It appears I have been slandering the good name of the birds that have been hanging around my patio. I mis-identified them as cowbirds ... but am now pretty sure, thanks to Audubon CA, that they are California Towhees.

Photo from tringa.org, via Google Images.
Looks just like 'em. Here is what "All About Birds" says about the California Towhee:
"Your first encounter with a California Towhee may be prompted by a tireless knocking at your window or car mirror: these common backyard birds habitually challenge their reflections."
That is most definitely true. The patio visitors also exhibit the clear and persistent chirp noted by several commenters to an Audubon post.
My dilemma is actually worse now. I was quite willing to abuse cowbirds, up to and including throwing things at them and letting my cats try to catch them, because they are predatory parasitic birds responsible for decimating songbird populations in some areas. I despise cowbirds only slightly less than I loathe crows.
However, towhees are - to the best of my quick researches' ability to determine - inoffensive.
Except that they are deterring my hummingbirds (MY hummingbirds!) from visiting the patio. What to do? It's a small space. If I do nothing, the hummers will stay away all summer. They're a big part of my pleasure in that outdoor space (all the view we have).
The hummers I've seen so far this year were all, I'm pretty sure, new birds - that is, not territorially invested in my patio. There are other, better, territories in our neighborhood.
But is there, in fact, anything I CAN do? I'm not feeding any other birds. There's a little dish of water. Should I just roll with it, put out some millet, and have towhees this year? I may have no choice in the matter.
At least the cats will be entertained.