Is the bill of the Hyacinthine Macaw too big?

by H.-J. KÜNNE. Published in Gefiederte Welt, Issue 10/96 S. 336-7.

I should say right away that I am about to discuss a matter that's not terribly serious. Evolution would hardly make such a mistake as to overlook an overlarge bill. Yet there are borderline cases, where a body part fulfils one task superbly, but only marginally succeeds in another undertaking. The Hyacinthine Macaw could have in my opinion such a problem with its bill. This bill is wonderfully equipped in dealing with its food supply, but appears to cause problems in feeding its young.

Largest parrot in the world

The Hyacinthine macaw with an overall length of one metre (40 ins) is the largest parrot on this planet and possesses a massive bill. Only the two subspecies of the Palm Cockatoo (Probosciger atterimus goliath and P.a.stenolophus has an upper mandible of similar size. However their bill is considerably narrower and the lower mandible is almost delicate in comparison with the Hyacinthine Macaw, which has a cutting edge on average 30.6 mm wide (1.2 ins).

Now why has the Hyacinthine Macaw such a large bill? As recent studies have shown the Hyacinthine Macaw like his near relatives the Lear's Macaw (Anodorhynchus leari)and the now probably extinct Glaucous Macaw (Anodorhynchus glaucus) appears to be a specialist feeder. The three blue macaws have specialised in feeding on palm nuts. The shells of these nuts are usually so hard that they can only be cracked by human beings with great effort. A normal nut cracker is not sufficient for this task, but a large hammer is required. A normal parrot bill such as that of an Amazon would have not chance in getting at the kernels of these nuts. C. Yamashita and M. de Paula have established that the Hyacinthine and Lear's Macaws open the nuts with great precision. The macaws use the cutting edge of the lower mandible to split the nuts. The other macaws, which have not specialised in feeding on palm nuts, have not developed a technique for opening the palm nuts, which is obvious from food remains. But the Anodorhynchus macaws are apparently really selective and do not eat all palm nuts, but only those of a certain size. The Hyacinthine Macaws prefer the fruits of the Mucuja palm (Acrocomia lasiopatha), the Bocaiuva palm (Acrocomia aculeata) and the Acuri palm (Attalea phalerata).

Specialist feeders are especially at risk

The specialisation on certain palm fruits guarantees the macaws a food source without competition since few other birds are able to crack the nuts open, but the continued existence of this macaw species is also dependent on the plant. At least for the Glaucous Macaw this specialisation appears to have been a dead-end. The disappearance of the yatay palm (Butia yatay) deprived it of its food supply. Unable to adapt to other food sources, it became extinct.

Large bill + small bill = trouble

Coming back to the bill of the Hyacinthine Macaw, it is, as shown above, particularly well adapted for the feeding habits of this species. But the macaws appear to have problems in feeding their young, especially in the first few weeks. I noticed this some years ago in my breeding pair and their first successful attempt at reproducing. During nestbox inspections in the first few weeks I observed that the young were heavily smeared with food remains as was the nesting material around them. At the time I assumed it was because of the female's inexperience in feeding young. However the situation did not improve in the following breeding years. As the problem did not arise with any of the other parrot species in my aviaries, I finally decided my Hyacinthine Macaw female was just unable to master the technique properly.

I was therefore very surprised to learn of similar problems with pairs of other successful breeders. They had also frequently discovered the young heavily soiled with food remains. Unfortunately it is very difficult to observe the behaviour of the macaws in nest hollows in the wild. Only the use of tiny video cameras will enable this and according to Hebel (verbal communication) this behaviour has recently been observed by this means.

In the first few days the female passed the food in a very thin liquid form into the bill of the young using her tongue. After that the young were fed in the usual parrot fashion by regurgitating food into their bills. However the female had problems in doing this as it could not make direct contact with the tiny bill of the young.

She simply regurgitated the food and dropped in the direction of the young's bill. Most of it missed and the adult tried to pass more food with the bill and tongue into the throat of the young macaw. Only a small part of the regurgitated food was actually passed on. The rest was eaten by the female. As the young got larger, the adult succeeded in getting the food into their bills with less waste.

Why do the Hyacinthine Macaws have problems with feeding their young in the first few weeks? The question can actually be answered very quickly when you consider the difference in size between the bills of the adults and the young. The difference is so great that it is difficult to identify the young as Hyacinthine Macaws. The young are very small in comparison with those of other parrot species. The smallest Hyacinthine chick I have bred weighed just 16.2 g and the heaviest only 24.6 g. The young are therefore no bigger than those of the African Grey or Amazon parrots for example. Because of their size Hyacinthine Macaws are therefore just not able to feed their newly hatched young easily.

Specialisation has advantages and disadvantages

However the difficulties in rearing young because of the massive bill will not affect the reproduction success of the Hyacinthine Macaw since they always manage somehow to satisfy their young. If this were not the case, those with smaller bills would have the advantage and their young would survive, resulting in the long term in smaller bills predominating. This is a clear example of where specialisation may be of advantage for certain aspects of life, but disadvantageous in others.

References

Yamashita, C. u. M. de Paula (1993): On the linkage between Anodorhynchus macaws and palm nuts. and the extinction of the Glaucus macaw. BOC-Bulletin,113(1).Pittman. T. (1994): Über die Ernährung der Anodorhynchus-Arten im Freileben. Papageien, 2/94, S. 54-56.

(Website Ed: Neiva Guedes has reported to me that the nests of Hyacinthine macaws and the young in the wild are always very clean. The problems mentioned above may be due to the diet we fed to our macaws in aviculture. Palm nuts, the natural food of the Hyacinthine Macaw, are very oily and the viscosity of this may assist the adults in feeding the young.

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( If you drive out nature with a pitchfork, she will soon find a way back)

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