Unexpectedly high frequencies of lead poisoning in eagles

Moving beyond traditional studies focused in one region, a new 38-state-wide analysis of lead exposure in bald and golden eagles across the U.S. reveals unexpectedly high frequencies of both chronic and acute lead poisoning, the incidence of which varied by age and, for bald eagles, by region and season.

“Our large-scale data set hints at drivers of spatial and sub-continental trends in the frequency of lead poisoning of eagles that would be impossible to detect in local studies,” say Vincent Slabe and colleagues.

Lead poisoning occurs in populations of predatory birds worldwide, but to date, exposure rates and population impacts are known only from regional studies. For eagles specifically, there is no understanding of spatial and temporal patterns of lead exposure at the larger scale. Here, using multiple lines of evidence including blood of live eagles and bone, liver, and feathers of those that had died, Slabe and colleagues sought to illuminate such patterns.

They quantified lead exposure of 1,210 bald and golden eagles from 2010 to 2018, reporting that 47% of bald eagles and 46% of golden eagles exhibited bone lead concentrations above thresholds for poisoning considered chronic (due to repeated lead exposure). For both species, adults were more frequently chronically poisoned than younger species. Showing a regional trend, bald eagles in the Central Flyway zone of the U.S. exhibited higher rates of chronic lead poisoning than those in the Atlantic and Pacific Flyways.

The authors also evaluated the frequency of acute lead poisoning – caused by a short-term, high-exposure event – and report that about 29% of bald and 9% of golden eagles had blood lead concentrations indicative of such poisoning. For bald eagles in particular, adults of the species were more frequently acutely poisoned, acute poisoning was less common in summer than in fall, and birds in the Central Flyway exhibited a higher rate of acute lead poisoning than those in the Atlantic and Mississippi Flyways.

Acute poisoning of both species was generally higher in winter months, when bald and golden eagles commonly scavenge and may directly ingest lead fragments from ammunition used in hunting season. “Our data show continent-wide temporal correspondence between acute lead poisoning of eagles and use of lead ammunition,” the authors write.

Their estimation of impacts of lead exposure at the population level, based on thresholds used by veterinary pathologists to define severe clinical poisoning, suggest the continent-wide population growth rates of these species are being suppressed by 3.8% for bald eagles and by 0.8% for golden eagles, with possible long-term impacts to the population.

Collectively, the authors’ large-scale data uncover trends harder to detect in local studies, such as the high frequency of acute and chronic poisoning for bald eagles in the Central Flyway zone, which can't be explained by differential sampling alone; "a more plausible explanation...lies in the potential for unexplained differential scavenging rates of bald eagles in the different flyways," say the authors.

The age-related patterns – higher chronic and acute poisoning in adults of one/both species – reflect the accumulation of lead in scavenging birds as they age, which, say the authors, creates an underappreciated demographic constraint for North American eagles. Lead poisoning of scavenging birds has been documented on every continent except Antarctica, although this is the first time that suppression of population growth rates has been documented on a continental scale. As such, these results identify important directions for future conservation action.