Tuesday, April 11, 2006

Birds of the Mid-Atlantic #6: Yellow-rumped Warbler

The yellow-rumped warbler (Dendroica coronata) is one of the most familiar warblers across most of North America. While other warblers have struggled or declined, this species has thrived, and maintains stable numbers on its breeding and wintering grounds. In the early stages of songbird migration, particularly in April, yellow-rumped warblers pass through the mid-Atlantic states in great numbers. Sometimes there will be so many that it will seem as if every small bird is a yellow-rumped warbler.

"Myrtle" Yellow-rumped Warbler / Photo by Jesse Achtenbach (USFWS)

Yellow-rumped warblers are divided into two subspecies, Myrtle and Audubon's. The Myrtle subspecies (D.c. coronata), pictured above, is primarily found in the eastern half of North America, with a breeding range across Canada and into Alaska, and some wintering territory in California. The Audubon's subspecies (D.c. auduboni), pictured below, is largely confined to the western United States. The primary way to tell the subspecies apart is by the yellow throat on most adults of the Audubon's subspecies, which the Myrtle subspecies lacks. These two subspecies were formerly separate species, and it is conceivable that they may be separated again. In the meantime, many birders - especially older ones - continue to eschew the species name for the subspecies names.

"Audubon's" Yellow-rumped Warbler / Photo by Dave Menke (USFWS)

The yellow-rumped warbler is one of the few warblers that will winter in the mid-Atlantic states, and the only warbler species that will winter in significant numbers. It is able to do this because it has a more varied diet than most other members of the family Parulidae. Most warblers depend exclusively upon insects and other arthropods. During cold winters, or even cold snaps in warmer winters, the arthropod supply will mostly disappear, meaning that insectivorous birds must vary their diets or head for warmer climes. The yellow-rumped warbler takes the strategy of varying its diet. In addition to insects, it will eat bayberries, wax myrtle, and poison ivy. The second of these earned the eastern subspecies its name.

Why would an insectivorous bird adopt this strategy? Migration is fraught with difficulty, especially when it involves long flights over open water. Species that can find a way to survive the winter without a long trip will have fewer losses due to exhaustion and stormy weather in flight. A shorter trip to breeding grounds in the spring also means a leg up on competing species. Yellow-rumped warblers can set up breeding territories and establish food sources while other warblers are still travelling, which in turn makes for a longer window for raising their young. It is no accident that this species is so abundant.

For upstate New York readers: If you are planning on birding in the Adirondack Mountains this summer, you might want to look into getting a copy of a new birding guide that covers the region. It gives dates and locations for finding many of the region's specialities and includes a birding map. The guide is published by the Franklin County Tourism Office and is being used to publicize the upcoming Great Adirondack Birding Celebration. I do not see any information about the guide on the Franklin County website at the moment, but it looks like you can request one from the site for Hamilton County's Adirondack Birding Festival.

Crossposted at Blue Ridge Gazette.