What is happening with eagles on the Mississippi River?

 

By Nancy Duncan, Natural Resource Program Manager, Mississippi National River and Recreation Area

In 2006, the National Park Service began studying eagles along the Mississippi National River and Recreation Area (Mississippi NRRA) to monitor levels of environmental contaminants in nestlings.  These human-created contaminants are released into air, water or on land and frequently end up in aquatic systems via runoff, ground water, and aerial deposition.  Because bald eagles are high on the aquatic food chain, they are known to accumulate contaminants that are persistent, bioaccumulative and toxic at high levels. These levels can be found and measured in eaglet blood and feather samples.  

The sky is blue, and a nest sits in a tree with someone ascending up to it to check on the fledglings.
A blood sample is being taken from a fledgling to test for lead levels and other contaminants.

Sampling was conducted on 5- to 9-week-old eagle nestlings at Mississippi NRRA, as well as in the Apostle Islands and along the Saint Croix and Namekagon rivers, from 2006 to 2015.  The contaminants measured are primarily accumulated by eaglets through the food they eat, which is mostly fish. The contaminants that were measured included:  

An eagle fledgling sitting next to a log. It has brown feathers with some black feathers. While it's eyes and beak haven't turned yellow yet as well.

  • Lead (from a variety of sources including contaminated soils and water, and ingestion of lead-based fishing sinkers and ammunition shot or fragments that are embedded or entangled in fish and other prey) 

  • Total mercury (primarily a result of burning fossil fuels)

  • DDT, including the breakdown chemicals DDE and DDD (insecticides banned in the 1970s and believed to be the primary cause of bald eagle declines in the 1960s)

  • PCBs (also banned in the 1970s. Were widely used as coolants and lubricants in transformers, capacitors, and other electrical equipment including old florescent lighting. PCBs are still produced as a bi-product of burning some plastics at low temperatures)

  • PBDEs (flame retardants that were put into many plastics and textiles)

  • PFCs (also known as PFAS; a large group of manufactured compounds that are widely used to make everyday products more resistant to stains, grease and water. PFCs may be used to keep food from sticking to cookware, to make sofas and carpets resistant to stains and to make clothes and mattresses more waterproof.)

All of these contaminants pose risks to humans as well as wildlife. The National Park Service wanted to determine the direction and extent of change in concentrations of these contaminants in eaglets, look at the incidence of developmental deformities in live and dead nestlings, and determine the number of nestlings produced per occupied bald eagle nest.  The results were intended to both assess how these chemicals affected eagles, but also as a way of assessing the potential exposure to humans who also use these park waters and consume fish. High levels of lead, along with PFCs and PCBs, were found in the Mississippi NRRA nestlings, and DDT/DDE was still found in nestlings across study areas. The National Park Service has produced several scientific articles and reports on these results, which can be found under Contaminant Monitoring Reports at:

Monitoring Reports (U.S. National Park Service) (nps.gov) 

Contaminants in Bald Eagle Nestlings at Apostle Islands National Lakeshore, Mississippi National River and Recreation Area, and the St. Croix National Scenic Riverway

An ariel shot of railroad tracks at Lilydale where the tracks run through the small lake at Lilydale.
An ariel shot of Banfill Island during the winter with trees lightly covered in snow.

In spring of each year, occupied bald eagle nests were mapped by observer/pilot teams from either a helicopter or a fixed-wing aircraft.  Then in May - June, field teams visited all occupied nests to collect blood and feather samples from nestlings in all nests where the trees were safe to climb. Each active nest contained from 1 to, more rarely, 4 nestlings.  Eaglets were measured, weighed, banded and checked for general health prior to being replaced in the nest.  

A survey helicopter is stationed on a landing zone. The helicopter is a small two seat helicopter that is white and has blue stripes on it.
A ranger is holding an eagle fledgling while another volunteer is holding one as well. There are other volunteers near them observing.

Since 2011, Mississippi Park Connection (MPC) has provided funding to the Mississippi NRRA so the spring flight surveys could be continued, with an April survey to determine which nests are occupied and a May survey to determine the productivity (number of eaglets) of the occupied nests.  This has provided a good barometer to measure how eagle populations are doing along the corridor.  During the course of the study the number of nestlings and nests steadily increased, reaching highs of 95 eaglets found in 2017 and 73 nests found in 2021 (roughly one nest per river mile).  There was no flight during 2020 due to COVID concerns.  This year (2022) eagle occupancy and productivity numbers were down, with only 44 occupied nests and 32 nestlings found.  John Moriarty, Senior Manager of Wildlife for Three Rivers Park District who has flown these surveys every spring, suspects the decrease in numbers are due to this year’s incidence of bird flu combined with a cold spring.  The surveys will continue next spring and we will see what happens!  As for the contaminants that were monitored, we now have a good baseline of information and a better understanding of the trends.  Many declined, but there are new chemicals being produced and entering the environment every year. The NPS or other partners may choose to conduct additional monitoring in the future to see how trends continue and whether new chemicals are of concern. 

 
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