Multi-year study notes steady population of Curelean Warblers in Yellow River State Forest/Effigy Mounds area while worldwide populations are in decline


Small in stature, but significant in nature ... Although small in size, the presence of the Cerulean Warbler noted in a 13-year study conducted in northeast Iowa signifies a steady population of the bird in the local area when other such studies have noted decreases in its population, including as much as an estimated 70% decline in population worldwide by Cornell University. Pictured above, is a Cerulean Warbler female and pictured below, a Cerulean Warbler male, with the bird’s presence in the northeast Iowa region playing a large role in the Effigy Mounds/Yellow River Forest Bird Conservation Area being designated as a “Globally Significant Bird Area” by the National Audubon Society and by the Birdlife International Important Bird Area program. Submitted photos courtesy of Kat Busse.

Findings lead to “Globally Important Bird Area” designation; Credit given to YRSF forest management

by Kelli Boylen

A small blue neotropical migrant bird in Allamakee County might not excite some people, but its presence might mean more than you think.

Cerulean Warblers are small, forest-dwelling birds. They are about four inches long, have a wingspan of eight inches and weigh a mere one-third of an ounce.

Estimates of experts vary, but Cornell University estimates there has been more than a 70 percent decline in the worldwide Cerulean Warbler breeding population between 1970 and 2014, mostly due to habitat loss and degradation in nesting and wintering areas. Overall, that is grim news for the species, but here in Allamakee and Clayton Counties, Cerulean Warblers are doing well.

Jon Stravers, Kathleen Carlyle and Dr. Paul Skrade of Driftless Area Bird Conservation recently released a report on the Cerulean Warbler that is the culmination of more than 12 years of investigation in several local study areas including Yellow River State Forest, Effigy Mounds National Monument, Pikes Peak State Park and Sny Magill Complex within Pool 10 of the Upper Mississippi River National Wildlife and Fish Refuge.

Their report is significant in that other similar studies in traditional breeding areas of the eastern United States have documented a decrease in the presence of this bird, but here in northeast Iowa the number of sites where they nest and the number of birds at the sites has held steady during the 13-year study.

“Our observations of high rates of reoccupation of Cerulean territories may be of special importance in light of dramatic declines of Cerulean Warblers reported in other studies in North America,” says Stravers.

Stravers credits Ceruleans doing well here to good forest management at Yellow River State Forest (YRSF) led by Forester Bruce Blair, as well as the presence of large tracts of forest and bordering land undisturbed by industry or intensive agriculture practices. Forestry practices such as long rotation timber extraction and selective logging to create natural canopy gaps and uneven-aged forest stands are very beneficial to the species.

Blair says, “There are most likely a number of factors combined that contribute to the abundance of Cerulean Warblers. We are aware there are many ceruleans warblers here and so we have the responsibility to manage the forest appropriately in a way to limit negative impacts and try and promote management practices that will yield positive impacts.”

Carlyle says one of the goals of this report is to shine a light on the forestry practices in YRSF that are helping Ceruleans. “This is thanks, in part, to Stravers building open channels of communication and respect with forest management. In the real world with the competing goals of forestry management, what they are doing can’t be underestimated. We are lucky to have Bruce, a forest manager who gives weight to avian habitat needs!”

In 2013, YRSF and Effigy Mounds became Iowa’s first “Globally Important Bird Area”, in a large part due to Stravers’ research on Ceruleans. It was known that there were a few Ceruleans in the local area, but his research showed populations far exceeding previously known numbers. The density of Cerulean Warblers documented in the region by these surveys was a key factor resulting in the designation of the Effigy Mounds/Yellow River Forest Bird Conservation Area as a “Globally Significant Bird Area” by the National Audubon Society and by the Birdlife International Important Bird Area program.

Cerulean Warblers are hard to see because they tend to stay up in the tree canopy and their light blue back and white belly make them hard to see against the sky. Stravers says it is actually easier to identify their presence by listening for their trill than trying to spot them. They eat insects and are not seen at bird feeders.

Even though they are hard to spot, bird watching enthusiasts are willing to travel here for the chance to log a sighting - or even just to hear their trill. This means this small creature could be beneficial to the local economy.

“Stravers’ 13 years of study of these birds documents Cerulean presence on the northwestern boundary of its range on a scale not known before,” says Carlyle. “This effort and data needed to be preserved and shared in avian science circles. Some of our goals for this report were to draw scientific interest in Ceruleans to the Effigy Mounds/Yellow River bird conservation area and to promote continued and expanding protection of habitat for Ceruleans, as well as funding for additional and continuing study.”

Stravers said he initially thought that the Ceruleans gravitated toward undisturbed old-growth forests that include white oaks and black walnuts, but after years of observation he has found they specifically like areas with these mature trees and a variety of young trees. They also like openings in the tree canopy and areas with protective cover. Management practices including prescribed burns and savannah restoration are helpful.

Cerulean warblers will return to the same nesting areas year after year and live in clusters of neighboring territories. Stravers says it seems that they have communities and they know their neighbors.

The only area Stravers surveyed where he had previously found Cerulean Warblers that they were absent in 2020 was in Sny Magill Complex, located south of McGregor. “The past decade has seen an increasing trend of extensive and prolonged flooding as a consequence of the increased frequency and severity of extreme rain events. This trend has resulted in visible damage to the tree species favored by Ceruleans, and flooding of the Sny Magill elevated ridge area well into the breeding season.”

When they are not hanging out in Iowa or other parts of the eastern United States during their May through July breeding season, they migrate 3,000 and winter in the northern Andes Mountains in Columbia.

Stravers noted that it’s not just the forestry management plan at Yellow River State Forest that makes it a good home, it’s the way it’s used by visitors. “Hiking, horseback riding, fishing and bird watching are all great, quiet activities that co-exist well with these birds.”
“Yellow River State Forest is really a special place and it’s quite fitting that such a special species is an emblem of the unique habitat that is protected here,” says Carlye.

Anyone who would like to learn more about the study or to make a donation toward research can write to the Driftless Area Bird Conservation at P.O. Box 309, McGregor, IA 52157.

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