Bird Survey, Forest Inventory

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For the past several years, bird researcher Jon Stravers has been boating and trekking into hard-to-reach areas to document clusters of cerulean warblers, neo-tropical migrants which inhabit the bluffs and ravines near the Upper Mississippi River for approximately three months each year. “I love this place,” said Stravers. (Photo by Ted Pennekamp)

Cerulean warblers are good indicators that species such as peregrine falcons and red-shouldered hawks, as well as various neo-tropical migrants, are also abundant in the area. (Photo by Kat Busse)

Intern Nate Vogt (left), biologist Billy Reiter-Marolf and intern Melissa Blasky work on the forest inventory in Sny Magill. (Photo by Dan Phillips Jr.)


Bird survey, forest inventory may shape 

habitat management actions in areas of Pool 10

By Ted Pennekamp


Area bird researcher Jon “Hawkman” Stravers and U.S. Fish and Wildlife biologist Billy Reiter-Marolf have recently completed a bird survey and a forest inventory in Sny Magill and Bagley Bottoms in order to document bird diversity, abundance and occupancy. 

The National Audubon Society, the Fish and Wildlife Service, the Upper Mississippi River National Wildlife and Fish Refuge, and the U.S. Army Corps of Engineers are partners in the survey and inventory which began on May 20 and ended on July 7.

Stravers said that all breeding birds encountered during his survey will be counted, but he is particularly interested in the cerulean warbler, which has been declining across North America in recent decades.

“We’ve been losing habitat for ceruleans in a lot of places around the country,” said Stravers. “We’ve also been losing wintering habitat in Peru and Venezuela, in part because coffee plantations have changed and coffee is now grown in more cleared areas.” Stravers said that ceruleans are among the rarest birds in the United States.

Stravers said he went out almost every day during the survey period to record all of the birds he saw and heard in a 10-minute period at numerous locations. Audio recordings were also made. The number of birds and what species they were, within a 100-meter radius at each location, were documented on a map.

Approximately 100 territories for cerulean warblers were recorded as GPS points in Sny Magill, said Stravers, who noted that the cerulean population in Bagley Bottoms was more sparse. Several years ago, about 200 territories were documented in the Yellow River Forest, a number not believed at the time by many biologists. The research proved true, however, and in 2014, it was instrumental in having the Effigy Mounds-Yellow River Forest Bird Conservation Area designated as Iowa’s first Globally Important Bird Area. The unit covers more than 14,000 acres of public land which includes Effigy Mounds National Monument, Yellow River State Forest, Pikes Peak State Park and the Bloody Run and Sny Magill-North Cedar state wildlife management areas. 

“The cerulean warbler is identified as a priority resource of concern in the Refuge’s Habitat Management Plan,” said Reiter-Marolf. “Jon has studied cerulean warblers for several years and red-shouldered hawks in this area since the 70s and we are fortunate that the Sny Magill area is a cerulean warbler hot spot, as is the Yellow River State Forest, and Effigy Mounds National Monument.”

Reiter-Marolf said that what the refuge would like to find out is what types of habitat characteristics are the cerulean warblers selecting to establish territories and commence nesting. 

“That is why we are also conducting a forest inventory in these areas too,” he said. “The forest inventory takes measurements on overstory and understory height, species diversity, and tree size, as well as forest structure, and forest health. We hope this research will give us a better understanding of cerulean warbler habitat preference, which would better inform our future habitat management actions.”

Anyone who has been out on the Mississippi River in Pool 10 can attest that silver maples are thick in most locations. Stravers noted that when the lock and dam system was put into place, it filled in the backwaters, and thus, silver maples have had an advantage for several decades in these watery environments.

Stravers said that the branches and leaves of silver maples are “thick” and that cerulean warblers prefer big trees such as black walnuts and oaks which have more open spaces between their branches. 

“Ceruleans need thick, horizontal branches to put their nests on,” he said. 

While on a survey in the Sny Magill area recently, Stravers pointed out that there are slightly different elevations of land. The silver maples tend to grow more closely to the water’s edge. There is then about a four-foot rise in the land, and the oaks and black walnuts grow on this higher ground a bit further back.

Stravers said that the bird survey points in the recently completed research are much more spread out than the forest inventory points. Stravers surveyed for birds every 300 meters. The forest inventory was done every 330 feet.

“Once, we can summarize and analyze the forest inventory data, then we can better understand the health, structure, and composition of different forest stands throughout the refuge,” said Reiter-Marolf. “We’ll be able to see where areas of low diversity or stressed or declining trees occur, as well as where areas of high diversity or good healthy trees occur. We can also compare different areas by the structure of the stands. Are there any young trees in the understory and mid-story, or is the area all one age class of trees, with little recruitment of young trees for the next generation of forest? This can help direct future habitat management, such as under-planting of new trees, or thinning of less desirable trees to benefit more desirable trees, etc.”

Then, said Reiter-Marolf, by comparing the forest inventory data to the bird survey data, they can get a better idea about the habitat that is preferred by nesting cerulean warblers, for example. 

“We could then take management actions elsewhere on the refuge to try and mimic the habitat characteristics that ceruleans prefer in order to help them expand into new breeding territories and potentially increase their reproductive output on the refuge,” said Reiter-Marolf.

Stravers, who is well known for his many years of raptor research, said that the presence of ceruleans is a good indicator that species such as peregrine falcons and red-shouldered hawks, as well as various neo-tropical migrants, are also abundant in the area.

Ceruleans, a threatened bird, are here in May, June and part of July, said Stravers. They begin courtship and breeding in May with the males singing to claim territory.

In 2006, the U.S. Fish and Wildlife Service estimated the total population of ceruleans at 400,000 and declining at 3 percent per year. Biologists have estimated that the population has declined by 70 percent over the past 40 years.

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