Refuge staff monitoring artifical nest box production at Union Slough NWR. - FWS photo by Becky Rasmussen

Refuge staff measure and test tree swallow specimens for pollution effects. -FWS photo by Becky Rasmussen

Biologists from Union Slough National Wildlife Refuge, Iowa, and Rock Island, Illinois Ecological Services Field Office, are gearing up for year two of a multi-year study to evaluate the affects of exposure to selenium in the wetland sediments at the Refuge. 

Last year biologists established about 50 artificial nest boxes around the Refuge to attract tree swallows close to the contaminated sediments.  The tree swallows are tested throughout the nesting period to measure the amount of selenium and other heavy metals in their system. 

The tree swallow test data along with reproduction data are used to determine if the exposure to these pollutants are sufficient to cause harm.  Staff will monitor the swallows again this year and then prepare our report next year once all of the data are analyzed.  The selenium is believed to come from decades of leaching of the soils through the agricultural tile drainage network. 

Selenium naturally occurs at trace concentrations in soils from this part of Iowa.   However, it has apparently built up in the wetland sediments because of the large volume of tile effluent that flush whole watersheds and drain into the Refuge.  Elevated selenium in birds causes a specific disease that leads to losses in reproduction and mortality.  

Staff noted higher than usual selenium concentrations in Refuge sediments during the mid 1990’s while conducting routine water quality studies.  Observations thus far are that tree swallow reproduction seems normal. 

Union Slough National Wildlife Refuge provides important prairie pothole habitats for Midwest breeding birds and supports large numbers of birds migrating between their southern wintering grounds and breeding grounds further to the north. 

Please contact Tom Skilling or Becky Rasmussen at Union Slough National Wildlife Refuge (515-928-2523) or Mike Coffey at the Rock Island, Illinois Ecological Services Field Office (309-757-5800 x206) if you have any questions.