I spent some time on the roof of my garage today. I needed to sweep off the defunct flowers of the local Live Oaks, but I took my recording gear along in hopes a bit of serendipity would save me from a shut-out on April 23. True, I do have a recording at a chickadee nest on 4/23/96, but occasional cheeps aren't terribly entertaining.
Serendipity could be nicely manifested by a Blue-Gray Gnatcatcher. I have repeatedly eyed gnatcatchers in the oak that shades that garage. I bet they have a nest saddled on top of a 2-inch limb somewhere in that canopy, and I plan to go up with video camera when they are feeding young to get some close-up action shots (from a safe distance of course). Needing sound now, I would have happily jumped the gun on that project.
In fact, serendipity came, not surprisingly, clothed in red. That cardinal sings from this tree often, and sometimes he and the gnatcatchers seem to be at odds. I thought he might show up. After a long chant by a Carolina Wren in the backyard, the cardinal popped into full view, belting out his what-cheer-cheer-cheer. I had to turn the gain down on my recorder, twice; he was so close he was pegging my VU meter.
Let's see what we can see in this cardinal's three minutes of fame. This serenade is impressively uniform. Singing cardinals often change song-types as well as increasing the length of successive songs. This bird was steady as you go. Even when a little disturbance next door threw him off pace, he resumed singing with the same song-type and same tempo. First he faced me, then he turned and faced away, then after the disturbance he moved a few feet, but nothing changes on the read-out. Today it was uniformity he wanted and he delivered it with aplomb.
But do not think what you see on your screen is prosaic just because it is uniform. Performance is everything for a singing bird. Stop the stream and look carefully at a single song. First, look at those two up-slurred whistles at the beginning of each song. Normally a birdsound like that would have multiple harmonics, each frequency sweep a whole-number multiple of the bottom one, the fundamental. Harmonics are absent, or greatly reduced in volume, because this bird is capable of filtering them out with a dynamic filter. a filter that tracks the fundamental and adjusts the frequency band it removes so as not to impact the fundamental. This was discovered by the magician-like researcher Rod Suthers and his colleagues at Indiana University. Next move on the third and subsequent notes in the song. Each is a very long frequency sweep, from 9500 Hz down to around 1500 Hz, three octaves, or about the range of a violin string. Pause to think of a single oscillator being tuned to change frequency over that range in 83 milliseconds, or about six times a second when you throw in the spaces between notes. A violinist would have to slide her finger the entire length of a string six times a second to equal that output. Most species of birds cannot do this. Cardinals do it with a little magic of their own. The upper part of the note is done by the right side of the syrnix, the lower part by the left. The left starts when the right stops, so that it looks like a single note, which it is. It takes months of practice for a young cardinal to get this down. Suthers et al. discovered this, too. The difficulty of producing a good song makes the song an honest signal of a singing bird's age and condition. Both sexes sing, and that may be the key to the rigor of this test: Sexual selection has been operating on both sexes, driving them both to strive for the perfect song.