U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS 2004-3042

April 2003

Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin

Tree Swallows: A Nationwide Sentinel Species for Assessing and 

Monitoring Aquatic Contamination

Environmental contaminants are often the byproduct 

of industry and agriculture. United States Geological 

Survey (USGS) scientists provide information necessary 

to protect and restore our Nation’s plants and animals 

from the effects of these contaminants. Federal and state 

management agencies use this information to determine 

if contamination has occurred and if any cleanup is 

necessary. 

Tree swallow on nest box

Nest box placed in typical tree swallow habitat

86%

14%

Aquatic Insects

Terrestrial Insects

Typical tree swallow diet 

Tree swallows are a useful bird species to measure 

exposure and effects of environmental contaminants: (1) 

They are widely distributed throughout the United States, 

(2) They will nest in boxes, so they can be attracted to 

study areas, and (3) They feed near their nest box on 

emergent aquatic insects. Aquatic insects emerge from 

river sediments, so the swallows’ tissues refl ect levels of 

contaminants in sediments. 

Upper Midwest Environmental Sciences Center 

(UMESC) scientists study swallows in many locations 

across the United States to assist other government 

agencies with their contaminant concerns. Scientists 

evaluate exposure and effects of many contaminants 

including pesticides (for example, DDT), industrial 

contaminants (dioxins and PCBs), trace metals (lead and 

mercury), and petroleum.

Contaminant studies using swallows are often approached 

step-by-step. Generally, the fi rst step is to fi nd out if 

contaminants are in the environment and are found in 

swallows. To do this, we analyze swallow eggs and 

other tissues for contaminants and compare them to 

uncontaminated locations. If contaminants are elevated in 

swallows, the next step is to fi nd out if these contaminants 

affect tree swallow reproduction or survival. Many 

environmental contaminants are toxic to bird embryos, 

so we measure the number of eggs that hatch. We also 

measure the growth rate and survival of young. Because 

swallows tend to come back to where they nested the 

previous year, we can band them at their nest box and 

recapture adults in subsequent years. The percent of adult 

swallows that return the following year can be compared 

between contaminated and clean sites. 

Percent of diet
Scientist checking swallow nest box

Tree swallow distribution across the United States Warning sign

The U.S. Fish and Wildlife Service, U.S. Environmental 

Protection Agency, and state agencies continue to 

request assistance from UMESC. The agencies use 

results like these to make decisions regarding our 

Nation’s environment.

For more information, contact —

Christine Custer

608.781.6247

ccuster@usgs.gov 

Thomas Custer

608.781.6375

tcuster@usgs.gov

USGS Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, WI 54603

Results from recent contaminant studies using tree 

swallows include the following: 

 Lead contamination from mining activities in the 

late 1800s along the Upper Arkansas River in 

Colorado is still inhibiting enzymes important to 

the functioning of red blood cells in tree swallows. 

 Hatching success of swallow eggs is lower on 

the highly contaminated Housatonic River in 

Massachusetts as a result of industrial pollution.

 Severe hatching problems are occurring at a site in 

Rhode Island that has some of the highest dioxin 

concentrations ever reported in tree swallow eggs. 

 Petroleum contamination was not detected in 

swallows at a former oil refi nery site in central 

Wyoming and we did not recommend cleanup. 

Location of swallow study sites by Upper Midwest Environmental 

Sciences Center scientists

http://www.umesc.usgs.gov/