Scavengers
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Increase abundance and diversity of avian scavengers at food sources
Data collection in progress
Greater Côa Valley
Portugal
Problem: Low numbers of wild animals in many parts of Europe mean that livestock carcasses have become a critical food source for scavengers such as vultures. However, in recent decades, regulations have banned the leaving of dead livestock in fields, drastically reducing food availability for vultures. A legal change in Portugal in 2019 permitted farmers to leave carcasses of small ruminants and equines in the field again. However, most Portuguese livestock breeders in the Greater Côa Valley are not aware of this change. Moreover, the licensing process is highly complex, bureaucratic and time-consuming.
Action: Rewilding Portugal are acting as facilitators in the process of applying for carcass deposition licences, by helping farmers write proposals and submit them to the relevant authorities. Rewilding Portugal is also supporting livestock breeders to implement carcass deposition.
Indicator: The abundance and diversity of avian scavengers at feeding points shows which species are feeding on carcasses and how different vulture species respond to the introduction of free livestock carcasses into the landscape.
Griffon vultures feeding at a carcass. Photo: Staffan Widstrand/Wild Wonders of Europe.
An Egyptian vulture. Photo: Rewilding Portugal.
A camera trap mounted on a tree. Photo: Rewilding Portugal.
Methods
In 2019, no free carcass deposition licences had yet been issued. Therefore, baseline data were collected from three vulture feeding stations in the area of the Greater Côa Valley (Table 6). Two of the feeding stations (Escalhão and Almofala) were active from March to September 2019 and December 2019 to mid-February 2020, with food provided once or twice each week. A third feeding station, Cumiera, was active from May 2019 to February 2020. A total of 40 feeding sessions were observed in Almofala, 49 in Escalhão, and 19 in Cumiera. 2261 kg carrion was provided in Almofala, 3564 kg in Escalhão, and 1241 kg in Cumiera.
Camera trapping was used to monitor scavengers visiting the feeding stations. The number of kilograms of carrion provided and the species richness and minimum abundance of avian scavengers were recorded during observed feeding sessions. The minimum number of vultures of each species was calculated as the maximum number of individuals caught in the same photograph.
These surveys will be repeated in future years to assess changes in vulture feeding behaviour, once more carcasses are being left in fields.
Results
Griffon vultures (Gyps fulvus) were the most frequent visitor at all three feeding stations (Fig. 1). Egyptian vulture (Neophron percnopterus) and Cinereous vulture (Aegypius monachus) also used all three feeding stations.
Figure 1: % of observed feeding sessions at which the three most common vulture species were recorded.
The minimum abundance of each species was variable between feeding stations and feeding sessions (Fig. 2). Griffon vultures were the most abundant species at all three stations. The number of Egyptian vultures was slightly higher than the number of Cinereous vultures observed at each site, and more birds were observed at Escalhão than at the other feeding stations.
Figure 2: Minimum abundance of vulture species at each site, based on the maximum number of vultures observed at the same time.
Total species richness observed at the sites was 10 species at Escalhão and Cumiera, and 7 species at Almofala.
Interpretation
This baseline describes the species and abundance of vultures seen at feeding stations in the Greater Côa Valley, where carcasses are provided relatively regularly in the same place each week. The number of griffon vultures seen at feeding stations is similar to those reported in other studies where the temporal provision of carrion was unpredictable (eg., Arrondo et al., 2015) and much lower than the high numbers observed in some intensive feeding stations (Cortés-Avizanda et al., 2012).
The indicator will be revisited in future years, once livestock carcasses are being left in the field more widely, to see how different species respond. In particular, cinerous vultures are expected to benefit, as they can be deterred from feeding stations by the presence of large numbers of griffon vulture, a more aggressive species.
Publications
Legalised carcass deposition promises brighter future for Greater Côa Valley vultures
Citations
Arrondo, E., Cortés-Avizanda, A. and Donázar, J.A. 2015. Temporally unpredictable supplementary feeding may benefit endangered scavengers. Ibis, 157, pp. 648-651.
Cortés Avizanda, A., Jovani, A., Carrete, M. and Donázar, J.A. 2012. Resource unpredictability promotes species diversity and coexistence in an avian scavenger guild: a field experiment. Ecology, 93, pp. 2570-2579.
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