Lead Ingestion in Eagles and Hawks from Shot Prarie Dogs in Wyoming

Many studies have established that raptors are poisoned from ingesting lead bullet fragments that remain in gutpiles of big-game that are harvested with lead-based bullets. Several studies, including our previous research, have directly linked lead exposure from this source to California Condors, Bald Eagles, Golden Eagles, and Common Ravens. Lead poisoning causes cardiac, respiratory, and neurological damage that can kill raptors or make them slower and more susceptible to risks like car strikes, predator attacks, and starvation. While the connection between lead-based ammunition for big-game hunting and blood lead levels in raptors is well established, there are several other sources of lead for which data are lacking, including upland game and varmint hunting.

The Thunder Basin National Grasslands (TBNG) in eastern Wyoming hosts large populations of black-tailed prairie dogs, Golden Eagles, and Ferruginous Hawks. Because of several management objectives, the TBNG has been closed to prairie dog shooting for over ten years. In 2017, TBNG temporarily lifted hunting restrictions in order to reduce prairie dog populations for the year and it is anticipated that shooting will continue in 2018. Many prarie dogs are shot and left behind, providing an easy meal for scavengers, like Golden Eagles and Ferruginous hawks. Unfortunately, these carcasses can be littered with lead bullet fragments (as shown to the right). Raptors scavenge on these carcasses and bring them back to their nest, exposing their young to lead. The initiation of prarie dog hunting in TBNG provides a unique opportunity to investigate the lead exposure risk from prairie dogs to nestling eagles and hawks in Wyoming.

The temporary allowance of shooting in the TBNG in 2017 and 2018 provides an opportunity to quantify lead exposure in nestlings caused from feeding on scavenged prairie dogs shot on TBNG. We are collecting blood and feather samples from nestling Golden Eagles and Ferruginous Hawks, allowing us to look at daily lead ingestion rates since lead is deposited in growing feathers. Blood will provide lead ingestion rates for the two weeks prior to sampling. We are simultaneously recording all ambient sounds surrounding each nest site to quantify shooting frequency and collecting shot prairie dogs to quantify the amount of lead within each prairie dog. Our collaborators will test all samples to directly link the isotopes of lead within the prairie dogs and eagles to the type of lead bullets used to hunt prairie dogs. By comparing lead levels in nestling eagles and hawks in 2017 to lead levels in nestling raptors in years when prairie dog hunting is not allowed, we can infer the impact of these hunting practices on lead levels in young raptors.

This study will be the first to quantify risk of lead exposure throughout the development period of young eaglets. Poisoning from lead and other sources is the third highest source of mortality in Golden Eagles. By quantifying the lead risk from prairie dog shooting, we will directly impact management decisions on TBNG and US Fish and Wildlife Service conservation objectives across the western US.

X-ray of shot prairie dog collected from TBNG in 2017 50 yards from an active eagle nest. White specs are lead fragments.

X-ray of shot prairie dog collected from TBNG in 2017 50 yards from an active eagle nest. White specs are lead fragments.

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