The difference between bird and human is that the gizzard is designed to use pebbles ( or just as easily use lead pellets/fragments ) instead of teeth to grind their food. And they have highly acidic conditions in which the grinding takes place. The 'grit" stays in the gizzard for many days. From an article published in the Alaska Dept of Wildlife: "The grinding action of the gizzard which breaks down food items is aided by the abrasive action of grit. Grit can be retained in the gizzard for several weeks and not passed to the intestine with the partially digested food. Lead resembles grit and may also be retained in the gizzard for two to three weeks." A lead pellet would stay in humans stomach for a few hours, and in the entire gut for no more than a day or two. Birds aren't so lucky. Their absorption of lead ingested is many many times higher than mammals ingesting the same relative amount because of how their digestive system works. And another unfortunate fact is that all toxins are dependent on dose / body size. Birds are small and light. They are at a disadvantage compared to most mammals in that way too. The ducks and geese species most affected are those who regularly eat items off the bottom of ponds. It is certainly true that the numbers of birds killed form lead poising is an estimate, but it is not a guess. It is quite easy to prove in a lab that one big lead pellet (BB) will kill a duck, or a two or three small ones (#6 ) If you find a duck with lead pellets in its gizzard, it was almost surely a doomed bird.
A quick search on google showed the following articles, and there are many many more:
From the Houston Chronicle: Prior to and immediately after the ban on the use of lead shot, studies that examined the gizzards of ducks wintering in Texas showed about 15 percent of all ducks had ingested at least one lead pellet, with some species having ingestion rates as high as 20 percent or more. Annually through most of the 20th century, an estimated 2 to 3 percent of North American's duck population - as many as 1 million to 3 million ducks a year - died from the effects of ingesting spent lead shot.
Recent studies conducted on the Texas coastal prairies and marshes indicate that while ducks continue ingesting spent shot pellets at generally the same rate they did before the lead ban, most of those pellets are steel (soft iron) and other non-toxic metals now used in waterfowl shotshells. Some recent studies of wintering ducks in Texas have found as few as 1 percent or fewer, depending on the species, had lead shot in their gizzards. ( my note: this lead is mostly left over from pre- ban years, it doesn't just " go away" but stays on the bottom of marshes until sediments build up enough to hide it from the birds)
and here's some data from a scientific journal in Britain: In the UK, long term wildfowl disease surveillance between 2000 and 2010 found lead poisoning to be responsible for 8% of all deaths41. A total of 73,750 wildfowl are estimated to die annually in the UK because they are poisoned by ingested lead shot. This represents about 3.1% of the wintering wildfowl population in the UK dying annually because of lead poisoning from ingested shot42. Waterfowl species vary considerably in their exposure to ingested shot because of differences in diet, foraging behaviour and habitat. The equivalent calculations to those used to calculate UK additional mortality for all wildfowl give considerably higher annual mortality rates caused by lead shot ingestion for the two wintering duck species with the highest prevalence of ingested shot. For common pochard (Aythya ferina) the estimated annual mortality rate from ingested lead is 7.6% and for northern pintail (Anas acuta) the annual rate is 11.7%43. Populations of both of these species have declined markedly in the UK in recent decades, and to a greater extent than other duck species with lower exposure to ingested lead44.