Study finds lead from ammunition threatens ‘scavenging raptors’

Sustainable Leaders | Oceania

By Catriona New, Freelance Writer

Published January 26th, 2024

Renowned for being one of the most toxic heavy metals, lead exposure can have a detrimental impact on wildlife and is contributing to the collapse of raptor populations around the world. The use of lead-based ammunition, in particular, is currently impacting one of Australia’s top avian predators.

A recent study published in 2023 investigated the impact that lead exposure has on one of Australia’s top avian predators, the wedge-tailed eagle (Aquila audax). Being facultative scavengers, these birds regularly take advantage of shot wildlife and hence, are vulnerable to the effects of lead-based ammunition.

Wedge-tailed eagles are facultative scavengers and take advantage of carcasses when they find them | John Robert McPherson / Wikimedia Commons

Lead pollution can occur from various anthropogenic sources, such as fishing sinkers, lead-based paint and dust from mining industries. However, lead-based ammunition is believed to be one of the main sources of contamination impacting avian species, including raptors.

Raptors are apex predators found across a wide range of geographical radii, who, across their lifetime, can accumulate environmental contaminants. Typically, these species are exposed to lead through the unintentional ingestion of gunshot fragments present when scavenging the carcasses of their prey.

‘Lead pollution can occur from various anthropogenic sources such as fishing sinkers, lead-based paint and dust from mining industries.’

This study found evidence for harmful levels of exposure to lead in this avian species. Lead was detected in over 80% of the sampled specimens, with almost 5% of birds categorised as having been exposed to severe concentrations (>20 parts per million) and 13% having had elevated (10 to -20 parts per million) exposure to lead.

Despite the pellets often being regurgitated, the heavy metals present within the shot can be absorbed quickly into the bloodstream, resulting in various adverse health implications.

Lead poisoning can cause both lethal and sublethal effects, which, depending on the concentration, can be responsible for reproductive, vascular, immunological, renal, and behavioural complications.

Although ingested heavy metals will come into contact with most organs and tissues in the body, the concentration of lead in bone is best used as a proxy for an organism’s lifetime exposure.

Heightened levels of this contaminant can be stored and accumulated in the skeletal system for years after the initial exposure and can be measured using portable X-ray fluorescence devices. By contrast, lead concentration diminishes considerably faster in blood and soft tissues, limiting their diagnostic abilities to only indicating more recent exposure.

Wedge-tailed eagle in flight. | Patrick Kavanagh / Wikimedia Common

The study also found age-related exposure patterns with higher concentrations of lead being more regularly reported in older wedge-tailed eagle samples, providing evidence for the accumulation of and repeated exposure to this contaminant over the lifetime of the organism.

Additional studies from around the world also support the prevalence of harmful concentrations of this metal in different scavenging raptor species. The range of exposure levels recorded, and the plethora of studies highlighting samples with repeated, high levels of exposure, means that there is potential for this environmental pollutant to be causing population-level effects.

In a 2022 study, Slabe showed using demographic modelling of lead poisoning in eagles across the US, that population growth rates could be restrained by almost 4% for bald eagles and nearly 1% for golden eagles.

‘Obligate and facultative scavenger species generally had the highest lead concentrations in liver, kidney, blood and bone.’

Inferences can be made about how this environmental toxicant might constrain the population sustainability of wedge-tailed eagles using the population models created for other similar species, such as golden and bald eagles. However, direct modelling of how lead exposure may be altering the population demography of this particular species is still required.

Further research is necessary for us to gain a more comprehensive understanding of the impacts that lead poisoning can have on avian species around the world to identify the species and regions that are most vulnerable to this pollution.

Lead-based ammunition is contributing to constraints in population sustainability of eagle species. | / Wikimedia Commons

The impact that different hunting regulations have on exposure levels may also be prevalent, with some regions having regulated hunting seasons and others allowing culling and hunting to take place year-round, as is found in south-eastern Australia.

In this case, seasonal trends in lead poisoning may not be seen as the abundance of unretrieved shot carcasses found throughout the year may increase the overall lead exposure of different facultative scavenging species.

In their lifetime, wedge-tailed eagles will encounter a wide range of anthropogenic threats including vehicle collisions, wind turbine strikes and power line electrocution. However, compared to the cost and adaptation required to mitigate some of the anthropogenic risks that they are confronted with, the use of lead-free ammunition is a relatively straightforward modification that can be made.

‘Removing this avoidable source of environmental contamination and suffering and mortality of wildlife is a matter of political will.’

Currently, action to ban lead ammunition in wetland areas has been granted in all 27 European Union countries, and Denmark has also agreed to a complete ban of lead-based ammunition, with several other countries considering following suit.

The benefits of Australia adopting the primary use of a non-toxic ammunition alternative will be seen in wedge-tailed eagles alongside other raptor species, scavenging mammals and the human consumers of meat that may have been impacted by lead contamination.

The United Nations acknowledges that we are currently experiencing a Triple Planetary Crisis, where environmental pollution is thought to be one of the top global issues, alongside climate change and biodiversity loss.

One example of environmental pollution is heavy metal exposure, with lead renowned for being one of the most virulent forms. Due to its abundance, toxicity and persistence within the environment, lead exposure can have detrimental impacts on wildlife, and, in some cases, the risks of lead contamination in birds can be used to highlight the threat posed to human health.

Despite countries introducing new laws and instigating local lead ammunition bans to mitigate the impact of lead poisoning in wildlife, there must be adequate enforcement of these laws.

Furthermore, with increased encouragement and education within the hunting community towards using alternative ammunition, we can raise awareness of conservation efforts and inspire more long-lasting change at an individual level.

Featured Image: Laurie Boyle | Flickr

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