Human-caused Factors of Abundance 



Human activities such as shooting, habitat alteration, and environmental 

 pollution have affected bald eagle populations. Bald eagles historically have 

 suffered from human persecution in Maine. Early settlers apparently used 

 eagles for food on occasion (Palmer 1949). Moorehead (1922) found eagle bones 

 among Indian shell heaps in Lamoine (region 5). The town of Vinalhaven 

 (region 4) approved a 20 cent/head bounty on bald eagles in 1806 (Lyons et al . 

 1889) but this precedent was not adopted statewide. Eagle eggs were collected 

 and offered for sale in the late 1800s. Spinney (1926) cited numerous 

 instances in which pine trees supporting eagle nests were cut for timber. 



Shooting has been the most common cause of mortality among Maine eagles in 

 recent years. Both adult and immature eagles are shot, indicating the problem 

 is not solely one of recognition. The frequency of shooting deaths among 

 known mortalities of Maine eagles is near the 407 o level observed nationwide. 

 Shooting incidence declined nationally (Coon et al. 1970; and Prouty et al. 

 1977) but not in Maine. At least five eagles have been shot in coastal Maine 

 since 1963. Other direct losses of eagles in Maine attributable to people are 

 trapping, electrocution, and lead poisoning (via ingestion of waterfowl 

 containing lead pellets). The impact of human-related mortality on an eagle 

 population may exceed that of the normal decline in recruitment (Young 1968) . 



Environmental contaminants found in Maine bald eagles and in their eggs 

 include 13 organochlorines and 5 heavy metals. Foremost are pesticides such 

 as DDT and dieldrin, industrial wastes such as PCBs (polychlorinated 

 biphenyls), and mercury. Residues of DDE and DDD (metabolic by-products of 

 DDT), dieldrin, PCBs, and mercury occur in all eagle egg and carcass samples 

 from Maine. Other contaminants appear at lower levels. 



Contaminants at high levels are toxic to some animals but their persistence 

 and cumulative effects at lower levels are not known for Maine eagles. They 

 accumulate in eagles through contaminated foods and may be a threat to 

 reproductive success. Reduced eggshell thickness and increased incidence of 

 egg breakage are related to organochlorines, particularly DDE. Shell 

 thickness of 34 eagle eggs collected in Maine between 1967 and 1979 averaged 

 0.52 mm, 15% below normal. No significant reduction in levels of DDE, PCBs, 

 mercury or associated thinning has occurred in Maine eagle eggs since 1967. 

 These contaminants probably have additive effects and their total impact is 

 unknown. Embryo mortality observed at various stages in unhatched eggs of 

 Maine eagles may be caused by DDE, PCBs and/or mercury. 



The impact of organochlorines on bald eagle productivity in Maine becomes 

 evident when the Maine eagle population is compared to those in other areas of 

 the country. The amounts of residues of DDE, DDD, DDT, and dieldrin in Maine 

 eagle eggs surpassed those of Florida and Wisconsin in 1968 (Krantz et al. 

 1970). Levels of contamination in Maine eagle eggs in 1969 were higher than 

 those in Minnesota and Alaska (Wiemeyer et al. 1972). Recruitment also is 

 lower among Maine eagles than in these four populations. Organochlorine 

 residues similar to those in Maine eagle eggs were reported in northwestern 

 Ontario, where productivity also was declining (Grier 1974). Detrimental 

 levels of mercury in bald eagle eggs are relatively unique to Maine (Wiemeyer, 

 unpublished ) . 



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