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The impact of supplementary feeding on the food searching strategies and social behaviour in an endangered top scavenger

Periodic Reporting for period 1 - SocForVul (The impact of supplementary feeding on the food searching strategies and social behaviour in anendangered top scavenger)

Reporting period: 2015-09-01 to 2017-08-31

The provision of surplus food at supplementary feeding stations is a worldwide practice to facilitate the recovery of endangered species like vultures, which have experienced dramatic population declines during the past century. Despite considerable research on the effects of supplementary feeding on the viability of populations, there is almost a total lack of knowledge on the effects of these practices at the individual level. The main aim of this project was to investigate the impact of supplementary feeding on food searching behaviour and the social dynamics of the endangered Canary Egyptian Vulture. The species is considered endangered worldwide, but 60 breeding pairs and a total population of about 300 individuals still occur on Fuerteventura (Canary Islands, Spain). Extensive field-work over the past 20 year has resulted in most birds being individually marked with plastic rings (>85%), 45 of them carrying GPS-transmitters. As such, this population is a case unique among vultures in the world allowing complex approaches to individual movement and foraging behaviour in relation to the spatial distribution of trophic resources. Changes in food predictability caused by supplementary feeding were found to lead to sex-specific patterns of resource partitioning within population through effects of social status. However, the presence of predictable feeding stations did not exert a constant influence on individual mobility, implying that, at least on Fuerteventura, the impact on population movement dynamics seems rather minimal. Although these findings contrast the popular claim that supplementary feeding may create a large-scale dependence on surplus food and leads to strong reductions in mobility, the asymmetric use of predictable food provide a cautionary note for the implementation of feeding stations. First, the sex-specific dominance effects on resources use may potentially lead to differential mortality traits among males and females. Secondly, we also found that dominant females bred closer to the central feeding station, indicating that predictable feeding sites may influence settlement patterns within populations. Future analyses should reveal the extent to which the use of surplus food confers any fitness benefits in terms of increased reproductive output. From a conservation point of view, this information may improve current guidelines for this conservation strategy and help sustaining healthy populations of scavengers as well as maintaining important ecological and evolutionary processes.
As part of a recovery plan of the species two feeding stations have been implemented on the island, where up to 150 marked individuals can be observed on a single day. Birds visiting the feeding station are completely at ease, allowing detailed observations on many aspects of their usually highly secret life. While the vultures are essentially attracted to the food, they also use these sites to engage in most of their other daily activities. An extensive data set on social status was collected of birds visiting the feeding station showing females to be the dominant sex, but with a relatively strong linear dominance hierarchy also being present within each sex (i.e. based on 4,593 displacements between 141 individuals during 400 h of observation. Data on social behaviour was combined with a high-resolution GPS data obtained from 45 individuals tracked for 1 year (± 1 million data-points).
Individual vultures were found to be highly repeatable in their time spent at sites where carcasses were supplied at predictable or semi-predictable rates (central feeding station vs. farms). Despite high levels of repeatability found for monthly home range size, and to a lesser extent, flight activity, mobility traits did not correlate with resource preferences at the between-individual level, implying that individuals did not exhibit consistent differences in food searching strategies in relation to their degree of resource specialization. By contrast, resource use and mobility parameters varied plastically within individuals on the short-term, with birds having larger home ranges and low flight activity in months where individuals spent relatively more time at the feeding station, but not at farms. Social dominance appeared an important individual-level predictor or resources preferences. In this system with reversed sexual size dimorphism, the dominant sex (females) preferred carcasses provided at the predictable feeding station, while males preferred carcasses provided at farms. However, within territorial males and females, a reversed effect of social status on resource use was found, showing a positive correlation in females, while a negative correlation was found in males. In territorial females, but not males, dominant birds bred closer to the feeding station, suggesting spatial constraints through central place foraging task to affect resource use patterns in this sex. Year-round analyses of the relationship between social status and mobility revealed several seasonal association that varied according to sex and territorial status, but only during specific parts of the year.
Apart from analysing individual space use patterns and foraging behaviours, more general aspects of the behavioural ecology of vultures were described. Basic information on sociality within vulture societies is still lacking for most, if not all species, hampering the full evaluation of the social consequences of vultures declines. Taking advantage of the unique properties of the study population, novel socialising behaviours were discovered and the causes and consequences of polygyny and polyandry analysed. Lastly, while performing fieldwork, it was noticed that individuals often display remarkable variation in reddish coloration on parts of their neck and head. By providing red mud at the feeding station, it appeared that this reddish colouration was the result of birds deliberately staining their feathers with red soil. Videos of this behaviour were the first ever recorded and reached the worldwide media.
This study represents one of the first attempts to integrate individual resource preferences into the spatial ecology of species. This approach allowed a detailed analyses of the influence of resource preferences on population movement dynamics, while simultaneously studying the social and behavioural mechanism which individuals may adopt to cope with spatiotemporal variation in resource predictability. Apart from providing novel insight into the ecological conditions promoting individual divergence in resource use, and the extent to which individual resource preferences may influence population dynamics, results obtained from this study provide novel insights into impact of supplementary feeding on the foraging dynamics of local vulture populations, thus, helping to further fine-tune this widely used conservation tool. In addition, by studying their socialising behaviours at feeding stations, flexible mating system and mud bathing behaviour, novel insights were gained about how these birds live and their adaptations, which may help to better understand current vulture declines. Lastly, by describing the remarkable behaviours of Egyptian vultures, this allowed to draw attention from the general public to the critical position of this highly threatened bird and the work carried out so far on Fuerteventura to save the bird from extinction.
Egyptian vultures fighting over food at the central feeding station on Fuerteventura