Adult coloration of the Bearded Vulture (Gypaetus barbatus) in the Pyrenees: relation to sex, mating system and productivity

Abstract

Cosmetic coloration is not a common phenomenon among bird species. Adult Bearded Vultures Gypaetus barbatus typically show orange coloration in head, chest, belly and tarsus feathers that is the result of mud baths in ferruginous substrates. Several non‐mutually exclusive visual signalling hypotheses can be proposed to explain this phenomenon. Coloration could be used to signal: (1) dominance towards conspecifics, with darker birds being dominant (status signalling hypothesis); (2) sex (gender signalling hypothesis) because females are usually more intensely coloured; (3) individual quality for mate choice (quality signalling hypothesis); or (4) different reproductive strategies between individuals (e.g. in polyandrous trios). Here we report results from systematic monitoring of breeding Bearded Vultures in the pre‐laying period in the central and western Pyrenees (Spain and France) between 2016 and 2020, including 162 individuals from 70 reproductive units (48 pairs and 22 trios). We classified individuals as being white, pale, medium or dark, and analysed the relationship with sex, mating system (pair or trios) and average productivity across years. Our results showed that the most common form was medium (67%), then pale (22%), dark (9%) and white (2%) morphs. Overall, females were darker than males, but with a large overlap in coloration. In trios, dominant males were darker than subordinate males, although differences were not statistically significant. The mating system (pair or trio) did not influence female coloration. A multimodel comparison showed that the number of years of territory occupation, mating system and their combination (but not coloration) were the best predictors of average productivity. Our results do not support the gender signalling hypothesis. The reproductive strategy signalling hypothesis might be supported because beta males were, on average, paler than males in pairs, but this topic requires further investigations because our results were inconclusive. Finally, that females (thought to be dominant because of their slightly larger size) were on average darker than males agrees with the status signalling hypothesis, although an experimental approach would be needed to formally test this hypothesis.

New Insights into the Cosmetic Behaviour of Bearded Vultures: Ferruginous Springs Are Shared Sequentially

“…Our observations are the first to document visits by individuals of all age-classes to a given ferruginous spring, constituting a significant part (2.5%) of the entire Pyrenean mountain range population (estimated at 1026 individuals; [26]). The only previous information available in the literature [23] showed that during 1483 days of monitoring a ferruginous spring in the French Pyrenees, bearded vultures were present on 112 occasions (7.55%) and ten individuals were identified (nine adults over six years old and one five-year-old).…”

“…For example, in captivity, the intensity of the ochre coloration generally correlates with the age and sex of the bird, with older individuals and females being more intensely pigmented [13]. In wild individuals, females (the dominant sex) also show more intense coloration than males and, in the case of polyandrous trios, alpha males are more heavily stained than beta males [1,18]. Thus, a more intense use of pigmentation sites by females could explain sexual differences in coloration.…”

“…Low-quality individuals (e.g., subordinate individuals with poorly maintained plumage) would benefit from staining, while high-quality individuals would pay a social (breeding) cost for hiding their immaculate plumage [16]. While signalling individual dominance status (hypothesis 2) has received wide support [1,[17][18][19], the other hypotheses have been rejected through observational and experimental approaches (hypotheses 1 and 3) or remain untested (hypotheses 4 and 5) [15,17,19,20]. However, no advances have been made in understanding this mysterious behaviour during the last two decades, and observational studies are needed to discover its function.…”

“…The information regarding the use of such resources and the exact function of cosmetic behaviour is still tentative due to the extreme rarity of field data [21][22][23] for bearded vultures and [24] for Egyptian vultures. This is in part due to this species secretive behaviour [4,13] and also the difficulty of obtaining high quality data on this uncommon threatened species in their mountainous habitats.…”