Brown pelicans have few natural enemies. Although ground nests are sometimes destroyed by hurricanes, flooding, or other natural disasters, the biggest threat to pelicans comes from people. Pelicans have been persecuted by humans for their perceived competition for fish, despite the fact that their diet overlaps little with fish caught by people. Starting in the 1880s, American white pelicans were clubbed and shot, their eggs and young were deliberately destroyed, and their feeding and nesting sites were degraded by water management schemes and wetland drainage In the late 19th and early 20th centuries, pelicans were hunted for their feathers, which adorned women’s clothing, particularly hats.
Several efforts in the early part of he 20th century were meant to curb the decline of brown pelicans. In 1903, President Theodore Roosevelt designated Florida’s Pelican Island as the first national wildlife refuge, a move that helped reduce the threat of plume hunters. Passage of the Migratory Bird Treaty Act in 1918 gave protection to pelicans and other birds and helped curb illegal killing.
DDT pollution in the environment was a major cause of decline of brown pelican populations in North America in the 1950s and 1960s. It entered the oceanic food web, contaminating and accumulating in several species, including one of the pelican’s primary food fish – the northern anchovy. Its metabolite DDE is a reproductive toxicant in pelicans, causing eggshell thinning and weakening, and consequent breeding failure through the eggs being accidentally crushed by brooding birds. Since an effective ban on the use of DDT was implemented in the US in 1972, the eggshells of breeding brown pelicans there have thickened and their populations have largely recovered.
As waterbirds that feed on fish, pelicans are highly susceptible to oil spills, both directly by being oiled, and indirectly by the impact on their food resources. A 2007 report to the California Fish and Game Commission estimated that, during the previous 20 years, some 500–1000 brown pelicans had been affected by oil spills in California alone. A 2011 report by the Center for Biological Diversity, a year after the April 2010 Deepwater Horizon oil spill, said 932 brown pelicans had been collected after being affected by oil and estimated that ten times that number had been harmed as a result of the spill.
Where pelicans interact with fishers, through either sharing the same waters or scavenging for fishing refuse, they are especially vulnerable to being hooked and entangled in both active and discarded fishing lines. Fish hooks are swallowed or caught in the skin of the pouch or webbed feet, and strong monofilament fishing line can become wound around bill, wings or legs, resulting in crippling, starvation, and often death. Local rescue organizations have been established in North America and Australia by volunteers to treat and rehabilitate injured pelicans and other wildlife.
In 1970, under a law that preceded the Endangered Species Act of 1973, the U.S. Fish and Wildlife Service listed the brown pelican as endangered, a term that means the species is in danger of extinction throughout all or a significant portion of its range. In 1972, the Environmental Protection Agency banned the use of DDT in the United States and restricted the use of other pesticides. Since then, there has been a decrease in the level of chemical contaminants in pelican eggs, and a corresponding increase in nesting success. As a result of the ban on the use of DDT in the United States, as well as complementary conservation efforts,
the species has made a strong comeback and, in view of its improved status, has been removed from the list of threatened and endangered species throughout its range. The U.S. Fish and Wildlife Service now estimates the global population of brown pelicans at 650,000 individuals.