Shooting and using poison baits (e.g., strychnine, zinc phosphide) are current management options for controlling Richardson's ground squirrels (Spermophilus richardsonii; RGS). Bullets used for shooting RGS contain lead, fragment upon impact, and RGS carcasses are not usually recovered after being shot. For these reasons, we hypothesized that scavenging birds of prey may be at risk of lead poisoning. To test this, we took radiographs of 15 shot RGS and analyzed the area around the path of the bullet for lead. Lead levels ranged from 0.01 to 17.21 mg/carcass (median = 3.23 mg), and fragments appeared as dust. Two common scavenging hawks (Swainson's and ferruginous hawks [Buteo swainsonii, B. regalis]) consume eviscerated RGS carcasses and would consequently ingest this amount of lead per feeding. In a previous study, an estimated 5.71 mg/kg of lead, eroded in vivo from ingested lead shot, was lethal to bald eagles (Haliaeetus leukocephalus). Fitting the residue values to a normal distribution and based on the mass of an average raptor, we determined that roughly 1 in 5 RGS carcasses had lead levels that exceeded this value. Based on the average amount of lead in carcasses, and assuming that uptake of lead from the carcass is as high as that of eroded lead, we suggest that hawks would have to eat roughly 6.5 carcasses, taking an average of 23 days of feeding on an uninterrupted supply of shot carcasses, to attain a lethal dose of lead. Uncertainties remain, but shot RGS carcasses appear to be an appreciable source of lead that could prove fatal to scavenging hawks. This hazard could be avoided with the collection and disposal of shot carcasses and with the use of (green) ammunition.
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Vol. 70 • No. 1