Strong floods drive warblers away from their known breeding sites

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[April 27, 2017]    Fewer migratory Swainson’s Warblers return to breed after high flood waters alter the quality of their wetland forest habitat, according to new University of Illinois research published in the journal PLOS ONE.

Bryan Reiley, a graduate research assistant at the Illinois Natural History Survey, studied the body condition and number of male warblers returning to two sites in southeast Arkansas during four breeding seasons. He investigated the warblers before the 2008 catastrophic flooding of the Mississippi River and three years after the flood.

Swainson’s Warblers (Limnothlypis swainsonii) are rare, occurring in the Caribbean basin during the winter months and primarily in the southeastern United States from April through August. They also reach southern Illinois and the Appalachian Mountains. This species typically lives in canebrakes adjacent to rivers in floodplain forests.

From 2005 to 2010, Reiley studied 278 males. He anticipated that their body condition would decrease in the years after the 2008 flood because warblers forage for insects under fallen leaves and debris. He also assumed that fewer birds would return to the previously flooded site in subsequent years.

He found that the condition of males was actually better in the years after the flood, perhaps because the La Niña climate pattern created favorable conditions at the birds’ wintering grounds.

Flooding did affect the number of males returning to the study site, however. Before the flood, 20 to 31 males returned to breed each spring, whereas in 2008, 18 males returned. The following year, Reiley counted only 7 birds, and in a follow-up observation in 2014, only 2 warblers were observed at the site.

For birds that attempted to breed in these habitats in the years following the flood, reduced leaf litter and shrub cover might have resulted in reduced habitat quality and decreased nesting success, leading to a significant drop in the number of birds occupying the area in the second and third years post-flood.

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“Habitats are never static,” Reiley said. “Swainson’s Warblers may reside in upland habitats following a significant flood. I’ve seen them feeding in the trees when they would normally forage on the ground. They can be flexible when their habitat is continually changing.”

To keep Swainson’s Warblers in a forest environment after a flood, increased forest management may be necessary, Reiley said. Flooding is not a unique condition, but the increased frequency and duration of flooding that may result from a changing climate can be stressful for birds.

“If the habitat changes too much, Swainson’s Warblers will not use it, and they may never return to that area,” Reiley said.

The study was funded by the U.S. Fish and Wildlife Service. Reiley is a PhD candidate in the Department of Natural Resources & Environmental Sciences, and his advisor is Thomas Benson, a wildlife ecologist in the Prairie Research Institute’s Illinois Natural History Survey. The article can be found at http://journals.plos. org/plosone/articleid=10.1371/journal. pone.0175179.

[Lisa A. Sheppard]


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