Who builds the nest?

First of all, it is amazing that a little bird that hatches out of an egg one June morning, can the next year build a nest of its own that is a typical specimen for its species (with no training even). Secondly, it is interesting that often it is only the female that builds it. The male presumably is above such domestic endeavors.

But not in all species. I'm not positive (and because this is a blog I'm not required to verify it), but I believe that the vireos and possibly other groups engage in dual-sex nest building. (Go check your Birders Handbook and post the answer for me below.) I seem to remember watching two blue-headed vireos both collecting nesting material together once somewhere near a hemlock-lined stream in Pennsylvania. Maybe it was a dream.

Now for the point of my story. Today I witnessed a common thing: a female bluebird making repeated trips to a nest box with a mouthful of nesting materials (grasses, pine needles, etc.). But the interesting thing was that the bright blue male followed her every step of the way, but did none of the work. He would fly with her to a spot several hundred meters away to collect stuff (staying within a few meters). Then he would fly all the way back to the nest while she put it in the box. While she was in there, he would perch on top and sometimes come down to look in.

This has two potential explanations, as far as I can tell: 1) he was guarding her against being cuckolded by rogue males (as she was likely approaching fertility at this point), and 2) well there is no 2. But it is also possible that this "male accompaniment" is some sort of transition between "male doesn't help at all" and "male helps build nest." No proof, but why not. I'm pretty sure there are also cases in which the "male helps a little bit."