A Song for August 19

I ran out to Folly Beach in the late afternoon to score an easy bird sound, maybe a Laughing Gull, House Sparrow, or one of three columbids that are common at the pier. Or, if I was lucky, a Boat-tailed Grackle. I was lucky. A gaggle of males sat at the corner of the flat roof of a hotel, singing. That sound is above. Below is a video of two males on the nearby pier. As you see, there is a lot more to the performance than sound. The singing male grouches and fluffs his head plumage as he sings. And he flaps his wings several times after each song. This is all part of the display.

Aware of the zaniness of that display when seen, I was a little startled when I first viewed the moving sonogram above. It seemed almost monotonous. Actually, per Pieplow, the acoustic part of the Boat-tailed display is rather uniform, unlike the singing of the Great-tailed Grackle, although the visual parts of the display are similar. These days, with so much good information on vocalizations available to birders, it may seem unbelievable that Boat-tailed and Great-tailed were once considered the same species. A study in the 1960s in Texas showed that they were sympatric without interbreeding. That means they lived in the same area, and chose actively not to interbreed. The different vocalizations are the key. Darwin's concept of sexual selection, which had never been effectively deployed by biologists, was "discovered" in the 1970s. The explosion of research and thinking on the role of displays in communicating species identity and individual quality helped us realize that species could look identical, as these two grackles do, and be "good species." This commonly-used term means real, distinct, and reproductively isolated.

Any time you see several males displaying together, you can expect two things: (1) the females do not look like males. (2) the males contribute little or nothing more than sperm to the females's reproductive effort. Don't pity the females, they wouldn't do it this way, if they needed the help of males in feeding their young. The females do need such help in the vast majority of bird species, from albatrosses to penguins to chickadees, which is the ecological reason most birds are monogamous. But females that nest in resource-rich habitats, such as marshes, can forego the domestic help and select males for other qualities, such as beauty and/or good genes. Sexual selection of this sort leads to elaborate displays, which we see in both Great-tailed and Boat-tailed Grackles.

As for those enormous tails, they must be part of the display. Each year in August birders along the Atlantic Coast of the U.S. are treated to the spectacle of large, black, tailless songbirds flying over the salt marshes. These birds are easy to identify as Boat-tailed Grackles when dragging their large tails along, but without tails they look nondescript and rather pitiful. Most birds molt their flight feathers every year at the end of the summer, but they usually replace them gradually and symmetrically. That way they can maintain the powers of flight while growing in replacement feathers. (Waterfowl are an exception to this norm.) I hypothesize that Boat-tailed Grackles drop all their tail feathers at once because the tail is so hypertrophied (by sexual selection) as to be useless aerodynamically. I can think of a couple of indirect tests of this hypothesis: (1) Do they have a gradual molt of the primaries and secondaries, which are still useful for flight? (2) Do Common Grackles, with somewhat less exaggerated tails, have a gradual molt of tail feathers? If you live in the range of the Boat-tailed Grackle, I hope you will look for tailless males for the next few weeks. And what about Great-tails? Are they tailless for a spell, too?