A current event explains what happened to the bald eagle in the first part of the 20th century.
The Salinas Journal reports,
The U.S. Fish and Wildlife Service (USFWS) is actively investigating the deaths of two bald eagles in Kansas. The first eagle was found in a borrow ditch on 370th Road, approximately ¼ mile north of the Ness County line, in southeastern Trego County, on Jan. 9. The USFWS worked with area veterinarians to X-ray the eagle and identified the presence of metal in the bird.
The second eagle was recovered near 1700 Road in northwest Montgomery County, approximately ½ mile south of the Wilson County line on Jan. 10. This eagle was also X-rayed and showed the presence of metal.
Bald eagles are protected by the federal Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. The killing of any eagle constitutes a violation of those acts.
Anyone with information regarding the death of these eagles is asked to contact the U.S. Fish and Wildlife Service, Office of Law Enforcement, in Topeka at 785-232-5149. The USFWS will pay for information leading to the arrest and conviction of the person or persons responsible for the killing of these or any other eagles or protected species. Anyone contributing information to authorities may choose to remain anonymous. Information may also be reported to the Kansas Department of Wildlife, Parks and Tourism’s Operation Game Thief hotline at 1-877-426-3843.
Approximately 50,000 pairs of bald eagles occupied the lower 48 states in pre-colonial times, but that number was reduced to 400 pairs by the 1960s. Biologists blame a loss of habitat, shooting, trapping, and the heavy use of pesticides such as DDT for the decline of the species, which was officially listed under the federal Endangered Species Act in 1976. After DDT was outlawed, bald eagles began making a comeback in the late 1980s, and the species was down-listed from endangered to threatened in 1996. In June of 2007, it was removed from the list completely. [Emphasis added]
With respect to the assertions in the highlighted paragraph, as pointed out in JunkScience.com’s “100 Things You Should Know About DDT“:
66. Bald eagles were reportedly threatened with extinction in 1921 — 25 years before widespread use of DDT. [Van Name, WG. 1921. Ecology 2:76]
67. Alaska paid over $100,000 in bounties for 115,000 bald eagles between 1917 and 1942. [Anon. Science News Letter, July 3, 1943]
68. The bald eagle had vanished from New England by 1937. [Bent, AC. 1937. Raptorial Birds of America. US National Museum Bull 167:321-349]
69. After 15 years of heavy and widespread usage of DDT, Audubon Society ornithologists counted 25 percent more eagles per observer in 1960 than during the pre-DDT 1941 bird census. [Marvin, PH. 1964 Birds on the rise. Bull Entomol Soc Amer 10(3):184-186; Wurster, CF. 1969 Congressional Record S4599, May 5, 1969; Anon. 1942. The 42nd Annual Christmas Bird Census. Audubon Magazine 44:1-75 (Jan/Feb 1942; Cruickshank, AD (Editor). 1961. The 61st Annual Christmas Bird Census. Audubon Field Notes 15(2):84-300; White-Stevens, R.. 1972. Statistical analyses of Audubon Christmas Bird censuses. Letter to New York Times, August 15, 1972]
70. No significant correlation between DDE residues and shell thickness was reported in a large series of bald eagle eggs. [Postupalsky, S. 1971. (DDE residues and shell thickness). Canadian Wildlife Service manuscript, April 8, 1971]
71. Thickness of eggshells from Florida, Maine and Wisconsin was found to not be correlated with DDT residues.
Data from Krantz, WC. 1970. Pesticides Monitoring Journal 4(3):136-140.
State Thickness (mm) DDE residue (ppm) Florida 0.50 About 10 Maine 0.53 About 22 Wisconsin 0.55 About 4
72. U.S. Forest Service studies reported an increase in nesting bald eagle productivity (51 in 1964 to 107 in 1970). [U.S. Forest Service (Milwaukee, WI). 1970. Annual Report on Bald Eagle Status]
73. U.S. Fish and Wildlife Service biologists fed large doses of DDT to captive bald eagles for 112 days and concluded that “DDT residues encountered by eagles in the environment would not adversely affect eagles or their eggs.” [Stickel, L. 1966. Bald eagle-pesticide relationships. Trans 31st N Amer Wildlife Conference, pp.190-200]
74. Wildlife authorities attributed bald eagle population reductions to a “widespread loss of suitable habitat”, but noted that “illegal shooting continues to be the leading cause of direct mortality in both adult and immature bald eagles.” [Anon.. 1978. U.S. Fish and Wildlife Service, Endangered Species Tech Bull 3:8-9]
75. Every bald eagle found dead in the U.S., between 1961-1977 (266 birds) was analyzed by U.S. Fish and Wildlife Service biologists who reported no adverse effects caused by DDT or its residues. [Reichel, WL. 1969. (Pesticide residues in 45 bald eagles found dead in the U.S. 1964-1965). Pesticides Monitoring J 3(3)142-144; Belisle, AA. 1972. (Pesticide residues and PCBs and mercury, in bald eagles found dead in the U.S. 1969-1970). Pesticides Monitoring J 6(3): 133-138; Cromartie, E. 1974. (Organochlorine pesticides and PCBs in 37 bald eagles found dead in the U.S. 1971-1972). Pesticides Monitoring J 9:11-14; Coon, NC. 1970. (Causes of bald eagle mortality in the US 1960-1065). Journal of Wildlife Diseases 6:72-76]
76. U.S. Fish and Wildlife Service biologists linked high intake of mercury from contaminated fish with eagle reproductive problems. [Spann, JW, RG Heath, JF Kreitzer, LN Locke. 1972. (Lethal and reproductive effects of mercury on birds) Science 175:328- 331]
77. Shooting, power line electrocution, collisions in flight and poisoning from eating ducks containing lead shot were ranked by the National Wildlife Federation as late as 1984 as the leading causes of eagle deaths. [Anon. 1984. National Wildlife Federation publication. (Eagle deaths)]