By Jack Nissen
That short burst of tweets you hear from wrens might be the best way to tell if they’re near, but it isn’t the only way.
A good way to predict the bird populations in the Great Lakes is to listen not for the songs of wrens, but for the roaring of car engines. A recent study published in the Journal of the Society of Wetland Scientists shows where humans are and where wren populations should be – but aren’t.
In one of the broadest research projects on two species of wrens in the Great Lakes region, urban development was found to have a primary influence on where the birds live.
“Human development of the landscape proved to be the best model for predicting where these species can be found,” said Hannah Panci, a member of the Natural Resources Institute at the University of Minnesota Duluth, and lead researcher in the study.
Sedge wrens look for shrubs and wet grasses to nest in while marsh wrens choose cattails and bulrushes.
For the most part, where you find people, is where you likely won’t find wrens. The Michigan Department of Environmental Quality attributes human development, like agriculture and industry, as key factors in the loss of wetlands, the primary habitat for these birds.
“Using this information is key to understanding where we can find these species,” Panci said. “If we know where these species live, we can better understand how to manage potential human development around the areas.”
Sedge and marsh wrens tend to avoid roads and settlements, the study found. The exception to this rule is when cropland is near. The marsh wrens in particular liked farmland even when people were nearby.
“Sedimentation and nutrient overload consequences of agriculture, creates a favorable climate for this habitat growth,” Panci said. And that habitat growth is exactly what the marsh wrens are looking for.
Panci notes the connection between the invasive cattail species Typha angustifolia and agriculture helps explain why cropland attracts marsh wrens. Where you find cropland is where you’ll find the marsh wrens.
Smaller studies in 2007 and 2010 also found that human development discouraged wrens.
“Human impact changes the dynamic of wetlands,” said Steven Miller an assistant professor in the Agricultural, Food and Resource Economics Department at Michigan State University. “Agriculture has a big impact because of the chemical runoff and erosion effects.”
Because the chemical runoff and erosion change the dynamic of the wetlands, it’s changing the preferable habitats these birds look for, a sentiment reflected in studies like Panci’s.
Panci’s study was conducted over three years. Data and maps that quantified vegetation, landscape-variables and roads were used in the 840 sites that were analyzed for the presence of the birds.
The loss of wetlands to roads and houses isn’t just bad for birds. Wetlands provide an abundance of benefits to a much broader environment.
“Wetlands provide tons of functions, like way off the charts in their value,” said Patrick Doran, the conservation director at the Nature Conservancy for Michigan. “They provide habitats for birds, they provide spawning areas for fish we like to catch, they filter water, they attenuate storm surge and energy.”
They’re also important for recreation, Doran said. From bird watching to fishing, wetlands have become hotspots for human development. This is a key component to Panci’s study in understanding the influence humans have on the wren populations.
Coastal wetlands stretch more than 9,000 miles in the Great Lakes region.
“A natural tension exists between human development and environmental sustainability that creates problems for the ecosystem,” Doran said. “People want to live near wetlands, but by doing this they are offsetting the balance that these environments need.”
The EPA’s “State of the Great Lakes” report estimates that 50 percent of the Great Lakes wetlands have been converted or lost.
Panci’s study was designed with the goal of refining the models used for predicting how wren populations interact with humans. And while hers wasn’t the first to point out this significance, it emphasized that point.