Meet John James Audubon’s Nemesis Bird: the Chestnut-sided Warbler

As a teenager, I once bragged to a friend that I had seen more Chestnut-sided Warblers than John James Audubon ever did.

It was true, too. But it wasn’t much of a boast: JJA’s lifetime tally was five, all seen one morning in May 1808 in eastern Pennsylvania. That was it. In another three decades of actively seeking birds all over eastern North America, he never encountered the Chestnut-sided Warbler again. He did wonder about it, though. “Where this species goes to breed I am unable to say,” he wrote. “I ransacked the borders of Lake Ontario, and those of Lakes Erie and Michigan, without meeting with it.”

(Kenn Kaufman's Notebook is a regular column featuring original artwork and essays by Kaufman, a field editor for Audubon, and a world-renowned bird expert, author, and environmentalist.) 

Today, birders who ransack the border of Lake Erie in mid-May can hardly avoid seeing Chestnut-sided Warblers. I can vouch for that, since I live just a few minutes away from the shoreline of Lake Erie in northwestern Ohio. At the Magee Marsh Wildlife Area near my house, it’s no surprise to see 20 or 30 Chestnut-sided Warblers on a May morning. Of course, Magee and nearby sites are exceptionally good hotspots: This region is often called the “Warbler Capital of the World,” and this is the scene of “The Biggest Week in American Birding,” arguably the largest birding festival on the continent. So warblers are unusually prevalent here. Anyone can walk out on the boardwalk at Magee Marsh in mid-May and see Chestnut-sideds and numerous other warblers almost too close for binoculars to focus.

But I’ve also seen double-digit counts of migrating Chestnut-sided Warblers elsewhere, from New York’s Central Park to random woodlots in Louisiana. In eastern Pennsylvania, where Audubon lived for several years, I’ve seen plenty of Chestnut-sided Warblers, both as migrants and as summer residents. Any way you look at it, this is not a rare bird, not even an uncommon one.

So how did this strikingly marked warbler manage to mostly elude Mr. Audubon?

There may have been several factors at play. For one thing, in his day, no one had binoculars. To see a small bird well enough to paint its portrait, it was necessary to catch it or shoot it. Audubon would shoot a bird when necessary for his art and science, but he wasn’t inclined to knock random birds down unless he had reason to believe they were new or different. Since warblers are generally small, elusive treetop birds, many must have slipped by without the artist making a serious attempt to ID them. 

A second reason involves a similarity of songs. Audubon was no slouch at birding by ear. He definitely paid attention to bird voices and sometimes tracked down new species that way. But the variable song of the Chestnut-sided Warbler is often surprisingly similar to that of the abundant Yellow Warbler. (Even after years of birding, I still sometimes have trouble telling them apart.) Audubon was quite familiar with the Yellow Warbler, so if he heard Chestnut-sideds singing, he might have passed them off as their bright yellow cousins.

But there’s a more intriguing explanation for why Audubon—and other pioneer birders of the early 1800s, like Alexander Wilson and Thomas Nuttall—mostly struck out on the Chestnut-sided Warbler. The bird may have been far less numerous in those days than it is now.

This is a warbler of scrubby second-growth woods, with short trees and open areas. When eastern North America was covered with old-growth forest, its habitat would have been limited. Then European settlers arrived and converted vast tracts of tall forest into open farm fields or pastures, which also didn’t help this warbler at all. It’s no wonder that Audubon, Wilson, and Nuttall all had trouble finding it, at least through the 1830s. 

By the late 1860s, exploring eastern Massachusetts, ornithologist William Brewster found that Chestnut-sided Warblers were common in summer. “I ascertained that they were breeding abundantly throughout most of the wilder parts of Belmont, Arlington, and Waltham,” he wrote. Frank Chapman, the founder of the Christmas Bird Count, made this observation in 1907 from Englewood, New Jersey:  “In my own experience, covering the past twenty-five years . . . I have seen this Warbler become established as an increasingly common summer resident.”

Finally, in 1929, a century after Audubon began publication of his Birds of America, the naturalist and ornithologist Edward Howe Forbush summed up the changes in the warbler’s status. In his classic Birds of Massachusetts (and in the poetic language that used to be more common in bird books), Forbush wrote this of the Chestnut:

“Its numbers have increased until it has become one of the commonest of eastern warblers. Its increase was favored by the destruction of the primeval forest and the continued cutting away of subsequent growths, and later by the increase of neglected fields and pastures with their growths of bushes and brambles, for it is not a frequenter of deep woods, nor yet of well-kept gardens, orchards or farmyards, but prefers neglected or cut-over lands, with a profusion of thickets and briers. So we may find it usually away from houses, in low roadside and brookside thickets, or in sprout-lands rather recently cut over.”

Today those sprout-lands and brambles and briers are still widespread, even if we don’t use those words as much, and the Chestnut-sided Warbler continues to be a reasonably common breeding bird over much of eastern North America.

I like to imagine bringing John James Audubon back and taking him to the Magee Marsh boardwalk on a spring morning. After he got used to using these new-fangled binocular things—and after he got over his surprise at learning that “the Audubon movement” now involves millions of people working to protect bird populations—I’ll bet he would love to see all those Chestnut-sided Warblers.