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Tennessee Warbler

By Gail Cleveland

Among the breeding wood warblers in Western Montana, the Tennessee Warbler has been the most elusive and difficult to see each summer. For 25 years, my husband Bruce and I have taken an annual May bike ride from the Trego area down Wolf Creek to the Fisher River and on to the Kootenai River. Each year we searched for this small, indistinctly marked warbler with a fine, sharp bill and a short tail with a distinctive three-part song. Finally, in 2010 we hit gold!

The Tennessee Warbler likes mixed woodlands, as long as there is a bushy and mossy understory. Our first and only glimpse of one in Montana before 2010 was in McGee Meadows in Glacier National Park. Along our yearly bike route are boggy areas and streams with prime habitat. They nest on or near the ground, often in sphagnum moss or hummocks in bogs.

Visually, Tennessee Warblers can be mistaken for Orange-crowned Warblers. The head is gray and the back is olive. The breast is whitish to yellowish. There is a pale white line above the eye and an indistinct grayish eye line. Orange-crowned warblers have yellow undertail coverts; Tennessee Warblers have white undertail coverts. This is the distinguishing feature to look for.
The songs of these two warblers are, thankfully, quite distinct. The Orange-crowned has a rapid trill that seems to lose energy toward the end, dropping in volume and pitch. The Tennessee is very vocal, having a loud, staccato three-part song.

After this first sighting of the season, we saw this vocal warbler singing near the tops of aspen trees in Glacier National Park, near a boat landing on the Flathead River, at Tally Lake campground, up Graves Creek near Eureka and on the Columbia Mountain trail. Were our eyes and ears just not tuned in to the Tennessee? Had they been here other years, and we just didn’t notice them? According to the Montana Field Guide developed by the Montana Natural Heritage Program and Montana Fish, Wildlife and Parks, sightings of Tennessee Warblers, both breeding and transient, were the highest in 2010 since observations began in 1980. Further research resulted in a viable answer to the increase in sightings. It all depends on the availability of the right food.

The Tennessee Warbler is a “spruce budworm specialist,” according to several sources. Their numbers vary from year to year based on budworm outbreaks. There is little doubt that western Montana is the middle of such an outbreak.

The western spruce budworm, Choristoneura occidentalis, is the most widely distributed and destructive forest defoliator in western North America. Their choice host in western Montana appears to be Douglas fir trees. Occasionally, they also attack spruce and subalpine fir.

Dead new foliage, webs on new foliage, and thinning needles are hallmarks of the spruce budworm. Outbreaks generally last three to five years. Young trees may die; older trees become stressed and susceptible to beetles. Glacier National Park around St. Mary’s Lake is an especially hard hit area where it is easy to see the damage done by this pest.

In late April or May the larvae, which hibernate in the bark or under lichen, migrate to the foliage, where they mine old needles or feed on host tree flowers. In a week or two, they enter developing buds, the habitat that gives them their name. As the new needles lengthen, the rapidly growing larvae continue to feed. They feed inside webbed foliage, where they are somewhat protected from predators. One such enemy is the Tennessee Warbler who feasts on the budworm and relies on them to raise their young.

How many Tennessee Warblers will be here in 2011 is difficult to know, but this plain, very vocal warbler is a “good” predator that we hope to see often again this summer.

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