Friday, April 20, 2012

Dastardly Duos - Yellow Female Warblers

By Larry Liese
From the Vermilion Vaults
Originally appeared: April 2003 issue

Ever notice how most exciting bird stories are about sighting and identifying some spectacular bird with bright red and blue and violet and …? But what about those tough-to-ID females that do all the work of getting the next generation going? Those of us who start getting serious about trying to ID every bird we get a good look at have to be familiar with females and young too. While males of many species advertise their fitness as mates with elaborate displays and bright plumage, females generally incubate the eggs so blander plumage is a plus.

This issue’s Duos are a threesome of mostly yellow female warblers. Two are common breeders and the other is one of our most common migrants. Guessed them yet? The Yellow Warbler (Dendroica petechia) and Common Yellowthroat (Geothlypis trichas) both breed here and pass through in significant numbers in migration. Along with the latter are many Wilson’s Warblers (Wilsonia pusilla). Though they don’t breed in Arizona, they come close, breeding in most western states and almost all of Canada and Alaska. Though Wilson’s Warblers have fairly consistent plumage patterns in their three subspecies, there are 43 Yellow Warbler and 13 Common Yellowthroat subspecies leading to a somewhat wide range of color patterns. In our area, the Yellowthroats have more yellow than in other parts of the country, but still have much less yellow than the other two. Wilson’s Warblers are always bright yellow underneath, while female Yellow Warblers usually are, but can look quite pale, particularly in first-year birds.

A good look at the undertail will easily distinguish Yellow from Wilson’s Warblers. The Yellow’s tail is quite short, with the undertail coverts extending closer to the tail tip than on the others. Furthermore and most evident is that the undertail is almost entirely yellow, with only dark corners on the tip. In contrast, Wilson’s have a solidly dark undertail that contrasts strongly with the bright yellow undertail coverts. Even a quick look is generally enough to see this difference. Common Yellowthroats are so often very low to the ground in their preferred marshy habitat that I can’t say that I remember ever seeing their undertails! Inspecting the specimens at the U of A Bird Lab showed a similar light/dark pattern to the Wilson’s, but with less contrast. The Wilson’s all showed an obvious notch, while the Yellowthroats did not and the Yellows were hard to determine. Various references didn’t seem to be consistent in this regard. It’s one field mark I plan on paying more attention to this spring!

A key difference to look for in Yellow Warblers is the yellow edgings to the feathers of the wings. The remiges[flight-feathers], coverts, and tertials all have this, giving a very recognizable pattern to the wing. Both the Wilson’s and the Yellowthroat have uniformly dark olive upperparts, with the Yellowthroat being darker with less color.

In overall size, the Wilson’s is smallest, though with its short tail the Yellow at first seems of similar size but weighs almost a quarter again as much. Yellowthroats are a smidge heavier than the Yellow and slightly longer.

Yellow warblers have a plain face and dark eye that give them a ‘blank’ look. Wilson’s have a dark area on the cheek and a dark crown (gets darker seasonally, at least on some subspecies), accentuating a lighter supercilium and forehead. Yellowthroat females have a whitish eyering if one looks closely, and usually show contrast between the lighter throat and darker malar area.

The males usually do the majority of the singing, but call notes can help. Yellows give a rich musical chip note, Wilson’s call is described as a husky or nasal chimp, and the Yellowthroat’s calls vary considerably but have a raspy quality. Take advantage of the abundance of these three species during migration to get familiar with their calls. As the really good birders-by-ear all say: watch the bird making the sounds and verbally describe what it sounds like to you. That is what locks the sight and sound together in your memory. Good advice!

So observe these three a little longer than normal as you run into them this spring. When you hear the male Yellow’s sweet, sweet, sweet – I’m so sweet; or the Yellowthroat’s witch-ity, witch-i-ty, witch-i-ty; look for the nearby female and take a good look at her field marks. This will help you become that better birder. Good luck!

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