Earlier post "Aliens?"

In a comment by belin104 to an earlier post, he suggested they might be purple martin houses. I did a little Googling and found this picture of purple martin houses at BestNest.com ...
I think belin104 nailed it. This looks pretty much like the picture from Glendale Lake.

Purple martins are a bird species that migrate from Brazil to North America and Canada. They are colony dwellers, which means they live as a group in one dwelling or house with many nest compartments. While traveling north in the spring they establish new colonies and also return to old housing to breed, raise their young and eventually migrate back to Brazil for the winter season. (from Purple Martin Central)

However, I'm not totally giving up on aliens just yet :)