As our birding group made its way to Jamaica Bay Wildlife Refuge’s West Pond one December day, a quick, dry Chup! sounded somewhere overhead. And again: Chup! “Another butter butt,” someone sighed. A few sharp-eyed birders glimpsed a brownish shape flit from shrub to shrub, brightened by a telltale flash of bright yellow at the base of the tail. But the group shuffled on, indifferent. We’d already seen a good many yellow rumps that morning.
Often taken for granted because it’s so plentiful, the Yellow-rumped Warbler, New York City’s only regularly wintering warbler, deserves a closer look. One of the most wide-ranging of North American warbler species, the yellow-rump, affectionately known as the “butter butt,” breeds across the entire continent and as far north as Alaska, and spends the winter as far south as Panama. Along the East Coast, its wintering territory extends north into Massachusetts. You can follow the yellow-rump’s movements throughout the year as recorded by eBirders here.
During their winter stay with us, males and females of this species look close to identical: drab brownish overall, with a bright yellow rump that can be obscured by the bird’s folded wings. All but the youngest female birds also sport a yellow patch on each side of the breast, bordered by patches of brownish striping—symmetrical markings that make ID possible even at a considerable distance.
In the spring, however, the yellow-rump comes into its glory. Males of the species, wearing a complex pattern of yellow, gray, and black, are among the most striking of spring warblers—and sport so many field marks, from wing bars to a partial eye-ring to a yellow crown patch, that I employ them as a “field mark guide” in my Beginning Birding course. (See photo on this issue's cover.) Spring females are similar in appearance, but not as brightly colored.
On some April and May days, it can seem as though winter never ended—birding seems to turn up nothing but butter butts! This species’ abundance and wide range is perhaps due to its adaptability: a “generalist,” it eats a wide variety of foods throughout the year. Yellow-rumps are often seen fly-catching when insects are plentiful. (Their Latin name, in fact, translates as “Crowned Moth-Eater.”) In fall, they turn to fruit. In the winter, they owe their survival along New York City’s chilly shoreline to their ability to digest the waxy coating of the small, hard fruit of Northern Bayberry, a plentiful maritime shrub, as well as the fruit of Eastern Red-Cedar and Poison Ivy. Seeds of goldenrod and sunflower species are also on the yellow-rump’s menu.
After departing our area in the spring, yellow-rumps head to northern breeding grounds, where they prefer nesting in evergreen trees. Like many northerly breeders, the species can be found nesting at higher elevations as far south as northern Pennsylvania. New York State’s Catskills are one such location not too far from the City, as I discovered a few years ago on a late spring bird walk up Hunter Mountain, three hours north of us. As we hiked from low meadows to a craggy mountaintop full of fir trees, we passed through breeding territory ringing with the songs of Wood Thrushes and Common Yellowthroats down low, Swainson’s Thrushes and Blackburnian Warblers in the middle, and finally a sunlit montane habitat where we heard singing Bicknell’s Thrushes, Purple Finches, Dark-eyed Juncos, and Yellow-rumped Warblers.
Both during migration and on breeding territory, yellow-rumps sing a few varieties of slightly descending warbles, which vary in pattern. One variant, to my ear, sounds a bit like the two-tiered song of a Nashville Warbler; while another, a less organized affair, sounds something like the meandering, unaccented variant of the Chestnut-sided Warbler’s song. I always hunt down those first singing yellow-rumps at the beginning of each spring migration, to get myself oriented! (Listen to the songs and calls of the yellow rump.)
Those who’ve had the luck to bird out west know that yellow-rumps are not all the same. When I first got to know this species, as an excitedly birding pre-teen, we called them “Myrtle Warblers,” a name derived from the Wax Myrtle, a southerly shrub related to our Northern Bayberry. Western yellow-rumps were then known as a separate species, “Audubon’s Warbler”—distinguished from its eastern counterpart by its bright yellow throat and plainer gray upperparts.
Ornithologists discovered, however, that Myrtle and Audubon’s Warblers interbreed freely in the northern Rocky Mountains, leading to the two species’ being lumped into one, demoting the two warblers to subspecies of the Yellow-rumped Warbler. Birders proud of their life list tallies groaned across the continent. There is occasional talk of splitting the species again into as many as four species, and birders often refer to the subspecies they are seeing. (Two additional subspecies are not seen in the U.S.: Read about all four subspecies and see photos.)
According to the North American Breeding Bird Survey, the Yellow-rumped Warbler’s population has remained stable over the past 50 years. However, yellow-rumps are frequent victims of window collisions (see this issue’s article on Project Safe Flight), and their breeding range may already be contracting as a result of climate change. We must do what we can do ensure birders continue to take plentiful butter butts for granted, for many years to come.
Explore how National Audubon scientists predict the Yellow-rumped Warbler’s range may be impacted by climate change.
