Two papers have been published recently with results from the monitoring of bearded vultures fitted with satellite tags: Gil et al. analysed the movements of six non-breeding bearded vultures in the Pyrenees, while Krüger et al. analysed movement patterns of 18 birds from 4 age classes in Southern Africa (see below).
Satellite tagging is a powerful method to study bearded vulture’s spatial ecology and behaviour. All individuals released in the current VCF-coordinated reintroduction projects are similarly fitted with satellite tags.
Home range areas reported in the Pyrenees were smaller than those reported in Southern Africa and slightly larger than those previously reported in the Pyrenees, and all included supplementary feeding stations, suggesting that predictable sources of food may be important, especially for inexperienced juvenile birds. The authors recorded also slightly larger home ranges than in the Alps. However, this comparison refers on the data of the Alps which were collected during only seven months after fledging which makes it difficult to draw robust conclusions.
Juvenile bearded vultures are highly mobile during their long juvenile dispersal period, often flying away from the natal mountain ranges. Once birds become adults and settle in a territory, home range seems to decrease abruptly.
In the Spanish study, more than 25% of the locations received were near supplementary feeding stations, which suggest that these are major determinants of bearded vulture ranging behaviour in the Pyrenees. Some of these feeding stations have been operative since the 1980s, and provide large amounts of food with regularity. These feeding stations may be fixing birds in the Pyrenees, and preventing young bearded vultures from leaving to other mountain ranges, thus preventing their natural recolonisation. One of the VCF’s main aims is to promote the establishment of a link between the Pyrenean population and the reintroduced alpine one. We have thus started in 2010 to reintroduce birds in Grands Causses (with our French partners LPO), and are working towards establishing a gene flow between these two populations through the French Massif Central, and would welcome a review of the current food provisioning system across the Pyrenees.
In Southern Africa, since a large number of birds were marked, it was possible to estimate the overall foraging range of the entire population, which was estimated to be 51 767 km2. There was a lot of overlap in the areas used by juvenile, immature and sub-adult birds, and average adult home ranges (286 km2) were only around 1% the size of the average foraging ranges of non-adults (10 540 –25 985 km2), with those of breeding adults being even smaller (95 km2).
Both studies support earlier findings that range size and use in bearded vultures increase with age as the individuals explore increasingly the territory, until they settled for breeding. In Southern Africa immature and sub-adults had larger range sizes during winter, but range of adults did not vary seasonally. There was also no difference on range size and use between the sexes in any of the age classes.
The average home range area of juveniles in South Africa (21 151 km2) was almost 10 times larger than those of juveniles in the Pyrenees (2 225 km2), although sample sizes were considerable different. This however seems to lend support to the suggestion that supplementary feeding station may be fixing populations in the European mountain range.
In both studies the home ranges calculated were quite larger than those estimated for the same populations using radio telemetry, most probably because of the use of improved tracking technology.
The study in Southern Africa is particularly useful to map conservation strategies for this declining species, as it included a very good sample size and covered all age classes, being thus representative and relevant for the entire population. It suggests that ideally a 10 km radius (90% kernel) around each of the 109 breeding territories, or at the very least a 4 km radius (50% kernel) represents the absolute minimum area for protection. The 90% kernel of the combined non-adult range (33 636 km2) marks the area in which resources for the implementation of conservation actions need to be focused to effectively address the needs of this species.
(photo by Hansruedi Weyrich)