U.S. Department of the Interior

U.S. Geological Survey

USGS Open-File Report 2005–1281

July 2005

Water Level Management and Contaminant Exposure in Tree 

Swallows Nesting on the Upper Mississippi River

by Thomas Custer and Christine Custer

The U.S. Army Corps of Engineers conducted a 

water level drawdown on Navigation Pool 8 of the Upper 

Mississippi River (fig. 1) in summers 2001 and 2002 

to increase aquatic vegetation production and, thereby, 

improve fish and wildlife habitat.

Flooding of previously dried out wetlands, such as 

a year following a drawdown, may increase the rate of 

mercury methylation and in turn make mercury more 

available to terrestrial vertebrates that feed in aquatic 

environments.

Tree swallows (Tachycineta bicolor) are a useful 

species for contaminant assessment of sediments. They 

feed on emergent aquatic insects and, therefore, their 

eggs and tissues reflect sediment contamination. Because 

tree swallows feed close to their nest boxes, contaminant 

concentrations in eggs and nestlings are indicative of local 

contamination.

A study was initiated in 2000 to determine if tree 

swallow exposure to mercury and other contaminants 

increased after the 2001 Pool 8 water level drawdown. 

If true, then we would predict the mercury concentrations 

in tree swallows would be higher in 2002 than 2000 or 

2001.

Methods

Bird nest boxes (fig. 2), 

suitable for tree swallows, 

were erected at two sites in 

Pool 8 of the Mississippi 

River (Lawrence Lake and 

Shellhorn) and at an inbasin reference location 

(Jans) in 2000, 2001, and 

2002 (fig. 1). In all 3 years, 

samples of tree swallow 

eggs (fig. 3) and nestlings 

were collected and analyzed 

for mercury and other 

contaminants. The egg and 

nestling collections in 2001 

were done before the 2001 

drawdown. Figure 1. Tree swallow study sites (red dots) in 2000, 2001, and 2002. 

Lawrence Lake and Shellhorn were on Pool 8 of the Mississippi River. 

Jans was a nearby reference area. 

Figure 2. Tree swallow on nest 

box.

Figure 3. Tree swallow nest with eggs.
USGS Open-File Report 2005–1281

July 2005

Findings

Mercury concentrations in tree swallow eggs averaged 

below 0.35 ppm dry weight and did not significantly 

increase after the Pool 8 drawdown (fig. 4). We have no 

explanation for the higher levels of mercury in swallow 

eggs at Lawrence Lake in 2001 compared to 2000. 

However, this difference does not suggest an increase 

following the drawdown because mercury concentrations in 

2002 were similar to 2000 and 2001.

Mercury concentrations in eggs (means 0.15 to 

0.31 ppm dry weight) were intermediate to levels 

reported in tree swallows from other North American 

locations and were not at toxic levels. Impairment of 

reproductive success in birds has been associated with egg 

concentrations of 2.5 to 10 ppm dry weight mercury. 

Mercury concentrations in tree swallow nestlings 

averaged <0.25 ppm dry weight and were not significantly 

different after the Pool 8 drawdown (fig. 5). 

Figure 4. Mercury concentrations in tree swallow 

eggs did not differ (ns.) among years for the 

reference location and Shellhorn. Mercury 

concentrations were greater in 2001 than 2000 at 

Lawrence Lake (A versus B).

Other trace elements, polychlorinated biphenyls, and 

organochlorine insecticides did not increase following the 

2001 drawdown and were not elevated compared to similar 

samples collected from other North American locations 

(fig. 6). Physiological and genetic bioindicators did not 

change after the drawdown. Hatching success of eggs also 

did not differ among years or locations and was comparable 

to a nationwide average.

Acknowledgments

We thank Paul Dummer and Craig Beckman for 

assistance with data collection, Jan Dagendesh for access 

to her property, and the U.S. Fish and Wildlife Service for 

access to sites on Pool 8. This study was funded through 

On-Refuge contaminants investigations of the U.S. Fish 

and Wildlife Service.

For more information, please contact:

Thomas Custer1 or Christine Custer2

U.S. Geological Survey

Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, Wisconsin 54603

Telephone: 1(608) 781-6375 E-mail: 1tcuster@usgs.gov

 2(608) 781-6247 2ccuster@usgs.gov

http://www.umesc.usgs.gov

Figure 5. Mercury concentrations in tree swallow 

nestlings did not differ (ns.) among years.

Figure 6. Location of tree swallow study sites by Upper Midwest 

Environmental Sciences Center scientists.