Then the use of the pesticide DDT in the 1940s and ’50s decimated the pelican population by thinning their eggshells.
DDT did the same thing to the eggs of the peregrine falcons. Almost made them extinct. Probably the DDT affected other bird species too. Fortunately/luckily the USA banned the usage of the DDT in 1972, and there is a worldwide ban on the agricultural usage of the DDT since 2004.