A Sparrow-Sized Mystery, Solved

My heart skipped a beat when I saw the report: A rare Henslow's Sparrow had been found in central New York at Shawangunk Grasslands National Wildlife Refuge. The excitement only grew as local birders puzzled over whether it was a new or returning individual. A Henslow's had shown up at the refuge around the same time last May, singing its heart out non-stop for a month before disappearing without a trace. Could the bird be the same male, or was this a second hint of the species' return to the area?

Just a few decades ago, this question wouldn't be so consequential. The Henslow’s Sparrow—a small, flat-headed, olive-faced songbird—was once common in Midwestern and Eastern grasslands. But the population has seen sharp declines over the past 50 years, mainly due to degraded or disappearing habitat. A study estimated an 8 percent annual slide between 1966 and 2006—the steepest for any grassland bird in North America. In 1999, the state of New York marked the species as threatened.

That was the same year the U.S. Fish and Wildlife Service (FWS) took charge of Shawangunk. The agency spent the next decade managing and restoring the 560-acre army training facility, airfield, and race course to support a variety of grassland birds, including some regionally rare ones. The FWS efforts helped shore up habitat already deemed an Important Bird Area by Audubon New York.

But even while the refuge flourished and topped the birding charts, Henslow’s Sparrows were nowhere to be seen. The last documented individual to nest in Shawangunk was in 1987—which is why the 2017 sighting had caused such a stir in the birding community. I, among many others, made the trek from New York City to the Hudson Valley last year to twitch the sparrow. Despite its inconspicuous and secretive nature, the lone male could be seen out in the open, perched on a tall piece of grass as it tossed its head back and sang its short, insect-like song.

Fast forward to this year's sighting. The photos and sound recording that Joseph DiCostanzo, who first reported the 2018 sparrow, included in his eBird checklist clearly showed a Henslow's. But what clues could differentiate it from last year's? A plumage comparison through photos might work—but birds swap their feathers from year to year, and details may vary with age and molt. Besides, I didn’t have enough experience with the species to rely on plumage. So, instead, I turned to the audio file.

More specifically, I checked out the spectrogram that eBird generated from  DiCostanzo’s recording. Reading a spectrogram is a bit like reading sheet music: It scans left to right with high pitches near the top of the graph and low pitches at the bottom. The graph itself is a plot of pitch (in kilohertz) versus time (in seconds). The sounds, however, don’t look like musical notes; they form lines that vary in length, shape, thickness, and density based on pitch, volume, and clarity.

The Henslow’s song is typically a buzzy two-note phrase that Roger Tory Peterson described as a “hiccoughing tsi-lick.” But the spectrogram for this year's Shawangunk sparrow revealed something more complex: a rapid downslur of four pitches delivered in under half a second. After playing the audio file a few times, I could hear the four distinct syllables.  

The next step was to see how this year’s song compared with last year’s. My initial search turned up 13 eBird reports with spectrograms for the first sparrow. Though recognizable as a Henslow’s hiccup, the 2017 song sounded like three syllable. But its spectrogram revealed a pitch sequence that fell, rose, and then fell again—a totally different visual pattern from 2018. The startling difference could be heard as well.  

As birders submitted more checklists for the 2018 bird, I gathered further evidence for my hunch. The subsequent clips sounded identical to DiCostanzo’s, making the two-sparrow argument more likely. But I couldn’t rule out a returning individual just yet. What if the 2017 bird had switched up its song between visits to the refuge? The intrigue deepened when some observers reported and documented two Henslow’s together. The second one, presumably a female because it wasn’t singing, had nesting material in its beak. Could the male simply be singing a different tune this year because he’d found a mate?  

Juggling all these ideas, I consulted Nathan Pieplow, who’s been studying birdsong since 2003 and wrote an entire book on spectrograms in 2017. His response from June 14 is as follows:

I'm about as certain as I can be that the 2018 bird is a different individual than the 2017 bird. Here’s why:

Henslow's is a poorly studied species, but in the research for my book, I never found a documented case of an individual Henslow's switching song types on a recording.

You've got a pretty good sample size of recordings here. I went through all the 2018 recordings and every rendition is identical. I didn't go through all the 2017 recordings, but in my sample, they were all identical to each other and different from the 2018 bird.

All the 2017 and 2018 recordings in your sample are stereotyped, not plastic [meaning the songs sound exactly the same each time]. This basically ensures the recordings come from adult birds. It has been shown in many passerine species that once birds are adults, they can’t learn new songs. A few birds have been shown to break this rule (like Northern Mockingbird), but it would be a surprise for Henslow's Sparrow.

The 2017 and 2018 songs differ in many details. If birders visit the same bird many times over the course of a season and never document any variation, it's very good evidence that each individual has a single song type. Plus, it happens to fit with the little we know about song in Henslow's—and a great deal that we know about song in passerines in general.

So, I'd say you have a new bird this year.

To me, the discovery was exciting for a number of reasons. First, it showed how eBird's growing database can be used to study little-known and imperiled species. Checklists submitted by citizen scientists often hold a wealth of information, and through the eBird website, anyone is free to analyze them.

More importantly, a second Henslow’s Sparrow in Shawangunk bodes well for both the refuge and the species. Ralph Tabor, a longtime volunteer at the site, tells me that after FWS took over the site, it covered the runways with three feet of soil and seeded them with native warm-season plants such as perennial ryegrass, big bluestem, panic grass, little bluestem, and Indian grass. The revamp was completed in 2011, and since then, visitors have seen a dramatic increase in Grasshopper Sparrows, Bobolinks, Eastern Meadowlarks, Savannah Sparrows, and other grassland birds. The reappearance of the Henslow’s, a species that favors tall grasses and dense vegetation, is icing on the cake.

I went back to the grasslands on September 2 with the dim hope of seeing a Henslow’s Sparrow before season’s end. (The last eBird sighting was July 31.)  But the trail was eerily quiet, and I left with a fresh wave of questions in mind. Did the sparrows breed successfully? Will they return from their wintering grounds next spring? One thing's for certain: Three different Henslow’s in two years is proof that habitat restoration is productive. Already the species may be experiencing a slight jump across the country, perhaps due to projects like Shawangunk’s. If you build it, they will come.

***

Audubon is a nonprofit working to save birds and the places they need. To support our work and conservation efforts, make a donation today.