McGREGOR – A remote expanse of northeast Iowa is home to one of the upper Midwest’s largest concentrations of the increasingly rare cerulean warbler.

The area is a happy home for the warblers for obvious reasons said Jon Stravers, a bird researcher who helped discover the pocket while conducting a bird census for the Iowa Department of Natural Resources.

“This habitat is big, old, deep, wide and seldom visited by people, all qualities favored by the cerulean,” Stravers said.

The “Cerulean Trench,” as Stravers dubbed it, runs along the North Cedar trout stream, a tributary of the better-known and more often visited Sny Magill, which empties into the Mississippi River just below McGregor.

The valley of the North Cedar is “chock full of high priority birds,” like the cerulean warbler, the veery, the scarlet tanager and the wood thrush, said Kelly McKay of Hampton, Ill., who also helped discover the pocket of birds while working for the DNR.

The U.S. Fish and Wildlife Service announced late last year that the cerulean warbler would not be listed as threatened under the Endangered Species Act, despite steep declines in the migratory songbirds.

Stravers and McKay spent 10 minutes watching and listening for birds at 13 sites spread throughout the 1,800-acre Sny Magill-North Cedar Wildlife Management Area. It was not unusual to find between eight and 15 ceruleans a day.

Named for the color of its plumage, the secretive bird typically resides in the canopy of tall trees, where it becomes virtually invisible against the backdrop of the cerulean sky.