Unlike some states, Missouri manages many thousands of acres to promote hunting opportunities. Some of those are for waterfowl, others for dove hunting - both involve assigned shooting spots for hunters. Whether it's a sunflower field mowed down for doves or flooded timber, because falling shot is concentrated, ingested expended shot is picked up by many species of birds. Because of the assigned shooting stands/blinds, over the course of season after season, expended shot is concentrated in known locations and never goes away.
Waterfowl hunting areas feature artificially flooded food plots, flooded standing corn, & timber. It's not only waterfowl exposed to lead shot while the areas are flooded, but also every other bird specie during the growing season prior to flooding. My son's a wildlife biologist, worked at Eagle Bluffs Conservation area, and explained what biologists found and why they manage the area as they do. All waterfowl areas not only have "refuge" safe areas closed to hunting during the season, but also serve as refuge areas throughout the spring and fall migrations to millions of migrating birds - everything from coots, pelicans, ducks, and geese to songbirds of many types.
As far as banning lead shot on upland areas - rabbit, squirrel, quail (as if there area any), etc. - to me, that's overkill. Wanna see dead birds? - Visit a wind farm.
