Together, or not together, that is the question. Hamlet is not the only one facing life-changing questions – wild animals have to make decisions pivotal to their survival on a daily basis. In a modelling case study, scientists of the GAIA Initiative investigated whether exchange of information among African white-backed vultures (Gyps africanus) bring more advantages than disadvantages to the individual vulture in its search for food. They found that social foraging strategies are overall more beneficial than non-social strategies, but that environmental conditions such as vulture and carcass densities greatly influence which strategy yields the best results.

In a paper published in the journal “Ecological Modelling” the GAIA team led by first author Teja Curk from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) modelled how different foraging strategies perform under different ecological conditions and compared findings with field observations. They calculated, among other variables, the searching efficiency (the time needed to find the carcass and proportion of population that found the carcass), the scavenging efficiency (the proportion of food left in the environment) and the degree of competition for resources (the number of vultures converging on a single carcass). To do this, they built agent-based models for three different 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.

Vultures are social and communicative – and find food more easily this way
The agent-based models were built and verified using data collected from 30 tagged vultures in Etosha National Park in Namibia, from which more than 26 million GPS positions and corresponding ACC bursts (acceleration data) were included in this evaluation. ACC sensors record small movements of the tag in three dimensions and allow fine-scale behaviours to be analysed. The GAIA scientists applied their own machine learning algorithms to classify the GPS and ACC data and identified feeding behaviour of the vultures and subsequently carcass locations in the landscape.

“In our models we compared the three strategies with the empirical data from Namibia and were thus able to reliably calculate parameters such as the searching efficiency”, explains Curk. “We found that both social strategies outperformed the non-social approach in terms of searching efficiency, as individuals located carcasses more quickly compared to the non-social model. The ‘chain of vultures’ is especially beneficial when there are many vultures in an area that can coordinate search efforts by transferring information over great distances”.

Effective ‘word of mouth’ can lead to overcrowded feeding locations
The evaluations also revealed that a chain of vultures comes with a trade-off, since it often results in large congregations of vultures at carcass sites which could reduce individual food intake. In contrast, the local enhancement strategy balanced moderate searching efficiency with reduced competition.

Comparisons of scavenging efficiency showed that with only few vultures in an area, the amount of potential food left in the environment is quite similar in social and non-social strategies. With higher vulture densities, both social strategies surpass the non-social approach to a similar degree. Only at very high vulture densities did the scavenging efficiency of the “chain of vultures” fall short of “local enhancement” – with very long vulture chains concentration on few feeding sites becomes frequent and many individuals feed on only a small number of carcasses, leaving others unoccupied.

Protection for the highly endangered vulture species is urgent and important
The authors conclude that vultures in the study area likely adopt diverse foraging strategies influenced by environmental variables such as vulture and carcass densities. This behavioural flexibility suggests that vultures can optimize their foraging success by adjusting their reliance on social information in response to changing ecological conditions. However, the confirmed benefits of social foraging strategies underline the importance of vulture conservation: when vulture densities fall below a certain threshold, there are not enough vultures to use social information. The searching and scavenging efficiencies drop and a notable amount of food remains undetected, further raising the challenges for vultures to feed, reproduce and, by extension, survive. From a vulture conservation perspective, it is therefore crucial to understand how environmental conditions shape foraging strategies and what are the associated costs and benefits.

In recent decades, the populations of many vulture species have declined sharply and are now acutely threatened with extinction. 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 percent in just three generations – equivalent to an average decline of 4 percent 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.