21 February 2017
The familiar real estate addage is "Location, location, location." For birds, the same is true of habitat. As a birdwatcher, you tend to learn what areas just "look good" for birds. If you're birding in the desert, and you see something like this, I recommend you stop and have a look around:
The pioneers knew that a good cluster of trees, often cottonwoods, meant water just underground. Naturally that was a good place to sink a well – especially if you had to dig it by hand! So, in western states, windmills are practically a signpost for "oasis." In this case, that was only a few hackberry trees, but sure enough this mini-oasis harbored a half dozen Western Bluebirds.
Looking around, I saw nothing but arid grassland in every direction. I think of Western Bluebirds as more of a forest species, but in the winter they do hang out at lower elevations where they can find adequate food and shelter. Netleaf hackberry (Celtis reticulata) is a great tree for all kinds of wintering birds, but in my mind hackberries = thrushes. Vagrant thrushes like Rufous-backed Robin are invariably discovered near hackberry trees or other fruiting trees and shrubs. So check 'em out!
When you look at the pea-sized fruit up close, it doesn't look like much. When you crack open the endocarp, there's just a thin layer of mesocarp. Most of the berry is really just a seed.
It must take a lot of these berries to fill up a bluebird belly! But, the fruits dry out and often hang on the branches throughout the winter, making them a reliable food source. And when they're newly ripened, they're actually somewhat palatable to humans, too. I don't recommend them by February, though...don't ask me how I know.
eBird list at: http://ebird.org/ebird/view/checklist/S34635985