Warblers and cowbirds

One segment of the Radiolab podcast Oops! directly concerns the near extinction of a species.  It tells the story of the efforts to save the Kirtland’s Warbler, a species of songbird that lives mostly in northern Michigan, whose population fell to less than 200 singing males by the late 1980s.

This species is a perfect example of a phenomenon described by Elizabeth Kolbert as both expanding biodiversity and making species vulnerable to extinction: adapatation to very specific environments.  In the case of the Kirtland’s Warbler, the Michigan Department of Natural Resources website lists its habitat requirements:

  • nests on the ground
  • which is almost always Grayling sand, which drains quickly protecting the nest from flooding and supporting the plantlife described below
  • under jack pines usually about 5′ tall
  • with living lower branches near the ground
  • in stands over 80 acres
  • with six to ten acres per breeding pair
  • where “dense clumps of trees” mix “with numerous small, grassy openings, sedges, ferns, and low shrubs.”

What brought the Kirtland Warbler to near extinction was the cowbird, a master of “nest parasitism.”  The cowbird makes no nest of its own, but places its eggs in the nests of other species.  Perhaps a warbler egg or two was removed by the cowbird, perhaps not.  In any case, the cowbird chicks hatch sooner and grow more quickly than the warbler’s.  Bigger and more aggressive they grab most of the food that the warbler parents bring to the nest.  One or more of the warbler chicks usually die from lack of food.

The cowbird originally lived on the Great Plains following the herds of bison to eat the insects and parasites on and around the larger animals.  They could not nest and follow the herds, so they developed this trick of using other bird species to raise their young.  As humans opened up and cleared the woods of Michigan for farmland, the cowbirds moved into the new fields near the jack pine stands where the Kirtland’s Warblers had nested for centuries.

This movement of the cowbirds into the Kirtland’s Warbler’s habitat and the subsequent devastation of the Warbler population is a perfect example of another point made by Kolbert, the point I discussed in the last post.  It’s not just the industrial revolution and the burning of hydrocarbons that is causing the Sixth Extinction.  The bucolic work of pioneering family farmers enabled the cowbird to spread into Michigan and prey on the Kirtland’s Warbler.

The Radiolab episode comes into the story after it was learned that the cowbird was the culprit in the decline of the Kirtland’s Warbler.  The forest service intervened and started a large program to catch and kill cowbirds.  The Michigan DNR site talks euphemistically of “euthanasia,” but the Radiolab interviewers were able to get one of the workers to describe very specifically how he crushed the windpipe of each cowbird.  Up to 12,000 were killed in a single year.

Still the Kirtland’s Warbler population did not rebound.  Go back to the list of its habitat requirements.  These birds need younger, shorter trees.  But the forest service had also intervened in the environment to stop forest fires.  Forests need fires to regenerate and clear space for new growth.  When the scientists realized this they started a program of controlled burns.  New jack pines followed and the Kirtland’s Warbler population started to rebound.

The Oops! in this story came one year when the service started a burn when winds were high, the woods dry, and they did not have enough personnel to control the fire, which took off and ended up destroying about 20,000 acres of woods.  After this disaster, it seemed as though the woods would never recover, but they did.  The population of Kirtland’s Warblers has grown tenfold and continues to thrive, as long as the forest service continues killing cowbirds and starting controlled burns.

The terrible fire, however, did not just destroy trees.  A forest service worker was killed in the fire.  Radiolab asked a very pointed question: was saving a rare species of warbler worth a human life?  As they said, there are thousands of other species of warbler.  Was saving the Kirtland’s Warbler worth the life of a young man?  Radiolab asked this question of forest service personnel, who weren’t sure it was worth it.  They interviewed the family of the dead man.  One of his relatives said that it might help if Kirtland’s Warbler were made the state bird.  The young man’s mother said, quite directly, don’t ask that question around here ever again.

Worth.  We have been talking about “dignity” in these last few posts.  “Dignity” comes from the Latin word, dignus, which means “worthy.”  What is a human life worth?  Or in the case of Louis Zamperini in a Japanese POW camp, what am I worth doing or not doing in order to stay alive, or what is staying alive worth before I do or don’t do something my captors are forcing on me?

Worth.  Is saving a rare species of warbler worth killing thousands of cowbirds?  Even the Michigan Audubon Society admits that “the Kirtland’s Warbler is a conservation reliant species and highly dependent on intensive human management and is unlikely capable of having a self-sustaining population.”

Like all nouns, “dignity” and “worth” obscure the actions that are really going on.


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