Thanks, Donna—a few paragraphs from that website:

What's the risk? Weigh the evidence

While it is hard to get an accurate count of water birds and birds of prey that die from ingesting lead tackle, current research indicates that lead poisoning is a serious concern.

Research on loons from six New England states has shown that on the majority of lakes where dead adult breeding loons were found between 1987 and 2002, about 26% of these loons died from lead poisoning. Some lakes were identified as hot spots with lead causing over 50% of documented causes of death.

In Michigan, another 15-year study examined 186 dead loons and revealed that lead poisoning — primarily from lead jigs — was the number one cause of death at 24% (44/186) of overall mortality. Limited research in Minnesota has also documented lead poisoning of loons. A study conducted by the Minnesota Pollution Control Agency concluded that lead poisoning accounted for 12 percent of the dead adult loons with known causes of death.

Jane Clark

Here's a link -- I think it's more likely the fish swallow lead jigs with "sinkers" on the end of the hook, rather than the traditional worm-on-a-hook with a sinker separately attached to the line. (see the x-ray of the loon).  Water birds that scoop up "pebbles" from the bottom could be eating sinkers from worm fishing.