Migrating Palm Warblers

It’s the middle of October and every day songbird migrants dwindle. Still, between rainstorms one morning the trees fill with warblers, especially yellow-rumps, which haven’t all that far to go. But I spot the pumping tail of a palm warbler too.

The following day three palm warblers sit and bob their tails in the one American elm beside our driveway. The day after that, our First Field plants shake with sparrows—song, field, chipping and at least two white-crowned sparrows.

In the midst of brown sparrow bodies, I pish in a yellow palm warbler, its yellow breast and belly making it the yellow palm warbler subspecies Setophaga palmarum hypochrysea, which nests in coniferous boreal forests east of Ottawa, Canada, and winters mostly along the Gulf Coast. The western palm warbler, S. p. palmarum, nests west of Ottawa and winters primarily along the southeastern coast of the United States and in the West Indies.

Their wintering grounds may account for their common name, but this is a species that nests on the ground or in short shrubs or trees in a black spruce bog environment from Newfoundland and southern Labrador to northeastern British Columbia in Canada, although occasional nests have been reported from northeastern Minnesota across to the northern New England states.

Still, according to the Boreal Songbird Initiative that is “committed to protecting the Canadian Boreal Forest—the largest intact forest on Earth—on behalf of the billions of migrating birds that rely on it,” according to their website, an estimated 98% of palm warblers breed in Canada’s boreal forest. This is the highest number of any warbler species and is followed by Tennessee warblers (97%), Connecticut warblers (91%), blackpoll warblers (82%) and Cape May warblers (83%).

Both subspecies of palm warblers have chestnut caps, giving them the nickname “redpoll warblers” and pump their tails more than any other warbler species so they are also called “tip-up” or “wag-tail” warblers. They also have yellow beneath their tails, but many have subdued colors in fall so it isn’t always possible for the casual observer to separate the subspecies.

However, the western subspecies has a whitish throat and belly and a grayish brown back and wings in contrast with the yellow throat and belly and yellow-green back and wings of the yellow subspecies.

Mostly the western palm warbler migrates west of the Appalachians in the spring, beginning later than the yellow palm warbler and southeastward to the Atlantic coast in the fall earlier than the yellow palm warbler, while the yellow palm warbler migrates earlier northeast along the Appalachian Mountains in spring and follows the Appalachians south later in fall.

In Pennsylvania it is possible to see either subspecies. According to bird banding records from the Powdermill Nature Reserve in western Pennsylvania from 1961 to 2000, westerns outnumbered yellow significantly during migration, but especially in autumn when their nets recorded 1,265 westerns versus 35 yellows. In spring they netted 24 westerns and 13 yellows.

W. Herbert Wilson, Jr., author of the definitive palm warbler account in Birds of North America, hypothesizes that “the earlier spring migration of yellows may reflect a more advanced phenology [blooming of plants] along their typical migration route, the Coastal Plain, compared to western, inland routes of western palm warblers…[and] maybe an earlier breeding season.”

During migration in southeastern Pennsylvania, for example, palm warblers feed on the ground in early spring but switch to feeding in trees as soon as they bud and flower and attract the insects palm warblers prefer.

Recent studies in 2013 by Frank LaSorte, a research associate at the Cornell Lab of Ornithology, and colleagues, discovered that many small, insectivorous North American bird species fly looping, clockwise migration routes. Those eastern species use strong southerly tailwinds in spring over the Gulf of Mexico and less severe headwinds in the fall. But they also seem to be following the flush of green vegetation in the spring in search of insects, which may help to explain the difference in migration rates between western and yellow palm warbler species.

Even though scientists have been studying bird migration for many decades and have found so far that they orient using magnetic fields, stars that rotate around the North Star, patterns of polarized light near sunset, as well as learning their route through experience, they are not certain if the birds can easily switch from one navigation process to another during a night when, for instance, it is overcast.

Palm warblers are, like many songbirds, night time migrants. In spring they travel with other early migrants—ruby-crowned kinglets, hermit thrushes, and their closest relative—yellow-rumped warblers. On migration either way they also mingle with other early migrants—pine warblers, chipping sparrows, and eastern bluebirds.

In the fall, they are among the last warblers, along with yellow-rumps, to migrate through Pennsylvania. Then they are usually seen in or near the ground in woodland edges, thickets, brushy areas or on the grassy sand dunes in Presque Isle State Park, according to The Birds of Pennsylvania.

Palm warblers don’t sing much on migration or even on their nesting ground, uttering five buzzy notes per second and reminding one observer of a debilitated chipping sparrow.

Even though much has been learned about migrating palm warblers, only one observer, D. A. Welsh, back in 1971, has studied palm warblers (in a Nova Scotia spruce bog) over their entire breeding period, although earlier observers in Maine, southern Quebec, and New Brunswick described nests they found.

Maine ornithologist, Ora Willis Knight, in 1904, discovered a yellow palm warbler nesting in a bog consisting “of large open expanses thickly carpeted with sphagnum mosses and dotted with numerous small trees and shrubs” including black spruce, tamarack, Labrador tea, swamp laurel and sedges.

Knight wrote that it is difficult to find nests because the incubating bird won’t flush until you practically step on it and he was not willing to do the alternative—“visit the bog and spend day after day during the nesting season, fighting the voracious mosquitoes.” I know about those mosquitoes. We lived in rural Maine from 1966 until 1971, and even though we were raised in New Jersey, we could not believe the clouds of mosquitoes in our woods.

Apparently, D.A. Welsh was made of sterner stuff. He reported that the average territory in Nova Scotia was between 2.61 and 6.61 acres and included trees around the margin of a bog.

Males established territory around the ten nests he studied by singing from high song posts and chasing interlopers, including American redstarts, dark-eyed juncos, magnolia warblers and common yellowthroats in addition to other palm warblers.

Nest-building occurred between May 26 and June 7 and was constructed in the sphagnum of a peat bog at the base of a small conifer. Even Welsh, though, never saw nest building and so it isn’t known which sex chooses the nest site, if the male assists the female in constructing the nest, what time of day it is built or how long it takes.

He and other researchers discovered that the nest is cup-shaped and hidden well in sphagnum and includes weed stalks, grass and sedges, and woody stems of Labrador tea. It is often lined with feathers including those of spruce and ruffed grouse, American bitterns, barred owls, blue jays, and American robins.

Females brood four to five eggs 11 days and are fed by the males. Both parents feed the nestlings for the 12 days they are in the nest, but Welsh found that the females did most of the feeding, with the males watching closely while the females fed and vice versa. Possible nest predators were gray jays, short-tailed weasels, and garter snakes.

Once the young fledged, they stayed with their parents at least eight additional days. Presumably, after those eight days, the young are on their own, but no researcher has verified this.

From 1966 to 1994, Breeding Bird Surveys reported increasing numbers of palm warblers. Christmas Bird Counts from 1995 to 2010 found a modest increase in Florida’s wintering palm warblers. However, at Manomet Banding Station in eastern Massachusetts, they recorded a 76% decline in western palm warblers from 1970 to 2001 and a 26% increase in yellow palm warblers during the spring.

While migrating, palm warblers are often killed in collisions with tall, lighted structures. Increased mining in peatland also may impact palm warblers because they need undisturbed vegetation for nesting, and once disturbed by mining, the peat takes many decades for even a partial recovery.

Still, unlike many songbird migrants, palm warblers are not threatened or endangered. As long as their nesting and wintering ground remain reasonably secure, and they can find plenty of food wherever they travel and live, we can look forward to seeing these birds during migration.