The lead (Pb) lining of agriculture‐related subsidies: enhanced Golden Eagle growth rates tempered by Pb exposure

Abstract

2020. The lead (Pb) lining of agriculture-related subsidies:enhanced Golden Eagle growth rates tempered by Pb exposure. Ecosphere 11(1):Abstract. Supplementary food resources (e.g., subsidies) associated with agriculture can benefit wildlife species, increasing predictability and availability of food. Avian scavengers including raptors often utilize subsidies associated with both recreational hunting and pest shooting on agricultural lands. However, these subsidies can contain lead (Pb) fragments if they are culled with Pb-based ammunition, potentially leading to Pb poisoning and physiological impairment in wildlife. Nesting Golden Eagles (Aquila chrysaetos) commonly forage in agricultural lands during the breeding season, and therefore, both adults and their nestlings are susceptible to Pb exposure from scavenging shot wildlife. We assessed drivers of Pb exposure in 258 nestling Golden Eagles (401 total blood samples), along with physiological and growth responses, in agricultural lands across four western states in the United States. We also evaluated the birds' Pb stable isotope signatures to inform exposure sources. Twenty-six percent of Golden Eagle nestlings contained Pb concentrations associated with subclinical poisoning for sensitive species (0.03-0.2 lg/g ww), 4% had Pb concentrations that exceeded subclinical poisoning benchmarks (0.2-0.5 lg/g ww), and <1% exceeded either concentrations associated with clinical poisoning (0.5-1.0 lg/g ww) and or those deemed to cause severe clinical poisoning (>1.0 lg/g ww). Lead concentrations were highest in nestlings with close proximity to fields that potentially provided subsidies and declined exponentially as distance to subsidies increased. However, close proximity to agriculture, and presumably subsidies, positively influenced nestling growth rates. Across the range of Pb exposure, nestlings experienced a 67% reduction in delta-aminolevulinic acid dehydratase (d-ALAD) activity, suggesting nestlings may have been anemic or experiencing cellular damage. Isotopic ratios of 206 Pb/ 207 Pb increased non-linearly with increasing blood Pb in Golden Eagle nestlings, and 45% of the birds were consistent with those of ammunition. However, above 0.10 lg/g ww, the proportion associated with ammunition increased to 89% of the nestlings. An improved understanding of how these positive (growth) and negative (physiology) effects associated with proximity to subsidies interact would be beneficial to managers when considering management scenarios and potentially evaluating any measures taken to reduce Pb exposure across the landscape.

High Frequency of Lead Exposure in the Population of an Endangered Australian Top Predator, the Tasmanian Wedge‐Tailed Eagle (Aquila audax fleayi)

“…A very high proportion (95%) of nestlings we tested had been exposed to lead. Although this rate is higher than that reported for other raptors (Harmata 2011; Katzner et al 2017; Bruggeman et al 2018; Herring et al 2020), that most exposure was at a low level is consistent with findings for other raptors (Carlson et al 2012). Nestling raptors can be exposed to lead from inhalation, ingestion, or maternal transfer (Pattee 1984; Katzner et al 2017; Bruggeman et al 2018).…”

“…However, the strong selection pressure in wild animals to hide signs of illness risks underestimating the effects. Furthermore, lead has been shown to have physiological effects even at low concentrations (Lanphear et al 2005; Espín et al 2015; Herring et al 2020), suggesting that even low exposure may be of concern.…”

Concentration and origin of lead (Pb) in liver and bone of Eurasian buzzards (Buteo buteo) in the United Kingdom

“…Swans with higher than moderate blood Pb were suggested to suffer sub-lethal effects which made them unlikely to fly and they were therefore unlikely to collide with structures. Regarding possible physiological effects, previous studies have detected an adverse effect of Pb on ALAD activity in birds at blood Pb levels below 20 µg dL -1 , and as low as 3 µg dL -1 (Finkelstein et al 2012, Martinez-Haro et al 2011, Espín et al 2015, Newth et al 2016, Herring et al 2020.…”

Consequences of migratory coupling of predators and prey when mediated by human actions

“…For example, when calf losses to eagles become too high, reindeer herding communities may apply to the authorities for protective hunting. Likewise, lead exposure can result in lethal or sublethal effects with demographic consequences (Golden et al., 2016; Herring et al., 2020; Newton, 2021). There is irony in the fact that these demographic consequences likely stem from an evolved behavioural response by eagles to increased food availability (Cooper and Blumstein, 2015).…”