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Photo of Tennessee warbler.

Opportunistic Tennessee Warblers

A spruce budworm outbreak is bad news for trees. But Tennessee warblers flock to feast on the caterpillars, and birders may flock too—to see a rare breeding species in Minnesota.

By Gerald J. Niemi

Tennessee warblers are a special thrill for Minnesota birders. Upon spotting one of these drab little warblers, or more likely hearing its distinctive staccato three-note song (ticka ticka ticka, swit swit, sit-sit-sit-sit-sit-sit-sit), astute birders have cause to toast the addition to their life lists. Under normal conditions, a birder can go years without seeing a Tennessee warbler during the June-to-July breeding season.

But during a spruce budworm outbreak in the forest, Tennessee warblers can be abundant, along with two other rare warbler species—Cape May and bay-breasted warblers. Then Tennessee warbler breeding-bird densities can spike to 80 pairs per 40 acres, or two pairs per acre. No wonder they can sometimes seem to be everywhere if you are within a budworm-infested area. Beyond delighting birders, the hordes of these dainty insect eaters are being found to benefit the trees under caterpillar attack.

Poorly Named.

Tennessee warblers are generally scarce in Minnesota; they occur in the northern portions of the state where we find coniferous trees and spruce budworm. Spruce budworm is actually a poor name because Choristoneura fumiferana, the scientific name of the Eastern spruce budworm, primarily attacks balsam fir trees, though it will feed on other conifers. Tennessee warbler is also a poor name. Naturalist Alexander Wilson observed and scientifically described the species on the banks of Tennessee's Cumberland River in 1832, giving it the name Tennessee warbler. The observed bird was obviously a migrant because this species of the boreal forest breeds across Canada and only rarely in the United States in Minnesota and portions of New England.

The Tennessee warbler is referred to as a Neotropical migrant. Neotropical relates to its habit of migrating back and forth between northern North America and its wintering grounds south of the U.S.–Mexico border. Tennessee warblers overwinter from southern Mexico through Central America and into northern South America. Alexander Skutch, a famous tropical naturalist, suggested that the Tennessee warbler actually be named the coffee warbler because of its strong attraction to coffee plantations in countries such as Costa Rica, Panama, Columbia, and Venezuela. It has been found to be most common in shade-grown coffee plantations. Hence, purchasing shade-grown coffee definitely benefits this species.

Mortality of Tennessee warblers is high during their more than 2,500-mile migration. The birds must survive moving through cities with buildings and windows and across landscapes with an increasing number of towers and wind turbines. As residential and agricultural development progresses, they find fewer suitable stopover habitats of forests and shrub areas to forage and replenish their fat stores. Without replenishment, it is impossible for them to make this annual journey.

In Minnesota.

The Tennessee warbler is among the later-arriving spring migrants to Minnesota. The majority of individuals migrate through Minnesota on their way to Canada from mid-May to early June. A few individuals have been observed as early as late April in southern Minnesota, and they often begin departing the state by mid-July.

Little is known about the nesting biology of Tennessee warblers in Minnesota or in its entire breeding range through much of Canada, because they are so secretive, typically have low breeding densities, and breed in remote places. We actually know more about their wintering biology in the tropics because they occur in places associated with humans (compared with the wilds of the boreal forest).

During the breeding bird surveys from 1991 to 2009 in Superior National Forest conducted by the University of Minnesota Duluth, an annual average of 21 Tennessee warblers (range from 1 to 83) have been counted each year. This is a small fraction of 1 percent of the total individual forest birds counted, making it one of the rarest warblers in northern Minnesota. The only state nesting record for this species was in St. Louis County, according to the Minnesota Ornithologists Union.

Like most species in the genus Vermivora, Tennessee warblers nest on the ground. The discovered nests have been embedded in sphagnum moss or other soft ground covers. The nest's outer portion is made of grasses, sedges, or weed stems and lined with fine grasses, rootlets, or mammalian hair from moose or deer (how they find moose hair is a mystery).

Tennessee warblers lay an average of five to six eggs per clutch. The female incubates the eggs alone, but the male has been observed feeding the female on the nest. Both sexes feed the nestlings. On the diminutive ground nests, the parent birds raise their young in the midst of tumultuous June thunderstorms. They must hope their nests will go unnoticed by roaming predators ranging from mice to wolves.

Looking for Budworms.

In the breeding season, Tennessee warblers search for spruce budworm outbreak areas. We do not know how they find them, but presumably either they return to suitable areas where they were born or previously nested, or they randomly search for new budworm areas. We presume they nest in budworm epidemic areas, but their overall breeding distribution across the vast boreal region is unknown.

Interestingly, scientists have observed that Tennessee warblers lay larger clutches of eggs, up to eight, during budworm outbreaks. This suggests they can respond quickly to the availability of abundant food, such as spruce budworm populations approaching a million per acre. Tennessee warblers weigh about 12 grams (as much as six grapes), so to produce eight eggs during less than two weeks is bio-energetically amazing. Each warbler egg averages about 1.5 grams, so this is like a human producing 12 percent of their body weight eight times in less than two weeks. For a 150-pound person, that'd be like having eight 18-pound babies within a two-week period!

To achieve this feat and to successfully raise larger clutches of young, Tennessee warblers need to consume a tremendous amount of food. High densities of spruce budworm make this possible. It has been estimated that each bird consumes about 6 grams of budworm, or between 65 and 100 caterpillars, per day. For a 150-pound human, that'd be like eating 75 pounds of hamburger a day.

Natural Pest Control.

Caterpillar-eating warblers reduce the intensity and the severity of insect damage to forests. C.S. Holling, respected ecological modeler and father of the principle of adaptive management, has done classical modeling of predator–prey dynamics over the past 50 years and in particular the spruce budworm cycle in Canada. He emphasized that insectivory by forest birds was a key component in maintaining the natural dynamics of spruce–fir forest ecosystems. Disruption of these predator–prey dynamics—such as the reduction of insect consumption because of declining Neotropical migrant populations—could lead to less resilient and more vulnerable ecosystems.

With spruce budworms affecting thousands of acres in Minnesota each year, the implications for forest health and management are immense. Caterpillars eat needles. Trees need needles for photosynthesis to grow. As they grow, trees consume atmospheric carbon dioxide, a primary greenhouse gas, releasing oxygen back into the air and storing carbon in wood. If there are fewer caterpillars and more needles, then there is greater tree growth, reduced carbon dioxide, and a healthier forest.

Not only can Tennessee warblers help control spruce budworm numbers, they also can help reduce the cost of control. One paper by Canadian scientists estimated that a pair of insect-eating birds equals about $4,720 per square mile in insecticide applications.

In return for the service of helping maintain Minnesota's 2 million acres of spruce–fir forests, Tennessee warblers gain a bounty of nutrition to raise their young. And spruce budworm provide a fascinating phenomenon for birders in search of the rare Tennessee (or coffee) warbler, the tiny drab bird that flitters through the spruce and fir gleaning caterpillars, going mostly unnoticed, except when it belts out a bold and piercing three-parted song—ticka ticka ticka, swit swit, sit-sit-sit-sit-sit-sit-sit!

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