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Mortalities at poisoned carcasses significantly contribute to the population decline of many vulture species. As vultures employ social strategies and follow each other in their search for food, one poisoned carcass can kill hundreds of individuals of endangered species such as the white-backed vulture.

Scientists from the GAIA Initiative at the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) have now shown that tagging vultures allows for the early detection of poisoning events and for the carcass to be removed. The findings are in the Journal of Applied Ecology.

As scavengers, vultures are resistant to many pathogens found in the environment. They fulfill, among other things, the ecologically important function of ridding landscapes of the pathogens present in carcasses such as Bacillus anthracis, which causes anthrax.

However, they are extremely susceptible to anthropogenic poisons such as agricultural pesticides and often die immediately after ingestion, for example via a poisoned carcass in the landscape. Poisoned carcasses are not uncommon; they are deployed, for example, in human-wildlife conflicts to control predators on farmland or to conceal illegal activities such as poaching.

Whether vultures are collateral damage or the direct target of poisoning, the effect is often a mass death of birds at a single location. These events play a major role in the dramatic population decline in some species; for example, by up to 90% within three generations in the white-backed vulture (Gyps africanus).

Tagging 5% of vultures can prevent 45% of further mortalities

Scientists from the GAIA Initiative have now identified an effective approach for mitigating the mass mortality of vultures from poisoned carcasses. Using data from white-backed vultures in Etosha National Park in Namibia, they demonstrated that the analysis of data from animal tags enables the early detection of cases of poisoning and rapid intervention.

If the poisoned carcass is removed within two hours, a considerable proportion of subsequent deaths can be avoided. The scientists calculated the cost-benefit ratio.

"According to our model tailored at our study system and species, tracking approximately 5% of the population (25 individuals in our system) offers a good balance between cost and effectiveness," says Teja Curk, scientist at the Leibniz-IZW and first author of the study.

"This approach can prevent 45% of poisoning-related deaths if intervention takes place within two hours. Our results therefore show that equipping only a small proportion of the vulture population with animal tags, which would act as a sentinel for the rest of the population, is sufficient to significantly reduce poisoning-related mortality."

The shorter the time it takes to remove the carcass, the more deaths can be prevented—if action is taken within an hour, more than 50% of deaths can be prevented; with a response time of 12 hours, this drops to 25%. In addition, the correlation between the proportion of vultures with animal tags and the number of animals potentially saved is not linear, meaning, for example, that with twice the effort (vultures with tags), considerably less than twice the benefit (avoided deaths) is achieved.

Analyses combine animal tag data, behavioral observations and simulations

The analyses are based on data from 30 white-backed vultures equipped with tags in Namibia over a period of 13 months. The tags recorded position data (GPS) and movement data via the so-called ACC sensor every minute. ACC data is acceleration data in three spatial dimensions and allows very precise conclusions to be drawn about the body movements of the animals at a particular position.

"With the analysis methods developed by the GAIA initiative, which are based on artificial intelligence, it was possible to reliably deduce both the behavior of the vultures (e.g. feeding) and the position of carcasses in the landscape," says GAIA project head Jörg Melzheimer, scientist at the Leibniz-IZW.

Using these carcass locations, vulture GPS trajectories, and information from previous vulture studies, the scientists simulated the vultures' foraging behavior in so-called agent-based models. Among other things, the models took into account the size of the foraging area of the Etosha population of white-backed vultures, the daily activity patterns, flight movements of the animals while searching for food and different scenarios of social foraging behavior.

Carcasses and vultures were randomly distributed in the landscape at the beginning of a simulation and randomly classified as poisoned/not poisoned or as tagged/not tagged in pre-defined shares. The scientists ran the models a total of 360 times with different parameterizations and then analyzed the results.

Social foraging strategies make vultures more susceptible to mass poisoning

The level of social behavior of vultures during foraging has a strong effect on the individual risk of poisoning. This was proven by three different models for three foraging strategies: non-social foraging, where every vulture relies on spotting carcasses in the landscape by itself; the "local enhancement" strategy, where vultures are attracted by directly observing conspecifics that are feeding in addition to detecting carcasses on their own; and the "chain of vultures" strategy, where vultures also sequentially follow other vultures in the sky that are potentially on their way to a carcass site.

In a previous study, the GAIA team demonstrated that social foraging strategies have more advantages than disadvantages for vultures—but in terms of poisoning risk, co-operation and interaction are a disadvantage, as the current study details. Both social strategies resulted in almost all vultures feeding on a carcass after one day, compared to only around 60% of vultures in the non-social model. This resulted in significantly higher proportions of poisoned vultures, regardless of the proportion of poisoned carcasses in the simulation.

"In recent decades, the populations of many vulture species have declined sharply and are now acutely threatened with extinction," says Ortwin Aschenborn, GAIA project head at the Leibniz-IZW. "The main causes are the loss of habitat and food in landscapes shaped by humans as well as a high number of direct or indirect incidences of poisoning.

"The population of the white-backed vulture, for example, declined by around 90% in just three generations—equivalent to an average decline of 4% per year."

The conservation status of the African white-backed vulture was reassessed from "least concern" to "near threatened" in the 2007 IUCN Red List. Only five years later, the species was further "upgraded" to "endangered," and in October 2015, its status was changed to "critically endangered" as the actual, continuing decline was more severe than previously expected.