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Yellow Warbler

by Linda de Kort

Yellow Warbler – Photo Credit: Dan Casey

Be looking soon for a woodland bird that weighs less than two nickels, is yellow like a canary (or the cartoon character “Tweety Bird”) and sings in a tumbling series of whistles that sounds like “ sweet sweet sweet I’m so sweet”.

These Yellow Warblers are among the first of the warblers to move up the North American continent in the spring from their wintering grounds. Many birders start looking for their FOY (first of year) warblers about this time.

Warblers are small insectivorous songbirds. There are about 14 species of warblers that breed in Montana and several more that migrate through in the spring and the fall. Contrary to their name, most warblers do not sing in trills; the Yellow Warbler is one of the beautiful, conspicuous exceptions.

As their common name implies, these warblers are extensively yellow, with face, throat and underparts a very bright lemon color. Upperparts are a more yellowish green, and males have chestnut streaking below the throat on the breast. Studies at Cornell Lab suggest that the more aggressive males may exhibit more streaking. Females and young are not quite as bright yellow as the males. All are also easily identified by their brilliant unmarked faces accentuating the very dark eye.

As the range map indicates, Yellow Warblers have a wide distribution in the Americas. Most varieties are migratory and leave their wintering grounds of mangrove forests in early spring. According to the newly published Birds of Montana (by Jeffrey S. Marks, Paul Hendricks, and Daniel Casey, and sponsored by Montana Audubon), the migrants  appear by the first week in May; the earliest recorded sighting in our state was at Rainy Lake on April 29, 2005. The southward migration peaks in August and virtually all have left the state by September, though there has been a verified sighting as late as November 15 in Missoula in 2012.

Like most warblers, they prefer the native willows and cottonwoods along stream banks and wetlands. They are found in abundance in these habitats from one end of the state to the other. Removal of native plants and the increase of Russian Willow have a negative impact on the nesting pairs. They can also be seen in shelterbelts and urban parks, especially those with native plant species. A pair will mate and stay together for the season. The nest is usually constructed by the female as the male persistently proclaims his sweetness from the treetops.

The nest begins with a woven ring in an upright fork of a tree and is filled and flattened with grasses, bark bits and plant fibers. Nest building can take a week or more but is usually completed in a couple of days. The first clutches are usually laid by early June and fledglings can by spotted by early July. Both parents feed the young, the female twice as often as the male; for he keeps other competitors and predators from his nest by exclaiming from the tree tops how sweet he is. The young and adults feed on insects, so you will not find them at your bird feeder. Look for them foraging frenetically in bushes as they glean caterpillars, spider egg cases and other delicacies. The tent caterpillar seems to be one of their favorites and possibly they help keep this annoying insect in check.

The Yellow Warbler has an interesting method of dealing with parasitism from Brown-headed Cowbirds. The female Cowbird lays her eggs in the warbler’s nest, attempting to pawn off the task of incubation and rearing to warbler parents. The warbler, however, will bury the interloper’s eggs and possibly her own by building another nest lining. The record is a six-storied nest with a total of eleven cowbird eggs buried between the layers.

A group of Yellow Warblers are collectively known as a “stream” or “sweetness” which is appropriate since by a stream is where you will find them, and “sweet sweet sweet” is what he will sing. Good luck in searching out YOUR “FOY” this month.

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