The bald eagle was classified as an endangered species in 1967, within 43 states due to a major decline in their population.  In Oregon, Washington, Minnesota, Wisconsin, and Michigan, the bald eagle became a threatened species.  Most investigations regarding the declining population of the bald eagle have focused on lower rates of reproduction associated with eggshell thinning, but few studies have investigated the relationship of diets and pollutants in the prey of bald eagles.  A study by Frenzel et al. (1989) examined organochlorine pesticide, lead and mercury concentrations in eagle blood and prey items (birds and voles) from the Lower Klamath Basin of northern California and southern Oregon.  The Klamath Basin has one of the highest concentrations of wintering bald eagles.  The Klamath-Tule Lake Basin also has a history of pesticide use and contaminant problems.  The diets of bald eagles were described by observing eagles foraging, examining castings from communal roosts, and identifying remains at foraging perches.  Blood samples and eagle carcasses were analyzed for several contaminants.  From the 21 bald eagles that were captured on the wintering area, Organochlorine residues were generally low or nondetectable.  Low concentrations of DDE were found in all but one of the eagle blood samples, residues of PCB's were found in less than half of the blood samples, and trans-nonachlor (0.01 ppm) was found in 1 subadult.  Lead shot embedded in water fowl is a major source of lead poisoning in eagles in the Klamath Basin.  The bald eagles wintering in the Klamath Basin are undoubtedly being  exposed to lead (Pb) shot through the waterfowl in their diet. Most eagles had low lead levels.  However, one of the 10 eagles found dead during the study was diagnosed as having died of lead poisoning.    More than three additional eagles in the basin have died of lead shot poisoning in this vicinity from 1975 to 1982.  Some high lead concentrations were also observed in mallards and ruddy ducks probably due to lead shot.  The use of steel shot instead of lead shot for waterfowl hunting should also decrease the incidence of ingestion of lead shot by bald eagles.  The eagles sampled from the Klamath Basin wintering area, who do not appear to nest in Oregon, did not have elevated levels of organochlorine.  This may be due to the wintering eagles diet of waterfowl and montane voles and the low concentrations of organochlorines in these prey.   The bald eagles that only winter in the Klamath Basin apparently nest in areas that have a relatively clean prey base or any contaminated prey species in the areas are of low dietary importance. Currently, the U.S. Fish and Wildlife Service has a proposal to take the bald eagle off of the endangered species list.  With the ban on DDT in the United States and over 30 years of protection, there are more than 5,748 pairs of bald eagles nesting in the lower 48 states.

Learn more by reading the full report:
Frenzel, R.W., and R. G. Anthony. 1989. Relationship of diets and environmental contaminants in wintering bald eagles. J.Wildl. Manage. 53:792-802 

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