I attended a seminar yesterday given by a former lab mate of mine, discussing her work to understand the duetting call of the California Towhee.
(An article in the Berkeley Alumni Magazine discusses the work and links to a sound file of the call.)
She described how this squeal-call is almost never given by just one bird, and is almost always a duet by the pairs who mate for life (and cheat on each other like mad).
It has been called the "reunion squeal" because the pair always fly towards each other while duetting, and end the duet side by side.
So this morning I was in my neighbor's backyard and noticed a pile of feathers on the ground. They have four outside cats, so I was not surprised. I bent over to examine the feathers and see who the victim was. Drab brown feathers. The wing and tail feathers were too small for a robin or a jay, but too big for most of the sparrows and finches around here. Then I noticed the clump of orange feathers, just the color of a towhee's undertail coverts. A California Towhee, one of the most common birds in the neighborhood, had bit it.
As I stood up, I heard the squeal. A rather half hearted squeal I thought. Then I looked up to see the towhee, sitting on the fence, squealing. It wasn't a half hearted squeal, it was that this bird was doing its half of the duet alone. It was initiating the reunion squeal, but couldn't reunite, because its mate wouldn't squeal back. It squealed twice more in the next several minutes, far more often than is common among pairs.
The cats sat on their lawn chairs and looked smug.