Bill—I don’t think you can assume that lead shot will be scattered widely and not be directly ingested by various bird species.
The USGS brochure on Lead Poisoning on Wild Birds notes that “Terrestrial bird species reported with ingested spent lead shot include mourning doves, ring-necked pheasants, northern bobwhite quail, wild turkey, and chukars.” (Note, no songbirds are mentioned here—most research is done on game species.)
Lead shot ingestion in mourning doves has been well documented in scientific research for more than 50 years. In 1999, the US Fish and Wildlife Service reported that "mourning doves are particularly likely to ingest spent lead shot." Ingestion of spent lead shot is recognized as a significant problem due to the harmful toxic effects and high mortality rate among victims. In wildlife, primary and secondary consumption is known to directly or indirectly impact populations through acute or chronic lead poisoning.
Iowa DNR and county conservation boards have planted fields of sunflower, etc. for dove hunting fields.
Todd Bogenschutz, upland game biologist for the Department of Natural Resources (DNR), in an April 9, 2011 Cedar Rapids Gazette article, said the DNR plans to establish food plots -- sunflowers, sorghum, millet and wheat -- on state-owned leased farm ground. (Dove shooting fields and "hunts" are managed to attract a large number of feeding mourning doves and keep them on the wing for shooters to target. Typically, plot management is implemented with the principal of planting and conditioning the birds to the fields prior to opening day.)
According to Missouri Department of Conservation resource scientist John Schulz in the Cedar Rapids Gazette article, a survey conducted on dove hunting at a managed field near Kansas City found 800 hunters fired 40,000 rounds to kill 1200 to 1400 doves.
Fields like this can be heavily hunted and according to Ted Williams who writes a column named “Incite” in Audubon Magazine, wrote an article about lead and below are excerpts from that article. Ted Williams is an avid hunter.
From Audubon magazine:
The U.S. Geological Survey reports that as many as 400,000 lead shotgun pellets per acre rain annually on popular hunting fields,
And,
Hunters shoot roughly 20 million mourning doves a year, but evidence suggests that nearly that many die from eating lead shot. A study at the James A. Reed Memorial Wildlife Area in Missouri revealed that 728 dove hunters had deposited 348,037 lead pellets per acre. And in an area in Arizona that wasn’t even managed for dove hunting, 19.9 percent of doves sampled had lead shot in their digestive tracts. Even if doves haven’t ingested lead shot, when hunters shoot them full of it, scavengers find lost carcasses.
And,
Ducks, swans, and geese actually key in on lead shot because to their sensitive bills it feels like seeds. ……..Mammalian scavengers are also at risk. In one study, 46 percent of blood samples from grizzly bears showed elevated lead.
And,
The federal government had known about it since at least 1894, when Audubon founder George Bird Grinnell first sounded the alarm in Forest & Stream magazine. But today—after the public has watched for 117 years as waterfowl and other wildlife die from swallowing lead shot and bullet fragments—the mantra from the gun lobby that plumbism publicity is a plot to disarm America remains unchanged. The question Americans need to be asking now is: Will that mantra, along with the toxic injections it has preserved, go unchanged for another century?
Jane Clark