If we look at the literature for humans, there is a growing body of evidence that any level of lead in the body is detrimental. According to a paper from the American Academy of Pediatrics (Prevention of Childhood Lead Toxicity, AAP 2016), blood levels...
There does seem to be some evidence that even very low levels of lead in human children can have deleterious effects, primarily related to learning and behavior.
That said, as you know, various organizations publish acceptable blood lead levels in humans. That is they publish blood lead levels below which no treatment is required (which is a good thing or we would all be in treatment) and the person is not advised that a change of behavior or environment is necessary to avoid harm.
More importantly, as you said earlier, extrapolation between species can be problematic.
Therefore, yes, exposure does lead to morbidity (harm). Any level of lead is considered toxic as lead serves no purpose in the body, unlike other heavy metals such as zinc, cobalt, chromium, selenium, etc which actually serve biological roles in the body.
Given your cautious statements earlier about extrapolating between species, it seems clear that you understand that a study showing behavioral/learning issues in human children at very low lead levels does not warrant the sweeping conclusion that all exposure to lead, regardless of the level and regardless of the species or developmental stage of the organism, causes harm.
However, considering that biological processes are 99% the same across all species, we can extrapolate out.
And if we were to do that from a practical standpoint, rather than an emotional one, it would follow that there are acceptable blood lead levels in other species, just as there are in humans. Levels below which the harm done is insufficient to warrant treatment, or even a change in behavior/environment.
I'm not saying that animals are not harmed by lead--it is clear that some are, just as some humans are. I'm saying this:
1. Equating exposure with harm is a bit much. Panic about childhood lead exposure notwithstanding, with any potentially toxic substance, the dose makes the poison.
That is a basic principle of toxicology.
2. Sourcing exposure is important since there are many sources of lead besides bullet/pellet fragments. The article I provided showing that the elimination of lead hunting bullets in the California Condor range hasn't resulted in lower exposure levels gives a good example of why it is worthwhile to raise that concern.
3. Any solution proposed will cause harm to hunters and hunting related industries and likely more generally to shooters and shooting related industries. Therefore, it's important not to just handwave about the level of harm done to animals and the environment. There needs to be a balance struck, and that can't happen until there's a genuine attempt to quantify the harm on both sides of the equation.
If the premise is that science shows that lead bullets are harming non-target animals, then let's really use science to prove or disprove the premise and to quantify the harm so it can be balanced against the harm caused by the solution rather than just implementing feelgood policies to provide the illusion of constructive activity.