Several years ago, a team of wildlife researchers tested the effect of California’s 2008 lead ammunition ban on the blood lead levels of two scavenging birds, the Golden Eagle and the Turkey Vulture (Kelly, et al. 2011). The lead ammunition ban was enacted to aid the recovery of a critically endangered bird, the California Condor, recovery of which has been impeded by lead poisoning. The ban restricted the use of lead ammunition in the range of the California Condor, for the hunting of big game (deer, bears, wild pigs, etc.) and non-game (coyotes, ground squirrels, etc.) species. The carcasses and entrails of these lead-killed animals are typically left in the field, embedded with lead fragments, so scavenging birds are highly susceptible to ingesting the lead that permeates a hunted carcass.
In this study, the researchers aimed to indirectly determine the ban’s effectiveness at reducing the lead exposure of condors, by testing the blood lead levels of two indicator species pre-ban and post-ban. Golden Eagles, while primarily predatory, will still scavenge carrion, particularly during winter. Turkey Vultures are scavengers by nature, almost exclusively eating dead animals. Scavenging behavior and abundance in the California Condor range make these two birds suitable indicators of the effectiveness of the lead ammunition ban.
The Golden Eagles and Turkey Vultures were captured in Kern County and Monterey County, respectively, two counties with extensive wild pig hunting. Golden Eagles were captured with carrion-baited bonnets and pit-traps, while Turkey Vultures were captured with a carrion-baited trap containing a live lure vulture. As Golden Eagles primarily eat carrion in the winter, they were gathered in the late fall and winter of 2007 and 2008.
Unfortunately, this coincided with their migration, so birds captured at these times could not be considered local birds and were classified as “unknown migrant status”. In order to capture local birds that were not likely to have come from regions outside the boundaries of the condor range, Golden Eagles were also captured in late spring of 2008 and 2009. Turkey Vultures eat carrion year-round, so they were captured in late spring of 2008 and 2009, and they were classified as local birds.
The Turkey Vultures were identified with injectable electronic microchips, and the Golden Eagles were banded, so that individuals could be identified if captured a second time. Blood was collected from the birds before eight hours elapsed following capture, and tested for lead concentration via graphite atomic absorption spectrophotometry or inductively coupled plasma mass spectrometry. Over 10 μg/dL was set as elevated lead exposure, while 10 μg/dL or lower was considered to be baseline lead exposure. Data on blood lead levels of Golden Eagles from previous research conducted 1985-1986 during the same seasons were also included in statistical analysis.
The percentage of Golden Eagles (either migrant status) with elevated lead exposure was decreased from 76% pre-ban to 32% post-ban, a significant change. The 1985-1986 research showed 66% of eagles with elevated lead exposure, similar to pre-ban data gathered in this study. With non-migrant eagles, the drop in lead exposure was more pronounced, decreasing from 83% to zero. The percentage of total Turkey Vultures with elevated lead exposure was 61% pre-ban, and 9% post-ban, also a significant change. Lead exposure incidence decreased in recaptured vultures from 60% to 14%. Adult Golden Eagles were found to have significantly higher lead concentrations than subadult eagles; the authors speculated that this was likely due to younger birds having faster lead uptake into their developing bones.
The data gathered in this study provide evidence that the 2008 California ban on lead ammunition effectively reduced blood lead concentrations in multiple scavenging bird species within the boundaries of the California Condor range. Lead exposure levels were reduced but not eliminated in Golden Eagles with unknown residency status, suggesting that some of these birds migrated from areas outside the condor range. These persistent levels (and those of the non-migrant Turkey Vultures) could also be explained by some hunter non- compliance, or scavenging of small game not covered by the lead ammunition ban. This study provides some hope for the recovery of the California Condor, showing that a lead ammunition ban can reduce the lead exposure of scavenging birds.
