The mockingbird leaps up, and lands only to leap up again, as if on springs, or as if the ground were too hot to stand on. Nearby another bird leaps up, then bounces down and up again. This dance for two plays to no music, but seems to follow an invisible line. Last summer the dance floor went right down our neighbor’s driveway, and the birds returned regularly to perform. Sometimes they spiraled up into the air, twirling together. Sometimes a third or fourth party came close. Sometimes one bird hopped the other right back away from the line. Some have caught this dance on film, here and here and here. Try not to listen to incorrect dialogue.
I tell my students to watch mockingbirds, to figure out how they use time and space, and how this differs between the sexes. They are not told that there will be no visible differences between the males and the females. They are also told to find something that they really want to understand, and to pursue it in more detail. Some choose this dance.
It must be courtship, I am told. The males are measuring the females in the dance. Or the females are judging the males. Maybe the swirling flight is the actual mating act, though they aren’t very convinced as to how the necessary connections could be made so rapidly in the air. I ask them how they know it is courtship, what other data they need, and they think.
There was a time back in the early 1990s when every mockingbird with a territory on the Rice University campus had a pair of neat little colored leg bands, and a silver band with a number. The colored bands were visible with binoculars from a distance. My graduate student, Debbie Moralez DeLoach, had banded them, and carefully followed them through the years. So, I can tell you the dance is between two males, and it marks the edge of a territory, a boundary that has to be actively maintained, or the neighboring male will take over.
Debbie used the dances and sightings and mapped all the territories on campus, for the mockingbirds had divided up all the open areas, from the academic quadrangle to the music school. The mockingbirds left only the more forested areas for the blue jays. At first these territories seemed oddly variable. One could be five or more times the size of the next, and Debbie struggled to figure this out. You see, a territory is an expensive business, and birds don’t have territories that are bigger than they need to rear their young. Time spent guarding a larger territory would come at the expense of increased foraging, or increased vigilance of the young, or the mate.
In a stable population like this one, it is a sobering thought that on average a pair of mockingbirds only replaces itself in its lifetime. This means that of all the babies they rear, and these can have two or three broods of three or four babies a year for six or more years, only two on average will ultimately survive. Three times four times six is 72 babies in a lifetime, and of those only two survive? Which two? The two that best balance their actions, the two who carry the genetic variants that result in optimal behavior.
So, why do territory sizes vary so much? It turns out there is a crucial resource, and each territory has to have enough of that resource. In the groomed grounds of Rice University, the resource is volume of hedge. The birds need hedges because that is where their fledged babies go when they leave the nest, but cannot yet fly well. They hide in the hedges, and from there try their young wings, gradually gaining the altitude necessary to fly into the safer trees.
So, keep your cats inside. Leave those fallen babies alone, or guide them to a nearby hedge where their parents can feed them. And help encourage Houston to plant more hedges, and let them grow high. On some birds and other animals, I’ll have one entry, but on my favorites, there will be more. To come on mockingbirds are stories of fidelity, bigamy, promiscuity by the fruit tree, the rewards of song, of age, and differences between the sexes.
I wonder what limits the mockingbirds in St. Louis, and know I would struggle to live somewhere lacking mockingbirds.
Wikimedia photo taken by Calibas in Lodi, California, GNU Free Documentation License, Version 1.2
A page from Debbie’s dissertation showing how she measured mockingbird territories.
Males that succeed in getting a mate have more hedge volume than those that don’t. Notice those pesky outlier points that always mess up our analyses!
Can you imagine how much work this was? Debbie’s excellent dissertation if full of such data, and is available from Ann Arbor Microfilms, but was unfortunately never published as papers.