Sunday, January 18, 2009

Warblers in Winter

Warbler watching in winter in Northern Virginia? Who does that, you say? Well, this year, I'm doing it! December and January are months when birders usually turn their attention to waterfowl, raptors, and sparrows, not warblers. Certainly, encountering Yellow-rumped Warblers along the trail is expected at this time, but seeing any other warbler species is far from a sure thing.

Beginning in early December, birders have been reporting sightings of warblers along the George Washington Memorial Parkway, from the LBJ Grove to Dyke Marsh. Joining Yellow-rumps have been Pine, Palm, Common Yellowthroat, Orange-crowned, Yellow-throated, Yellow, and Northern Parula. Although not all at one location on one day, that’s still eight warbler species!

Many birders' hearts go thumpety-thump when a rarity is reported. The listers among them run out to snag a new bird for their life lists, year lists, month lists, whatever lists. None of these birds are true rarities, however, just uncommon to rare at this time in this place, except for the widespread Yellow-rumps. So birders haven't been flocking to the parkway to view warblers who are trying to tough it out through what may turn out to be a colder-than-normal winter.

I thought it might be interesting to see how often these eight species turn up on the D.C. Christmas Bird Count which encompasses this part of the GW Parkway.

Although it must be assumed that birds are often missed during CBCs, looking at the last 10 years of data, 1997-2007, still provides some sense of how often individuals of these 8 species can be found in December along the Potomac River, just south of Washington DC.

Here's what I learned:

Yellow-rumps: found every year in numbers ranging from 34-78.

Palms: found 7 of last 10 years, with numbers varying widely, from 1 to as many as 28 in 2002.

Common Yellowthroat: found most years, but just 1 to 4 of them.

Pine: found in half the years, similar to Common Yellowthroats in numbers, 1 to 5.

Orange-crowned: found in half the years, but just one individual in each of those years.

Yellow: found just once, one bird in 2001.

Northern Parula: never found.

Yellow-throated Warbler: never found.

I haven't seen all of these eight species myself, but a Yellow, Palm, and Orange-crowned, pictured below, were along the GW Parkway between the Belle Haven parking lot and the stone bridge on Jan. 13. A Common Yellowthroat was there too, but declined my invitation to appear in my blog. All seemed active and appeared to be foraging successfully.

Unquestionably, CBC data does not provide anything close to the whole picture. Last year, a Yellow-throated Warbler spent most of the winter at a park in Arlington, where it used the feeders, picked up suet crumbs on the ground, and poked around the eaves of the nature center, presumably finding over-wintering insects. I photographed this bird in February of 2008 at Long Branch Park.

Insects are the primary food of these eight warbler species, but many of them rely on a variety of food sources in winter, such as berries, tree sap, nectar, plant galls, even seeds and other plant material. Surprisingly, the Yellow-throated is one for which alternative food sources are not listed in The Birders Handbook, by Ehrlich, Dobkin, and Wheye. The authors say the diet of this species is not well known, but may include spiders. I guess it can be safe to say that seeds and suet can be alternatives in winter!

Temperatures dropped to as low as 8 degrees a couple of nights ago. I worried about these tiny birds, hoping that they had eaten enough to stoke their metabolic fires and keep them warm through the night.

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