Here's why the Turkey Vulture isn't a good indicator species for lead toxicity in other raptors:

http://www.jwildlifedis.org/cgi/reprint/39/1/96.pdf

In summary: "considerable tolerance by turkey vultures to the deleterious effects of lead ingestion.
Based on these observations, turkey vultures appear to be poor models for assessing the risk of
lead poisoning to California condors or predicting their physiologic response."