Spring bird song

A liquid, twisting bird song sails out from the willows. I stop walking, search the treetops and see no birds. The song burbles out again, a string of curling, clear phrases. I know it's a black-headed grosbeak because it sounds, just as it's often described, like a robin that has taken voice lessons. But I can't see it. A week later, on the same trail near Jewel Pond in Tilden Park, I hear the song again, and this time I see a few of the birds—robin-orange chests, black faces, chunky (gros!) beaks. They stay mostly tucked among the branches but I get a few not-quite-focused photos.

Male black-headed grosbeaks arrive to the Bay Area from their winter habitat in Mexico a few days before females do. At this point, the males sing as part of their competition to stake out territory. Apparently, if the hierarchy is not settled by singing, they will also attack each other in the air. As the females arrive, males sing to attract mates. Once pairs are established, females sing as well, with both males' and females' songs working to alert other black-headed grosbeaks to stay out of their territory. Perhaps their singing also functions to strengthen their bond to each other?

It feels contradictory—the song's utilitarian (even aggressive at times) functions next to its ornate, extravagant beauty. There is disagreement among evolutionary biologists about how beauty evolves. Some argue that it is always a measure of fitness: a beautiful trait, such as an elaborate song, is  an indicator that helps one individual (usually the female) choose the mate that is likely to have genes for strength, health, survival. Others argue that mate selection may favor beautiful traits that have nothing to do with practical advantages: a species' brain evolves to prefer some quality, perhaps somewhat incidentally and for a reason unrelated to mating at all, then this preference bleeds into mate selection, and over time this leads to more and more extreme forms of that trait.

In our springtime forests and fields, blackheaded grosbeaks are far from the only bird you will hear singing. The song I hear most is the song sparrow: tik tik tik tsweeeeeeee chirry chirrchir The first three simple tik notes are like a warm up to its virtuoso trills. It has the endearing habit of posing at the top of a bush and ignoring nearby humans as it sings.

Our brains  as well have evolved to appreciate beauty. And it seems that we and the song birds have evolved similar aesthetics. The songs meant to woo grosbeaks or sparrows touch me and my breath catches in my throat.

LINKS:
New York Times article about the evolution of beauty
Black-headed grosbeak information from Cornell Lab of Ornithology
Song sparrow information from Cornell Lab of Ornithology
Tilden Regional Park