The breakdown product DDE, from the pesticide DDT, interferes with calcium deposition in eggshells. Brown pelicans incubate by resting the webs of their feet on the eggs, so thin shells fail directly under the parent's weight. Through the 1950s and 1960s colonies produced almost no young, and the Louisiana breeding population, in a state whose flag carries the bird, was extirpated.
The brown pelican was listed as endangered in the United States, DDT was banned there for agricultural use in 1972, and reintroduction from Florida stock restored Louisiana colonies. Eggshell thickness recovered as DDE residues declined, breeding success returned, and the species was delisted from the Endangered Species Act in 2009.
The recovery is often told as a straightforward victory, which flattens two things. Populations remain exposed to oil spills, storm loss of nesting islands and fishing-gear entanglement, and coastal Louisiana continues to lose the islands themselves. DDT is also still used for malaria vector control in some countries, so the residue story is not globally finished.