Sorry, I misread from data earlier presented in this thread, here's the proper explanation:
"The bloodwork regularly shows two of the four isotopes of lead. One isotope is the one found in leaded gasoline, and it is never found in the bloodwork. Second is a background isotope found naturally occurring in soils and, typically, is waters where glacial deposits left cinnabarite (a lead bearing ore), and this occurs in low levels. But the third and fourth isotopes are found in galena ores that are mined for lead smelting and is used in all kinds of industrial uses, including bullets. THAT is the one that shows up in high levels in sick birds.
Now, to narrow down possible sourcing of that lead in the bloodstream, they also began testing for copper and zinc. Why those two? Gilding metal, 95% copper, 5% zinc. That's the common jacket for dang near every modern bullet manufactured. Copper and zinc also shows up in the blood of lead poisoned raptors at the exact ratio of 95:5. This is what is known in science as a one to one correlation. "