A dog whistle makes a tweet too high-pitched for human ears to hear. It turns out that the Song Sparrow can do that too. At least this one did. Sometimes I could hear a very high note just at the start of its song, and a few choppy notes at the end, but I heard nothing in between. I have to assume that the female has ears that can hear this song. I suppose it’s possible that the bird is just pantomiming singing to fool the photographer and is actually not making any sound, but that seems really unlikely, doesn’t it?
Comparing this bird’s song at that time to its song the next day, and to the song of other Song Sparrows, it becomes clear that singing at humanly inaudible pitch is just one option. In the video below, the second bird singing is (almost certainly) the same bird as in the video above, viewed the next day, and most of its song is in the human range. Two other Song Sparrows from different parts of the park sing almost entirely in human range, with only the occasional note up in the dog whistle register.
Did the weather have something to do with this? The supersonic song came on a sunny day. The audible tunes in the video just above came under a cloud cover. Something to study, there.