Reference: Yellow-Rumped Warbler

In springtime, the last snow drifts melt away, the first tiny woodland flowers poke through the dirt, and yellow-rumped warblers (Setophaga coronata) head north for the summer.

Warblers are the jewels of the birding world, but most species reach their nesting grounds after the trees have leafed out, making them hard to see. Not so for the yellow-rumped warbler. This sharply dressed bird—colored in beautiful shades of blue-gray, black, and bright yellow—isn’t worried about a little snow or a few cold, rainy days, because it has a superpower that other warblers don’t: it can eat things that would make other warblers sick. Most warblers gorge on caterpillars, grubs, flies, and beetles; yellow-rumped warblers add things like wax myrtle and poison-ivy berries, frozen apples, suet, and seeds to their list. Once in a while, they even stop in at bird feeders. While other warblers hide among the treetops, this one comes to you.

There are four subspecies of yellow-rumped warblers in North and Central America. Myrtle warblers with white throats populate eastern North America; Audubon’s warblers are the western version, and they have yellow throats. Western Mexico has, appropriately, the West Mexico yellow-rumped warbler, a darker version of the Audubon’s warbler; and Guatemala has the striking Goldman’s warbler, an almost pure black version of the yellow-rumped with a white belly and yellow sides, head, throat, and rump. The groups may have slightly different colors, but they all share a yellow rump, leading to their best-known common name, the Butter Butts.

Butter Butts live from the northernmost parts of Alaska and the Yukon to the jungles of South America, but when it is time to nest, they head to evergreen forests filled with balsam fir, pine, spruce, and tamarack trees. Males get there before the females to search for likely nesting territories. Then, they start singing with a bouncy trill, hoping to catch the heart of a female. When a female lands nearby, the male excitedly bounces from branch to branch, fluffing and shaking his body feathers to show off his colors. Of course, he makes sure that all his bright-yellow parts are showing.

Yellow-rumped warblers are monogamous, but not always with their own species. Eleven different types of warblers have been documented mating and raising families with yellow-rumped partners. Identifying warbler species can be a challenge—accurately identifying the hybrid types is overwhelming. However, today there are online sources and Facebook pages with experts dedicated to warbler and hybrid identification. The hard part is getting photos that are clear enough to identify.

Female warblers build the nests. The male brings her an occasional twig or piece of grass, but usually he just follows her around and sings as she works. The nest is a cup of twigs, grass, moss, and lichen. A few feathers and fur line the nest with a little softness.

Hens keep the eggs warm and healthy for about two weeks. Males don’t incubate eggs, but they aggressively defend their territories from other warblers. They are not as good at defending the area from brown-headed cowbirds. In fact, they pay little attention to cowbirds at all, even though cowbirds are notorious for laying a single egg in another bird’s nest and letting the foster parents raise the chick, often at the expense of their own young. This is a great tactic for the cowbirds, because they live to follow bison herds—but away from the Great Plains, cowbirds can be a nuisance. However, yellow-rumped warblers can somehow delay and even prevent a cowbird egg from hatching. Even if a cowbird egg does hatch, the adult warblers easily raise the nestling alongside their own brood.

Nestlings hatch naked, blind, and mostly helpless. They grow fast, so the parents work from sunup to sundown to give them a steady diet of high-protein and high-fat foods such as ants, caterpillars, beetles, and spiders. In one study, parents made 108 separate trips to the nest in one day, bringing food for the young birds. Heavy, high-calorie eating is essential; these chicks grow from weak, defenseless birds the size of a jellybean into partially feathered fledglings as large as their parents in only two weeks.

Even after leaving the nest, the chicks won’t fully form flight feathers for another couple of weeks. They wander through the branches near the nest, alert and learning about the world. Parents keep feeding the young as their flight feathers grow and they begin their first clumsy attempts to fly. They flap fast but not well, looking like drunken moths crashing into trees. The clumsy days don’t last long, as their breast muscles grow and flight feathers lengthen. Soon, the fledglings are flying as well as their parents.

After nesting season is over, it is time to fatten up and go south for the winter. Heading south for the season is not as frenzied as the spring migration, but it still takes a lot of energy. Yellow-rumped warblers form small flocks with other birds, feeding and resting through the day and flying through the night. A few stay along the Atlantic coast as far north as Nova Scotia, but most winter in the southern United States, Mexico, or the Caribbean.

And at the end of every winter, they get tired of the quiet, warm, sunny days in the south and head back north. They show up in our woodlands and at our feeders, and they always bring a smile, because when the Butter Butts come back, spring is never far behind.

MEASUREMENTS* (in mm)

	Culmen	Wing	Tail	Tarsus
Female	9.4	72.4	54.2	18.7
Male	9.5	74.7	55.2	18.8

*Adapted from Hunt, et. al.

REFERENCE

Hunt, P. D. and D.J. Flaspohler (2020). Yellow-rumped Warbler (Setophaga coronata), version 1.0. In Birds of the World (P.G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.yerwar.01

A yellow-rumped warbler stares down the camera in Prospect Park Lake in Brooklyn, New York.
Credit: Rhododendrites/Wikimedia Commons