THIRD REVISED EDITION
PARROTS OF THE WORLD

Pages 23 – 28

PHYSICAL ATTRIBUTES OF PARROTS

Some conspicuous external features of parrots have already been mentioned, but in this section these are less obvious characteristics will be described in more detail. Figures 3, 4, and 5 are an integral part of this section and are intended to replace wordy definitions and descriptions of technical terms such as nape, lower mandible, metatarsus, crop, etc. They are also designed as explanatory figures for plumage descriptions and anatomical features referred to in the main text.

PLUMAGE
The plumage is the most important element in the external appearance of a bird. It plays a major role in intra-specific recognition, and parts are often specially adapted for use in display. The unit of plumage is the feather and there are about six types, of which the main two are contour feathers and down feathers. Contour feathers constitute the ordinarily visible plumage and include flight feathers, tail feathers, ear-coverts, tail-coverts and feathers on other parts of the body. Down feathers form an undercoat in most birds and are generally not visible. In parrots and some other birds there is another peculiar type of feather called a powder-down. A powder-down feather is a modified down feather which grows throughout the life of the bird, the barbs continually disintegrating into a fine powder; this powder is used by the birds for cleaning the feathers and gives the plumage a characteristic bloom.

In most birds contour feathers grow only from definite tracts of skin called pterylae, while the intervening areas, or apteria, are bare or with down but are covered by the overlapping of the contour feathers. Except in the ‘ratite’ birds, the penguins, and the toucans, there is a definite distribution of feather tracts and the study of the different patterns is known as pterylography or pterylosis. Pterylography is sometimes useful as a taxonomic character.

The plumage of parrots is sparse. This means that the pterylae are sparsely distributed and the apteria very prominent. Down feathers grow profusely from all parts of the skin, except on the neck where in many species the apteria are bare.

COLOUR
The brilliant colours of many species of parrots demonstrate dramatically the processes of plumage colouration. In birds there are two types of colours: (i) structural colours and (ii) pigmentary colours. The structures responsible for structural colours are present in the barbs and barbules of the feathers. The structures responsible for structural colours are present in the barbs and barbules of the feathers. Dyck (1971) examined feathers from the Peach-faced Lovebird Agapornis roseicollis and came to the conclusion that in that species, and probably most other birds, blue and blue-green colours are due principally to back-scattering of light from the numerous hollow, randomly oriented keratin cylinders which make up the spongy structure of the barbs. The range of colours which may be produced by the spongy structure probably is not limited to blue and bluish-green; it is possible that other barb colours are produced by varying the dimensions of the spongy structure. He further points out that green barbs differ from the blue barbs in having a yellow-pigmented cortex and a denser spongy structure with wider keratin rods and correspondingly narrower air-filled channels. Vevers (1964) says that iridescent colours result from barbules being flattened for parts of their length and twisted at right angles. Pigmentary colours, as the name suggests, are due to pigments, of which the commonest is called melanin. The exact chemical composition of melanin is not known, but despite the name it is not always black and may be brown, red-brown, or even yellow. Other pigments include turacin and the carotenoids.

In parrots there are two types of pigments of unknown composition. In one type the pigment is pale yellow in visible light and fluorescent yellow-gold, sulphur-yellow or green in ultra-violet light. Volker (1937) points out that these pigments are not present in the Loriinae or in the genus Eclectus, have been found in two South American species (Bolborhynchus lineola and Pionites leucogaster), but are common in Australasian parrots and cockatoos; crest feathers of some Cacatua spp. Are coloured by one of these pigments. The second type consists of non-fluorescent red or yellow pigments found in some parrots, including the Budgerigar Melopsittacus undulates and the Cuban Amazon Amazona leucocephala.

Many colours are due to a combination of two or more pigmentary colours or to a combination of pigmentary and structural colours. For example, purple on the head of the Plum-headed Parakeet Psittacula cyanocephala is the result of the barbules of the feathers containing red pigment and the structure of the barbs producing blue (Vevers, 1964) Dyck notes that dark green back feathers of the Peach-faced Lovebird Agapornis roseicollis reflect approximately half as much light throughout the visible spectrum as do the paler green abdominal feathers, the difference being due to variations in yellow and black pigmentation of the barbules.

BILL AND TOUNGUE
The bill of a parrot is characteristic and comprises a down-curved upper mandible fitting neatly over a broad, up-curved lower mandible. The upper mandible is attached to the skull by a ‘hinge-like’ arrangement thus allowing extensive movement of both mandibles Kinesis, that is movement of the upper mandible in relation to the skull, is present in most birds, but is especially marked in the parrots and the resulting increase in leverage enables parrots to crush the seeds and nuts that constitute the diet of so many species. The are some minor modifications in bill shape for different feeding habits, but in all species the basis structure is identical. For example, the upper mandible of the Slender-billed Corrella Cacatua tenuirostris is elongated and less curved and is used to dig up roots and corms; the upper mandibles of the spurius are also elongated and less curved and seem to be identical for extracting certain seeds, while those species which feed extensively on pollen and nectar have narrow, protruding bills (e.g. the Loriinae and Loriculus spp., Touit spp. and Brotogeris spp.).

Parrots have a thick, fleshy tongue, generally with a thick and horny epithelium towards the tip. In the Loriinae and Lathamus discolour it is tipped with ‘brush-like’ papillae used for gathering pollen (see fig. 7).

DIGESTIVE TRACT
The digestive tract of a bird is basically the same as that common to all vertebrates, and consists essentially of a coiled tube or gut leading from the mouth to the anus. In common with other grain-eating birds parrots have well-developed crops and gizzards, and these deserve special mention here. The crop is a thin walled, distensible elaboration of the oesophagus, where food is stored for subsequent digestion of feeding of the young by regurgitation. The proventriculus and gizzard (ventriculus) together correspond to the stomach in mammals. The gizzard is weak and not muscular in the Loriinae, particularly in species that feed almost exclusively on pollen and nectar, but in other parrots it is highly developed with thick walls and massive muscles. Digestive juices are secreted by the glandular walls of the proventriculus.

A protein secretion similar to that originating from cells lining the crop in a pigeon and known as ‘pigeon milk’ has recently been described from the Budgerigar Melopsittacus undulates, though in that species it may be proventricular in origin.