Aims: Aggressive interactions between the Bearded Vulture Gypaetus barbatus and the Common Raven Corvus corax are frequent in the Pyrenean nesting sectors shared by both species. The Bearded Vulture´s nesting sectors are vulnerable to kleptoparasitism (food is stored in a visible and predictable manner in nests, perching sites and ossuaries), and the Raven´s parasitic-predatory abilities are well known. How both species interact was examined by studying their behaviour during the nestling period and analysing the factors that affect this behaviour.
Location: Central Pyrenees (Catalonia, NE Spain).
Methods: The aggressive interspecific encounters in 10 nesting sites (area = 3750 km2) was quantified. The observations were carried out from locations where the nests, perching sites and ossuaries could be viewed simultaneously. The data were compared using non-parametric statistical tests.
Results: Most of the attacks on the Ravens occurred from nests and the highest percentage of defensive behaviour in the Bearded Vultures was observed when the chicks were only a few days old. The Ravens preferred to attack when the Bearded Vultures were near the nests, whether the latter were carrying food or not. The Ravens were only relatively effective in their kleptoparasitic attempts when they attacked in a group at times when the Bearded Vultures were manipulating the food in open areas.
Conclusions: The difference in size between both species, and the type of food affected the Ravens´parasitic efficiency. For the Bearded Vultures, the negative effects of coexisting with the Ravens are associated with the energetic costs derived from nest defence and the disturbance generated by the Ravens´kleptoparasitic attempts. Nevertheless, the Bearded Vultures´ defensive behaviour suggests that risks of predation exist, mainly during the initial stages of the breeding period, which is when the chicks are likely to be more vulnerable.
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