The incident suggests that the birds may be becoming victims of their own success. Though scientists could not pinpoint the source of the lead, they suspect the birds fed on an animal carcass full of lead shot-showing that they are increasingly finding their own food sources instead of relying on carcasses left by the recovery team. Farry's team has observed the birds feeding together and suspects the poisoning could come from a single carcass, although they don't exactly know where the carcass was located because the radio collar signals used to track the condors are interrupted when they fly below the canyon's rim.
Scientists can only speculate as to why the carcass was full of lead shot.
Hunting is allowed on some federal lands surrounding the Grand Canyon, but no big game seasons were open when the birds began dying in June. Bill Heinrich, species restoration manager for The Peregrine Fund, said that it's possible the birds found the carrion on private land. All of the condors captured in July had lead in their bloodstreams. At press time, ten of the 16 birds had been let go, and the other six were awaiting release. Heinrich said that the poisonings are also a concern because they are an indication that the toxic environment could be harming other creatures that are not being monitored.