Toward the end of April, soon after the fall of Baghdad, I was in Tilden Park on a rare dry Saturday watching hostilities of a different kind. The black-headed grosbeaks were back from their wintering grounds in western Mexico and setting up territories for the nesting season. All the grosbeaks I saw were males; the females might have been due in a second wave, or may just have been staying out of sight.
Intense song-duels were in progress all along the trail to Jewel Lake. The grosbeak’s is one of the songs I have to relearn every spring: robin-like, but slower and more deliberate. Just off the boardwalk section of the trail, two males were vying for a bent-over willow that looked like prime real estate. From nearby perches in the same small tree, they sang vehemently at each other. One would chase its competitor off, then both would return and start up again, oblivious to my presence a few feet away.
The grosbeaks were just one set of voices in the chorus. All around, other returning migrants—warbling vireos, Wilson’s warblers—had joined the year-round residents like song sparrows and juncos. Even the Anna’s hummingbirds were belting out their scratchy, squeaky excuse for a song. Dozens of birds broadcasting: “I’m a grosbeak (or finch, or towhee); I’m a real stud, and I have this amazing nest site staked out.”
Which is of course to anthropomorphize a bit. Birdsong is a multifunctional thing. Whole books have been devoted to its esthetics alone. Listen to a Swainson’s thrush bouncing its voice off the caves above Wildcat Creek in late spring; does it sound as good to a female thrush us it does to us?
Ornithologists used to assume song was pretty much hardwired: every male bird leaves the egg with the “right” song in its genes. That may be true for some species, but not across the board.
Many birds have to learn their songs, and there’s an optimal window for learning. The late Luis Baptista, curator of birds and mammals at the California Academy of Sciences, is responsible for much of what we know about that process. He found that some birds have local song traditions—dialects, in a way. To a discerning ear, a white-crowned sparrow from Golden Gate Park will sound different than one from Tilden Park, as will a bird from across the hills in Orinda.
We’re also learning what female birds listen for; how they judge the contestants. They’re attuned to the right kind of male, of course; differences in song can keep closely related species from hybridizing. And experiments have shown that some females, like Mae West, prefer a male with a big vocabulary. Western marsh wrens can run through a repertoire of 200 or more song types, and females seem attracted to those with the most variety. This may be what has driven the evolution of song mimicry in mockingbirds and their relatives.
Recent research by Stephen Nowicki and William Searcy on song sparrows, a common species in the Bay Area and across most of North America, has also shown that mate choice reflects how well a male has learned the local dialect. That seems counterintuitive when you think about the bird’s history. Song sparrows are sedentary in the extreme, never wandering more than a few miles from their birthplaces. Females rarely encounter a male singing an unfamiliar song. So what’s the point of their selectivity?
It turns out that a male’s ability to deliver the right version of the song reflects his general fitness. Nowicki and Searcy reduced the food intake of captive-reared male song sparrows during the critical song-acquisition stage. They grew up physically normal but never mastered the local song dialect, despite exposure to appropriate models. And they had no luck at all with the females. Song-dialect fidelity appears to be one of the cues females use to determine if males have the right genetic stuff.
I have no idea whether this also holds for black-headed grosbeaks . But it’s possible that any females present that Saturday in Tilden Park were keeping score of those dueling males’ mastery of Tildenese.