Standing a couple of yards from a singing male western meadowlark a couple of weeks ago, I was struck by several things. First, the sheer virtuosity of the song: this bird is consistently ranked among the best North American singers, and I couldn’t argue with that. Second, the volume: western meadowlarks are loud. Their vocal performance is “often the only song that can be heard from a car driving at highway speed with the windows open,” writes Alvaro Jaramillo in New World Blackbirds: The Icterids. Couldn’t argue with that either.
The third reaction was how different this song was from that of the eastern meadowlarks I used to hear in Georgia. Eastern birders tend to be surprised when first exposed to a western meadowlark’s song. Arthur Cleveland Bent described his response on his first day in the North Dakota prairies in 1901: “I could hardly believe it was a meadowlark singing…until I saw the plump bird perched on a telegraph pole, facing the sun, his yellow breast and black cravat gleaming in the clear prairie sunlight. His sweet voice fairly thrilled us and seemed to combine the flutelike quality of the wood thrush with the rich melody of the Baltimore oriole.”
No one would ever compare an eastern meadowlark with a wood thrush. Its song, a series of high-pitched whistles, is comparatively wimpy. The western’s song, on the other hand, is consistently described as flutelike or gurgling.
Why such a difference? In an eccentric but interesting book called Born to Sing, Charles Hartshorne ventured an environmental explanation, speculating that open arid country might favor loud and distinctive songs “by causing birds to space unusually far apart from one another in order to find sufficient food….Possibly this is why Western Meadowlarks in the drier parts of North America have more powerful, lower-pitched, and longer, some say, more beautiful songs than Eastern Meadowlarks in the moister portions.”
That’s plausible, but the difference I hear goes beyond volume, pitch, and duration.
As in many songbirds, the songs of both species aren’t genetically hardwired; they have to be learned. Some males acquire songs of the wrong species. One captive male western meadowlark picked up the vocal style of a Baltimore oriole it had heard through an open window.
To the frustration of birders, eastern and western meadowlarks are extremely similar in appearance. The visual field marks are subtle indeed. Logically, differences in song quality would help the birds sort themselves out in the broad area of the Midwest where their breeding ranges overlap.
Field studies show that male meadowlarks don’t differentiate between males of their own species and of the other. A male western meadowlark will respond aggressively to an intruding eastern male, and vice versa. In both species, the females are the discriminating sex. Males arrive at the breeding grounds first, setting up territories on which females settle. As far as can be determined, natural hybrids between easterns and westerns are rare. There’s a good reason for that: the two species are genetically incompatible. Mixed captive pairs will mate, but the offspring tend to be sterile.
So has female choice driven the evolution of species-distinctive songs? To begin to answer that question, you’d have to know what female meadowlarks are listening for. Field research in Manitoba suggests that they’re at least paying attention to song repertoire.
Again like many other songbirds, male western meadowlarks can perform more than one type of song. The average male has six identifiable song types; some have as many as 12. Biologists Andrew Horn, Thomas Dickinson, and J. Bruce Falls of the University of Toronto measured the wing length of 29 male westerns (a handy index for body size), tallied their song types, and monitored their territory sizes and reproductive successes.
Their conclusion: “Our study shows that males with larger repertoires tended to pair earlier, were more likely to be bigamous, and fledged more young per female. These effects were independent of territory size, which was not related to repertoire size.” If I may trot out that Mae West quote one more time, female western meadowlarks do seem to prefer males with big vocabularies. Song type repertoire size appears to function as a signal of good genes.
Interesting, but that doesn’t speak to the divergent evolution of song quality-which would be harder to study in any kind of quantitative way. We may have a runaway sexual selection effect going on, in which female preferences for loud fluty songs increase the frequency of both the genes that code for such songs and the genes that code for such preferences.
It would be satisfying to know the history, but it’s not necessary for the appreciation of the performance. Another old-school naturalist, Donald Culross Peattie, called the western meadowlark’s song “the most joyful voice in all the world of birds.” That’s another assertion I can’t really argue with.