Wild Side: The yellow-rumped warbler

A lovely day led to swarms of happy birds and birders on Saturday.

Yellow-rumped warbler in fall plumage, taken in Aquinnah. —Matt Pelikan

Last Saturday, Oct. 28, saw a flurry of activity around the parking loop at the Gay Head Cliffs in Aquinnah. Some of the activity was human: A field trip run by the Martha’s Vineyard Bird Club brought a group of local birders, augmented by some visiting South Shore Bird Club members, to this most famous of Vineyard birding sites. And some of the activity was avian: Mild temperatures and winds from the Southwest put a temporary brake on migration, and the area around the Cliffs loaded up with southbound birds awaiting favorable winds for moving on.

The full gamut of migratory birds was evident, from loons and sea ducks to sparrows and finches. In one sense, the bird of the day was clearly a sedge wren. Roughly a once-a-decade rarity on the Island, this colorful little devil was a first Island sighting for me and several other local birders, and an out-and-out life bird for some others. Birders including Nancy Nordin and Lanny McDowell managed photographs of this reclusive gem, either on Saturday or when the bird was located again the following morning.

But in another sense, the bird of the day was just as clearly the yellow-rumped warbler. Typical for the date, warbler diversity was low; aside from yellow-rumps, the only warblers I recorded that day were a few palm warblers, and one mystery bird that might have been an immature Wilson’s warbler. But yellow-rumps made up for the lack of diversity with numbers: This species was everywhere.

My personal count for the two locations I visited in Aquinnah was 72 yellow-rumps. But that is a conservative figure, leaving out individuals posing the least concern about double-counting, and also leaving out hundreds of birds seen overhead that were unidentifiable but almost certainly this species. Thousands of these small songbirds must have been on the Vineyard that day, concentrated along the south shore, the Vineyard Sound shore, and around the western tip of the Island.

It seemed that there wasn’t a moment when this species wasn’t evident, passing overhead, flycatching around thickets, sometimes in sight, sometimes just revealed by its distinctive “check!” call note. The abundance may have been unusual, but not by much. Easily the most numerous warbler species that visits the Vineyard, the yellow-rumped warbler almost always reaches notable peaks of abundance here in late October and early November. Hundreds may still be here for the Christmas Bird Count around the turn of the year; at least a few will probably overwinter here successfully.

The sheer abundance of the species accounts for some of the numbers we experience. This is a successful generalist, breeding across a wide range of both latitude and longitude. And unlike some specialized warbler species, yellow-rump breeds successfully in a wide range of habitat types. The result of this versatility is a huge population.

Moreover, the fall migration of the eastern yellow-rumped warbler population is largely coastal, concentrating this species in coastal thickets, and bringing it to the Vineyard in droves. A neat dietary trick is associated with this migration route. While yellow-rumped warblers are largely insectivorous, like other warblers, they are also far more ready than most of their relatives to subsist on fruit and berries. The fruit of the bayberry is a particular favorite of migrant yellow-rumps, which successfully digest these tough, waxy berries.

The yellow-rumped warbler also has a different migratory pattern than most of its relatives. While there are other exceptions, the warbler clan in general consists of long-distance migrants that move between breeding sites in northern hardwood or boreal conifer forest for breeding, and subtropical or tropical sites for wintering. The yellow-rump, in part because of its ability to live on fruit, tends to move much shorter distances when migrating, and to migrate in easy stages in response to the weather. When mild weather settles in during the peak of this bird’s migration, huge numbers of individuals simply stop moving south until they feel they need to resume their journey. As was the case last Saturday, the result can be a huge concentration of lingering birds, fattening up on the berries and bugs to be found in our coastal thickets until an influx of cold air triggers a resumption in migration.

I mentioned the call note, which is how I detect the vast majority of yellow-rumps I encounter. Visually, fall yellow-rumps are rather drab, except for the bold, eponymous yellow patch above the tail. This patch is usually evident when you see one of these birds at close range. Even on birds far overhead, it’s sometimes visible as the bird turns or has its flight twisted by wind.

Otherwise, the species is mostly grayish-brown above in fall, and white below. There is often a hint of a black face mask; two white wing-bars mark the wings. The breast is boldly streaked and features subtle yellow patches just below the folded wings.

So while not exactly striking, yellow-rumps at this point in the season are easily identified by both sight and sound. And any birder is wise to be familiar with this species. You’ll be seeing a lot of them. And if you hope to find one of the more rare warblers that can still be mixed in with flocks of yellow-rumps, doing so will depend on picking out the one bird among hundreds that is not like the rest.