Thursday, July 5, 2012

How to become an endangered species

2015 update: I did some experiments and found that little bee-eaters, African hosts of another deadly brood parasitic bird, are also remarkably maladapted when it comes to avoiding parasitism.

I recently made a pilgrimage to see a very special bird, the Kirtland's warbler (Setophaga kirtlandii), one of the rarest and most endangered wood warblers
Male Kirtland's warbler in all his breeding glory

This wee creature has decidedly unfortunate tastes in where to breed. It is both fussy and unaesthetic, nesting only in the scrubbiest, shabbiest stands of young jack pines (Pinus banksiana) that grow in vast stands on sandy soil in northern Michigan. The success of Smokey Bear, the US Forest Service's campaign to stop wildfires, allowed all the jack pines to grow big and strong, leaving almost no young trees for the warblers to nest amongst. 

Cowbird egg (the big one) in a Kirtland's warbler nest

To make matters worse, human development has greatly expanded the range of the brown-headed cowbird (Molothrus ater), a brood parasite that lays its eggs in the nests of other species, thereby dispensing with the bother of incubation and childcare. The Kirtland's warbler was not historically exposed to cowbirds -- named for their habit of following bison (and subsequently cows) to eat insects disturbed by the moving herds. Without having evolved in tandem with the cowbirds, Kirkland's warblers are hopeless when faced with a cowbird egg in the nest. Instead of tossing the foreign egg or chick out, these tiny birds proceed to care for their parasitic offspring, which grabs all the food and starves its smaller warbler foster siblings to death. By the time they are fully grown, cowbirds are 3-5 times heavier than their foster parents.
Courting cowbirds

With habitat loss and intense parasitism by cowbirds, the Kirtland's warbler almost went extinct about 50 years ago. However, it has since become one of the best conservation successes in America. The U.S. Department of Agriculture Forest Service and the Michigan Department of Natural Resources began logging jack pines to simulate the disturbance caused by wildfires, and to produce large enough (at least 160 acres) areas of short young jack pine to suite the Kirtland's warblers. In addition, they trapped and killed thousands of cowbirds. As a result of these ongoing efforts, the Kirtland's warbler has now expanded its breeding range to bits of Wisconsin and Ontario.

The Kirtland's warbler is now the subject of considerable local attention, partly because it is an eco-tourist (or at least mad birder) attraction. There is a Warbler's Way Inn, and until recently, a Kirtland's warbler festival. The jack pine habitat is protected, so one is forced to join a tour to see birds. That said the tour was excellent, and well worth waking up at 2am and driving for 3 hours to see several male Kirtland's singing with gusto.

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