Nature’s Secrets

Climate is overheating for thrush

July 15, 2012 

Meg Lowman is an N.C. State University professor and forest canopy expert who directs the Nature Research Center, N.C. Museum of Natural Sciences.

Few animal sounds are as beautiful as bird song. Once you’ve heard a whippoorwill throwing the boomerang of its voice across the summer marshes, you listen with a new sense of privilege.
Diane Ackerman, “A Natural History of the Five Senses”

North of Stowe, Vt., along some steep mountain trails of Mount Mansfield, is a two-decade-long research site investigating the impacts of climate change on songbirds. Not surprisingly, the ornithologists leading this project have documented birds shifting uphill to escape increasingly warmer temperatures at lower elevations.

Ten years ago, no Swainson’s thrushes nested near the top of Vermont’s tallest mountain; now they are abundant. Project leader Chris Rimmer directs the Vermont Center for Ecostudies, and has dedicated a large portion of his life to researching the Bicknell’s thrush. This rare and threatened species, a relative of the commoner wood thrush, is limited to alpine mountain tops of New England and Canada. Where does an alpine bird living at the top of a mountain go when warmer temperatures force it to seek cooler temperatures? Unfortunately, the Bicknell’s thrush and other mountain-top denizens may face extinction.

Rimmer explained that not only are the bird distributions squeezed by warming temperatures, but the firs require cool temperatures for survival, and are predicted to die off on many New England mountaintops. The loss of forest habitat will be another nail in the coffin of the Bicknell’s thrush, which seeks this evergreen tree for nesting.

A third environmental issue threatening this bird’s future involves its wintering grounds. These thrushes fly almost exclusively to Haiti and the Dominican Republic for the winter, and both islands have experienced extreme deforestation. With losses of both summer and winter habitats, this rare songbird is in jeopardy.

The Bicknell’s thrush is one of only several North American birds renowned for a unique sexual behavior: Not only do females mate with more than one male, but the males mate with multiple females. To ensure the survival of their progeny, males busily feed young in multiple nests. Who is the father of each young? Neither females nor the males know for sure, but as a consequence, thrush babies are well-fed by a gang of potential fathers, each hoping to insure the survival of his genetic lineage. This may confer better survival rates for the young, who receive multiple meals in this harsh environment.

Rimmer predicts that Bicknell’s thrushes will be gone from Mount Mansfield by the year 2100. Not only would North America lose a wonderful songbird, but one of Mother Nature’s most unique mating and child-rearing behaviors may become a distant memory in the ornithology textbooks.

Meg Lowman is an N.C. State University professor and forest canopy expert who directs the Nature Research Center, N.C. Museum of Natural Sciences. Online: www.canopymeg.com.

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