Springtime courtship: A tale of grackle tails
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By David Spector Gazette Contributing Writer
Among the earliest birds to move north as the days lengthen in winter are common grackles, members of the New World blackbird family. Here in western Massachusetts, we often see noticeable increases in their numbers as early as February. These are among the most abundant birds in North America. But a species doesn't have to be rare to offer fascination. The courtship behavior of male grackles is a case in point.
After they arrive and then through the spring, glossy common grackles engage in courtship flights: A female is followed by one to several longer-tailed males with their tails folded into a "V" shape suggestive of a boat's keel. The male can look like a blackbird with a sparrow in tow.
Aerodynamic considerations suggest that the V shape reduces lift and increases drag. The fact that female grackles do not fold their tails into the extreme Vs, and that males rarely do so other than in courtship flights, are further evidence that this shape is inefficient. So why would a male fold his tail in this way?
Extreme tail-folding in courtship flights suggests that the tail posture is a signal, and advances in the understanding of animal signaling over the past few decades offer a plausible explanation of this male behavior.
In 1975 the Israeli evolutionary biologist Amotz Zahavi proposed his "handicap principle" to explain such ornaments: A signal, to be reliable, must be honest, and to be honest it can't be bluffed. In other words, any male can claim to be a fit mate, but it pays off for the female if he can prove it. Perhaps male common grackles gain an advantage in mating if they show they are so fit that they can keep up in courtship flight despite the handicap of dragging a long, folded tail.
This explanation, however, raises another question. As a result of predation, disease and harsh weather conditions, natural selection tends to weed out the weak, so why aren't all males about equally fit as mates? In the early 1980s, British biologist William Hamilton and his student Marlene Zuk suggested the answer had to do with the ever-changing threat of disease.
Diseases change in frequency and the molecules of disease-causing organisms that their hosts' immune systems recognize evolve rapidly. Because of these changes, the relative advantages of genes that affect a host's ability to deal with specific disease organisms also change: One form of a gene may be advantageous in the presence of a particular strain of pathogen, while another form becomes more widespread when a different pathogen (or a different strain) comes to predominate-just as we encounter year-to-year changes in the strains of flu or the common cold.
Validating a hypothesis
By assessing a male's ability to keep up in courtship flight despite his long-tail handicap, a female common grackle might therefore be able to assess whether he has the appropriate forms of genes for dealing with currently common grackle diseases. This idea has been supported in studies of other bird species, but, as far as I know, has not yet been tested for common grackles.
Of course, plausibility alone does not validate a hypothesis. To feel confident that the scenario sketched above applied to the evolution of the tail of the common grackle, one would want to know many things, including whether the ability of a male to fly with a long, down-folded tail correlated with his health and with the health of his offspring. Although the research to test such a hypothesis would be long and complicated, it would offer the pleasure of close observation of a bird species and unexpected discoveries along the way.
Such open questions, and the joy of exploration in trying to answer them, are what make ecology and evolutionary biology-indeed, any science-so rewarding.
David Spector is a former board president of the Hitchcock Center for the Environment. He teaches biology at Central Connecticut State University.
Earth Matters, written by staff and associates of The Hitchcock Center for the Environment, appears every other week. The center's mission is to foster a greater awareness of the environment. It is located at 525 South Pleasant St., Amherst. For more information, visit www.hitchcockcenter.org, call 256-6006 or write to column@hitchcockcenter.org.
