From the article:
Researchers suspect that hormone-disrupting pollutants such as flame retardants may have eroded the birds' delicate social structure and contributed to a mysterious drop in Squam Lake’s loon population. In other parts of the Northeast, scientists have implicated acid rain and mercury in declining numbers of chicks. Loons with high mercury levels lay and hatch fewer eggs, and they’re not good parents.
Once facing steep declines in much of their U.S. range, loons have made a comeback in the last 40 years. Now this success story is in jeopardy.