Look, the bottom line is that we know birds ingest lead pellets in the field. We know that from controlled tests in the lab, that birds fed lead pellets get lead poisoning. It would be blatantly stupid to believe that birds in the field eating lead pellets won't get led poisoning. Plenty of birds taken from the field that ate lead pellets have been shown to have lead poisoning, but via symptoms and through tissue analysis.
Are there potentially other sources for lead as well? Sure. That doesn't negate the fact that consumption of lead pellets can cause lead poisoning.
So is lead toxicity from hunting ammo real as per the OP's query? How could it not be?
Even humans get elevated lead levels from eating meat from animals shot with lead bullets.
https://www.outdoorlife.com/is-game-meat-shot-with-lead-safe-to-eat/
The counter argument to the CDC's warning was that the level of lead, on average, did not reach the level of toxicity. This is huge on many levels of consideration. One, it is recognized by folks on both sides of the argument that consuming meat with lead fragments resulted in elevated lead levels. Nobody is claiming that is can't happen.
The ironic thing is that the CDC was mostly issued their warning for pregnant women and children under 6, because for developing kids fetuses and kids, the risk is much higher even at lower levels.
You have a particular issue with the biologists doing the studies or are you making a gross generalization like the left saying we can't be trusted with guns because there are a lot of gun nuts out there?
Well, it has been demonstrated, but the testing was done by biologists and you have discounted the claims of biologists. Do you trust medical doctors, or do you just consider them to be glorified biologists with a narrow field of study?
https://journals.lww.com/pec-online...llet_Ingestion_in_3_Children__Another.15.aspx