So this is the big weakness of the raptor and lead studies out there. They assume that particles of metallic lead in carcasses are the source of eagle and raptor lead levels. While that is not an unrealistic initial assumption, it should be tested. I have not seen that work anywhere to date.The site was huntfortruth.org - it's still accessible on archive.org, I believe.
And I don't know about the condor isotope isssue, but I do know that the guys at the North American Non-Lead Partnership admitted when I went to their training that the proportion of condor mortality due to lead toxicity remained unchanged after banning lead bullets for hunting in the condor range - that fits with the different isotope issue.
They seemed shocked when I pointed out that their own data indicate that changing to non lead bullets does not improve avian lead toxicity mortality...