The reason for the bare cheeks of all the macaws is often discussed and the answers have never been satisfactory. Often they are cited as a means of signalling between members of the same species as well as those of other macaw species. However, I find it improbable that such a physical development in a few parrot species should be just for communication purposes.

A rational explanation for the bare cheeks could be regulation of body temperature. As birds capable of flight cannot store large quantities of water in their bodies to perspire as a means of lowering body temperature when it overheats, this must be achieved in other ways. To assist in this birds have anyway a higher body temperature than mammals (at rest 40 ± 1°C). This requires greater energy consumption to maintain, but provides advantages when the ambient temperature is high. If this exceeds body temperature, then the bird must reduce its own temperature. This can occur through evaporation of water in the body, of which the bird has very little. However the ambient temperature seldom exceeds body temperature even in the tropics and therefore the bird is not frequently required to reduce its water content by evaporation.

However, good insulation is necessary to maintain a high body temperature at rest. Birds have this in their plumage. On the other hand excess heat must be dissipated in cases of physical exertion. Bare body parts such as the bill and feet will assist in this greatly. These parts will have a relatively good blood supply, thereby allowing heat to be dissipated. Large birds have more difficulty than small birds in achieving this as their ratio of volume to surface area is not so favourable. It is therefore necessary for a much larger bird to have some other means of dissipating excess body heat. Bare facial skin areas with a good blood supply are especially suitable for this purpose. If the bird has in addition dark plumage as most macaws have, then thermal regulation is especially important. Dark feathers reflect less light (and heat) and body temperature rises more quickly. A similar development is seen in the Palm Cockatoo (Probosciger aterrimus), which also has dark plumage and bare cheeks with good blood supply.

These bare heat-dissipating areas have to be protected in order not to lose body heat unnecessarily when at rest. Parrots and incidentally most other birds have solved this problem by burying their feet and bill in their plumage and thereby insulating them. The burying of the head in the plumage when roosting is for thermal regulation and not to hide a dangerous weapon (the bill) when in close contact with others as Lantermann (1990) claims. Cockatoos also have special feathers, which can be raised to cover the lower mandible.

Bare cheeks need not just be for thermal regulation, since in the course of time they could have acquired additional functions such as signalling other individuals. This has, however, not been researched much and appears to be only possible by undertaking long tedious study. I am preparing some experiments with Hyacinthine Macaws, which I shall report on eventually if successful. I should be grateful for any information others may have on this topic, even if it seems trivial.

Why do the members of the Anodorhynchus group have such conspicuous yellow cheeks? This group includes the well-known Hyacinthine Macaw (Anodorhynchus hyacinthinus), the lesser known Lear's Macaw (Anodorhynchus leari ) and the probably extinct Glaucous Macaw (Anodorhynchus glaucus). These three macaws differ in appearance and behaviour greatly from the remaining macaws and have probably undergone separate development long ago. As they belong to the group of the large macaws and have dark plumage, the problem of thermal regulation is particularly pertinent to them. On the other hand the yellow cheeks and also eye surround lend themselves to use for signalling as they are so conspicuous. I also believe the yellow tongue stripe to be of even greater importance for communication. It is easily seen when the bill is open and would be presented for threatening others. Incidentally not all individuals have a brightly coloured tongue stripe. I have been informed of a female, which has a tongue stripe, that is almost invisible even though she has bred successfully. Possibly the tongue stripe plays an important role in communication within a group of Hyacinthine Macaws. This may be confirmed by the fact that Hyacinthine Macaw young only develop a yellow tongue at a far later stage than the yellow cheeks. Another explanation for the existence of the yellow tongue stripe has not yet occurred to me.

But is the yellow colouring of the cheeks and periophthalmic ring really only for signalling purposes? There are reasons for me to doubt this. Low (1987), for example, describes a Hyacinthine Macaw, which plucked itself and the resulting bare skin area became yellow through exposure to the sun. I have observed something similar in another Hyacinthine Macaw. This macaw was plucked on the head and the bare skin turned yellow. It was not so intense as the cheeks and the periophthalmic ring, but it was clearly visible. It is possible that this effect is a form of protection against the sun as in the case of human skin. This would mean it only happens to macaws exposed to the sun. I know of no Hyacinthine Macaws, which are kept exclusively indoors. I have noticed that the yellow colouring is more intense in summer, but as my macaws breed in summer, this could be due to breeding condition.

Yellow skin areas becoming paler cannot be due to breeding condition as Robiller & Trogisch (1992) allege. Also Lantermann's theory that females with pale cheeks in the nest hole are less visible to predators cannot possibly be valid since according to my observations females with pale cheeks are just as if not more visible in a nestbox than those with deeper yellow colouring. In the meantime it is generally accepted that the cheeks of sick macaws rapidly become paler. I could not say here whether this is comparable to the paleness of humans when ill. The cheeks of my females are always more intense than the males, even during the breeding period.

Apart from the Hyacinthine Macaws a colouring of the bare skin has also been described in the Greater Vasa Parrot (Coracopsis vasa. Here the female loses the feathers to the head before breeding and the resulting bare skin becomes yellowish. I do not know if this happens to birds kept indoors and therefore not exposed to sunlight. It is also not clear whether this colouring is at all comparable to that of the Hyacinthine Macaw. It is also open to question why the skin of most other parrots is not yellow.

In future it will be necessary to survey a larger number of birds to obtain exact data on this subject. Therefore I should like to urge all keepers of Hyacinthine Macaws and Greater Vasa Parrots to send me their observations, especially of plucked individuals. Do the skin areas always become yellow or only in birds kept outside? My considerations lead me to believe that Hyacinthine macaws kept indoors do not acquire yellow colouring. All information will, of course, be dealt with in strictest confidence.

References

Lantermann, W. (1990): Großpapageien. Franckh-Kosmos Verlag, Stuttgart.
Low, R. (1989): Das Papageienbuch. Verlag E. Ulmer, Stuttgart.
Robiller, F., u. K. Trogisch (1982): Ein Beitrag zum Verhalten des Hyazintharas. Die Voliere 5. 6/82, S. 207-208.