Hairy or Downy comes all day long every day. Grrrrrr.

I agree with Old Forester's comments above. I have been observing and photographing bluebirds (professionally) for over 20 year and I can confirm that they are very tough and very resourceful. Mealworms are almost always only a small portion of their diet (like a treat or a snack). They are constantly catching insects and worms all day long (literally hundreds of times each day!) And, if necessary, a Bluebird will hold it own. In fact, when they are nesting, I have seen them attacking and successfully chasing away Robins, Thrashers, Cardinals and a variety of smaller birds. They will even attack squirrels and cats.

Here is the link to my site. In case anyone likes bluebird pictures as much as I do! Link: BluebirdsPlus

The aggressive robin? Shoot him.

I know this is an old thread but I have about 16 robins sometimes this winter (Febr 2025) while they're in winter flock mode in New Hampshire. They guard the feeding area but now 2 days using a bluebird feeder that excludes them. Today they are still chasing away bluebirds but not as many robins as before.

I think in this deeply snow covered winter, the robins will have to leave for their own survival soon.