Cerulean warblers in Iowa get a break

HARPERS FERRY - Mature walnut trees that might otherwise have been harvested will be spared to provide habitat for the beautiful, mysterious and threatened cerulean warbler.

'This is definitely the first time we have considered the presence of ceruleans in tree harvest decisions,” said Bruce Blair, the Department of Natural Resources forester who manages the Yellow River State Forest, a cerulean stronghold.

'The ceruleans are in my management flow chart because they are here and need protection,” he said.

Bird researcher Jon Stravers of McGregor, the first to document unsuspected throngs of increasingly rare cerulean warblers in remote bluffs and ravines in northeast Iowa, believes that large walnut trees may be a key component of the habitat ceruleans find so attractive.

When he learned of the prospective harvest, he asked that certain specified trees be spared.

'We are aware of the big walnut trees in some of the cerulean hot spots, and we are trying to balance all the interests,” Blair said.

The harvest, he said, was prompted by recent severe gully erosion that has either toppled some big walnuts, or threatens to do so soon.

Rather than let valuable trees go to waste, the DNR solicits bids from commercial loggers. Such sales, Blair said, typically would include other nearby trees that - while in no immediate peril - either have health problems or limited life expectancy.

'I'm in no hurry to cut healthy walnuts. We'll save from half to two-thirds of the healthy walnuts” in the cerulean hot spots, he said.

'The DNR deserves credit for considering these birds in their management plans,” said Stravers, whose cerulean research was instrumental in securing for the DNR's Effigy Mounds-Yellow River Forest Bird Conservation Area the state's first designation as a Globally Important Bird Area.

The hot spots - clusters of cerulean nesting territories - often center around what Stravers calls 'wolf trees,” large mature specimens set slightly apart from neighboring trees, many of which are black walnuts.

Stravers research and complementary research led by Upper Iowa University biology professor Paul Skrade will attempt to pinpoint the importance of walnut trees to nesting ceruleans.

Upon first hearing Stravers's data on the cerulean density in the Yellow River State Forest, Skrade said he uttered a common barnyard epithet. But his own independent research since has confirmed Stravers's findings.

Last year with 462 point-count surveys and 152 interpoint observations, Skrade and colleagues detected 334 ceruleans, which enabled them to map 182 active male cerulean territories in Yellow River State Forest. Stravers in 2014 documented 192 active cerulean territories.

Also consistent with Stravers's research, Skrade and colleagues found that cerulean territories often were clustered near water in areas close to bluffs or other steep inclines among large mature trees with open patches in the upper branches.

Skrade said the research suggests a correlation between clusters of cerulean territories and the presence of mature black walnut trees - a focus of his continuing research.