Lead exposure in bald eagles from big game hunting, the continental implications and successful mitigation efforts

Abstract

Studies suggest hunter discarded viscera of big game animals (i.e., offal) is a source of lead available to scavengers. We investigated the incidence of lead exposure in bald eagles in Wyoming during the big game hunting season, the influx of eagles into our study area during the hunt, the geographic origins of eagles exposed to lead, and the efficacy of using non-lead rifle ammunition to reduce lead in eagles. We tested 81 blood samples from bald eagles before, during and after the big game hunting seasons in 2005-2010, excluding 2008, and found eagles had significantly higher lead levels during the hunt. We found 24% of eagles tested had levels indicating at least clinical exposure (>60 ug/dL) during the hunt while no birds did during the non-hunting seasons. We performed driving surveys from 2009-2010 to measure eagle abundance and found evidence to suggest that eagles are attracted to the study area during the hunt. We fitted 10 eagles with satellite transmitters captured during the hunt and all migrated south after the cessation of the hunt. One returned to our study area while the remaining nine traveled north to summer/breed in Canada. The following fall, 80% returned to our study area for the hunting season, indicating that offal provides a seasonal attractant for eagles. We fitted three local breeding eagles with satellite transmitters and none left their breeding territories to feed on offal during the hunt, indicating that lead ingestion may be affecting migrants to a greater degree. During the 2009 and 2010 hunting seasons we provided non-lead rifle ammunition to local hunters and recorded that 24% and 31% of successful hunters used non-lead ammunition, respectively. We found the use of non-lead ammunition significantly reduced lead exposure in eagles, suggesting this is a viable solution to reduce lead exposure in eagles.

Figure 1. Study area with eagle count transects.

Boundary (red) and land ownership of the study area. Eagle count transects outlined in black. The majority of elk harvest in Grand Teton National Park occurs within the study area boundary and the study area includes all hunt zones on the National Elk Refuge.

Figure 2. Big game harvest and eagle blood lead levels.

Total big game harvest (lead and non-lead) by year (black bars, 2005–2008, 2009–2010) and harvest of big game using only lead-based ammunition (white bars) in Jackson Hole, Wyoming. Log transformed mean seasonal blood lead levels of bald eagles (gray line). Note the x-axis is not time contiguous since eagles were not sampled during the 2008 hunting season.

Figure 3. Summer and wintering locations of bald eagles.

Winter (2009–2010) and summer locations (2010) of 10 bald eagles fitted with satellite transmitters during the 2009 big game hunting season in Jackson Hole, Wyoming.

Figure 4. Bald eagle fall migration routes.

Fall migratory (August 2010– January 2011) routes of bald eagles captured the previous fall (2009) in Jackson Hole, Wyoming. Four of the five eagles returned to Jackson Hole during the big game hunting season a year after being tagged.