Foraging Strategies

Fuerteventura, guirres 2014

Egyptian Vulture visiting a tradional goatfarm

Food shortage due to intensification of livestock management is affecting local vulture populations worldwide. Even on Fuerteventura, with its relatively small-scaled goat-farms, a similar  negative trend towards a more industrialized way of farming is currently taking place, with potential negative effects on the availability of food resources for Egyptian vultures. To counter these effects, two feeding stations have been implemented on the island since 1998 and 2008. Although the provision of surplus food at these sites may facilitate access to food, and hence, improve the survival probabilities of vultures, the long-term benefits of these feeding practices nevertheless still remain controversial because negative effects at population and community levels could appear (Cortés-Avizanda et al 2016).

In this project (funded by the EU through a Marie Skłodowska-Curie Fellowship to M. van Overveld), we evaluate the effectiveness of this conservation tool, by studying in detail the impact of feeding stations on individual food searching behaviour and social dynamics of the Egyptian vulture population.


Cabildo feeding

Food provisioning at a feeding station on Fuerteventura

Why studying effects of supplementary feeding

Feeding stations often involve a profound modification of habitat quality by changing both the quantity and the predictability of food resources. Thus, they may have a great impact on the spatial dynamics and social structure of local vulture populations, which are typically heavily relying on cues and information gathered from conspecifics to locate rare and unpredictable food resources (carcasses). Yet, the extent to which these artificial food resources are truly ‘supplemental’ and the degree to which these feeding practices influence the natural foraging habits of individuals still remain poorly studied. Since populations are formed by individuals with variable responses to similar environmental and social pressures (Réale et al. 2007), a deeper understanding of those responses on an individual level are necessary in order to optimize the management of supplementary feeding programmes to enhance the viability of threatened populations.


study_systemStudy System

Both ‘natural’ and human-provided carcasses are available through respectively goat farms and two feeding stations (at approximately 40 km distance), generating a quasi-experimental landscape  consisting of areas differing in food predictability. As shown in Figure 2 , there are three almost ‘permanent’ food resources on Fuerteventura, i.e., two feeding stations and a garbage dump (blue dots, garbage dump in the middle). Apart from this, there are many goat-farms (up to 600, small dots), which function as food resources that are predictable in space but not time. The green line shows the main power line, which birds use for roosting. The 50% Kernel density estimates (all locations of individuals tagged with GPS-loggers, n = 46), shows that space use is centred around predictable food resources.


Data Collection

To quantify individual variation in the use of  different food resources (feeding stations, goatfarms or natural food resources), we make use of the newest advances in bio-logging techniques (GPS/acceleration-loggers, see Movement Ecology). This data is combined with behavioural observations from a hide at feeding stations.

40C_movements

Movements of 3XV, an 11 years old high-ranked female that has a breeding territory near the feeding station in the central part of the island. This female is found at the feeding station on an almost daily basis.

2XX_movements

Movements of 2XX  a 5 years old, unpaired, an low-ranked male. This bird moves throughout the island and occasionally explores the island of Lanzarote. It feeds at goatfarms, (both) feeding stations and the garbage dump.

 

 

 

 

 

 

 

 

Behavioural observations

Observations from the hide are mainly aimed at quantifying which birds visit the feeding stations and how often. These observations have taking place almost each month since 1998 with number of observation days ranging between 1-19 days. During some years, particularly in the beginning of the study period, more detailed observations have been performed to obtain information on the amount of time individuals spent at feeding stations and to determine the social status of these birds. Vultures typically push or kick away birds from food (pork heads/goats), and by noting both the identity of initiators and ‘losers’, it is possible to collect very detailed data on social hierarchies.

visiting rates

Graphs shows data obtained from 19 days of observation (± 200 hours). A clear two-day peak in number of individuals visiting the feeding stations is visible  after ‘fresh’ food supply (5/6, 12/13 and 23/24 February. In total 146 individuals visited the feeding stations (± half the population, 49 ad females, 47 ad males, 17 immature females, 33 immature males). Note that adult females visit the feeding stations more frequently (see also García-Heras et al, 2013). Food provided included 54 Pork heads, 4 goats, 1 sheep, and unknown amount of intestines.

20160224_164243

Rare example of an escalated fight between a juvenile and adult female. In most occasions  conflicts are settled quickly and the social hierarchy is well-respected.

 

 

References

Supplementray feeding and endangerd avian scavengers: benefits, caveats, and controversies (2016) Cortés‐Avizanda A., Blanco G., DeVault T.L., Markandya A., Virani M.Z., Brandt J, Donázar J.A.. Frontiers in Ecology and the Environment 4:191-198

Integrating animal temperament within ecology and evolution (2007) Réale D., Reader S.M., Sol D., McDougall P.T., Dingemanse N.J. Biological reviews 82:291-318

Advertisements