The use of red muds to paint feathers is a rare phenomenon among birds, and so far only described in Egyptian and Bearded vultures. Why these vultures take mud baths is still a mystery. It has been argued that feather painting serves as a protection against bacteria or viruses. However, recent laboratory experiments failed to prove such function. In Bearded Vulture, feather painting has been proposes to serve as a signal of dominance, since females (the dominant sex) are often more reddish than males, especially in polyandrous trios. However, using actual data on social ranks, there is no indication for such function in Egyptian Vultures.
Given the overall lack of field-data, a poor understanding still exist of many aspects of this behaviour. In recent years, we succeeded in collecting quantitative data on mud bathing behaviour in Canarian Egyptian vultures. By creating natural looking mud pools at the central feeding station on Fuerteventura , and by using camera-traps, we recorded over 600 mud bathing events during different periods of the year (May/June, August, October, November, February) .
It is now clear that mud bathing is a very common aspect of the behaviour of Egyptian vultures. However, considerable variation exist among individuals in the frequency and intensity of mud bathing. Some have little interest in mud bathing, others only paint their head feathers and some take very extensive baths. Interestingly, before taking mud baths, they often first scratch the mud with their feet, presumably to check certain properties of the mud, such as its stickiness or coloration. This may suggest that bathing is ‘tuned’ to specific needs. After mud bathing, vultures often yawn extensively. It is unclear why they do this. Possibly, mud bathing has a relaxing effects, or alternatively, given the unusual position of the body to paint head feather, this may lead to contraction of abdominal muscles. For more camera-trap photos of bathing vultures click here, or bathing ravens click here.
Observations of collective bathing events (i.e., up to 25 birds taking mud baths in a few hours) in summer suggests that cooling effects and/or protection against UV radiation may play a role. However, frequent mud bathing of up to 1-8 birds daily is also observed later in the year when temperatures are lower or during rainy periods, indicating they paint feathers also for other purposes. An important next step is to examine individual-specific painting frequencies, and also quantify which individuals visit mud pools but do not paint feather, and link this info factors such as sex, age, and pairing status.
As shown in the picture above, there can be large differences in the degree of painting between individuals. Part of this variation could be simply the result of chance, for example, depending on whether birds find a temporal mud pool (such as illustrated below). However, it may well be that some birds specifically search for such pools, or that some individuals are not interested in painting feathers.
Even in rainy periods, when there are many opportunities to paint feathers (see pictures below), there is considerable variation in the degree of painting. This seems to suggests that painting may not necessarily be constrained by environmental factors.
Maintaining artificial mud pools in summer is a lot of work. In this period, mud is probably a scarce resource.