This is the abstract of the Keel study, the first of the last list:

"Abstract. We estimated total lead shotshell pellets expended,
resultant pellet availability near soil surface, and the frequency
of pellet ingestion by northern bobwhites (Colinus virginianus)
attributable to nearly a quarter century of bobwhite hunting on
a 202-ha upland habitat at Tall Timbers Research Station, Leon
County, Florida. A total of 7,776 shots were fired, resulting in
the expenditure of approximately 4.5 million pellets ( 22,519/
ha). Sixteen of 235 (6.8%) soil samples collected in 1989 and
1992 contained one or two pellets. Soil samples indicated that
approximately 7,800 pellets/ha (about 35% of the projected
24-year deposition) were within 2.54 cm of the soil surface.
Pellet ingestion by bobwhites was evaluated by examining 241
gizzards collected from 1989–92. Three bobwhites (1.3%) had
ingested pellets (x  1.3 pellets). No instances of suspected
lead poisoning were noted in bobwhites over the 24-year period.
Sport hunting of wild bobwhite populations on upland
habitats appears to produce a low potential for lead poisoning
compared to lead deposition in association with waterfowl and
dove hunting."