How did the pesticide DDT threatened the population of the brown pelican?
DDT changes the way in which birds metabolize calcium, resulting in unusually thin-shelled eggs which sadly can't support the weight of a mother during the incubation period.
The effects of DDT are still felt today, as even nowadays there are birds in the USA incapable of producing eggs that have shells thick enough not to crack. A good example is the California condor.
The lesson to be learned here is that if a product is incredibly effective at fixing one issue, we shouldn't assume it magically has no side effects, both in the short and long term. The use of DDT as a pesticide began in 1939 and continued for nearly 2.5 decades, devastating wildlife not just in that time period but for decades thereafter. Convenience come at a cost and we should learn from our mistakes with DDT usage.