Everyone understands the general meaning of health, but it is not easy to define the state precisely. Dictionaries give varying definitions: "a soundness of body" or "a normal condition of body with all parts functioning well", whilst one authority merely states that it is "an absence of disease". The diagnosis of disease in all live animals, but especially in birds, is particularly difficult compared with man. A sick person is able to describe his symptoms to the doctor, but the veterinarian relies entirely upon his powers of observation and those of the patient's owner. When consulting a vet. therefore, it is very important to be able to provide as much information as possible. To give the fullest information entails being fully conversant with the normal behaviour of the species, otherwise it may not be possible to recognize certain signs of sickness or unusual behaviour. Sometimes it can be very difficult to tell the difference between a healthy and a sick animal, particularly in the early stages of an illness.
Veterinarians, however, are specially trained to be aware of the signs of ill-health in domestic animals and much of their knowledge can be applied to other species, including birds. Although your vet may not have personal experience of the species of birds which you keep, he may well, with your help and his specialized knowledge, be able to diagnose the cause of sickness in your bird. The metabolic rate or "rate of living" varies considerably among the different species of birds and determines the normal heart and respiratory rates and the body temperature, as well as physical and mental activity. Therefore, before attempting to keep any species it is essential to become familiar with its normal behaviour, including its temperament and dietary requirements. Generally speaking, healthy birds have a sleek "tight" plumage unless moulting. They are also inquisitive, bright-eyed and hold their wings close to the body. Many birds, for example, draw one leg up beneath the abdomen during sleep and tuck the bill under the wing.
The appearance of the excreta varies with different species and according to the diet. Most birds, especially seedeaters, have relatively soft but formed droppings, with a dark greenish part representing the feces and a white portion being the urate excretion. Fruit-eating birds such as lorikeets, however, normally have wet droppings which must not be confused with diarrhea. The maintenance of health depends on environment, diet and heredity. Environment is an all-embracing term which includes such factors as temperature and humidity of the air, ventilation, draughts, infections, injuries, intensity and nature of light, presence or absence of predators, and circumstances causing fear, excitement and boredom. Diet greatly affects health, although its effects may not always be realized. A bird caged alone for example with a liberal supply of suitable food may fail to thrive because it may become excessively fat. This applies particularly to budgerigars. On the other hand recently captured birds given a well balanced but unvaried diet may waste away and die through failure to eat sufficient food. These dietetic and other nutritional problems, however, are discussed elsewhere.
Heredity can be defined as "the inheritance of qualities or of diseases from the parents and previous generations". It determines such characteristics as the bird's sex, shape, size, color, resistance to disease, muscular build and, to a great extent, its behaviour, food preferences and other activities. Many of these characteristics, however, are also modified by environment. Genetics is the science which deals with the origin of the characteristics of the individual. It is the study of heredity and is a complicated subject which can be dealt with here only very briefly. In fact, relatively little is known about the subject in birds--with the exception of the domestic fowl and to a lesser extent the pigeon, budgerigar and canary. All the cells which make up the body contain chromosomes within their nuclei, the number of which is the same in every cell and constant for each species. All the body chromosomes are paired, and with the exception of the sex chromosomes, are identical. The latter, occurring in the reproductive or germ cells of the sperm and ovum, carry the hereditary characteristics from one generation to the next; to explain the inheritance of sex it is customary to label them with capital letters.
Hen birds have a pair of sex chromosomes, one being an X and the other a Y chromosome in each ovum or egg, whereas cocks have a pair consisting of two X chromosomes for each sperm, this being the opposite to the situation in mammals. It is important to note that prior to fertilization the number of chromosomes in each germ cell is halved, one of each pair separating, otherwise the resultant zygote formed by the fusion of a sperm and an ovum would contain twice the number of chromosomes for that particular species. When fertilization occurs and a sperm unites with an ovum in the upper part of the oviduct, the chromosomes of each parent separate so that one X chromosome from the cock unites with one X from the hen, to produce XX (a male), or alternatively one X from the cock unites with one Y from the hen to produce XY which is a female. It can be seen, therefore, that in theory the chances of equal numbers of both sexes being produced in a brood are equal. In practice, however, this seldom occurs. The same laws of chance apply as when a coin is tossed in the air an equal number of times. Theoretically, the results should be 50 per cent "heads" and 50 per cent "tails", but this does not always happen in the short run, the chances of it doing so increasing the greater the number of tosses. In addition to statistical reasons, however, there are many others why equal numbers of both sexes are not produced.
The chromosomes, as stated previously, carry all the characteristics of the individual. Specific characters or "genes", of which there are many, are normally located in the same position on a chromosome. Geneticists have been able to produce chromosome maps for some species showing the sites of genes which represent many different physical characters such as color of eyes, color of body, etc. Sometimes a whole group of genes or genetic units is involved in producing a single character such as color. This is the case when it is not a straightforward specific color such as the yellow of a budgerigar, but based on a variety of factors, as in the plumage coloration of a peacock. Sex chromosomes may also carry genes other than those for sex and if these genes (for certain colors for example) are confined to one type of sex chromosome, then the characters are said to be sex-linked; examples in budgerigars being albinos and the colors known as opaline and lutinos. These factors are always linked to the X and never to the Y chromosomes. When factors such as color are not sex-linked, they are inherited according to the Law of Mendel. The dominant factor is always designated by using a capital letter, e.g. GG represents a pure green budgerigar of any of the three shades. Gy is a green bird which is the result of crossing green and yellow, the latter factor being recessive. If GG is crossed with yy (yellow) the offspring will all be Gy. This is called by geneticists the first filial or F1 generation. The birds will look green, but they will have inherited the yellow factor. If a Gy is mated with a Gy, then according to the Law of Mendel, 25 per cent will be pure green, 50 per cent green inheriting yellow, and 25 per cent pure yellow. This second generation is called the F2 and it is impossible from the outward appearance to tell which of the green birds (representing 75 per cent) are pure green and which carry yellow genes. It is easy to see that working out the inheritance of characters in the next and subsequent generations becomes increasingly complex.
Parents (Pure Green GG x Yellow Yy)
F1, 1st filial generation 100% Green
|Gy Green||Gy Green|
|Gy Green||Gy Green|
Parents (Green Gy x Green Gy)
F2, 2nd filial generation 75% Green, 25% Yellow
|GG Green||Gy Green|
|Gy Green||yy Yellow|
In the Fa generation for example, there are theoretically 16 possibilities, by mating GG with Gy, GG with yy, Gy with Gy and Gy with yy. When a germ cell contains genes that produce like characters, e.g. GG or yy as illustrated above, it is said to be homozygous, whereas if it contains different characters, e.g. Gy, it is stated to be heterozygous. In order to reduce the number of individuals that are heterozygous for any one pair of genes and to increase the number that are homozygous for one or other member of the gene pair, many breeders practice a system known as in-breeding. This is the mating together of individuals which are related to each other through having one or more ancestors in common. It ultimately produces uniformity of genetic constitution; but because undesirable as well as desirable characters can become concentrated in the stock, the method requires a great deal of skill in order to eliminate the undesirable characters. Inheritance of specific characters is not always as straightforward and predictable as it may seem. Sometimes for example, a derangement of the genes occurs at the moment of fertilization when sperm and ovum unite, due to the chromosomes crossing over each other, instead of lying side by side. Any pair of chromosomes may be involved in this way, thus giving rise to unexpected characters in the resultant offspring. Occasionally the chemical nature of a gene becomes altered, resulting in a permanent change of some specific character. The process is known as mutation and the resultant individual is called a sport or mutant. Mutations, however, unlike crossing-over of chromosomes, occur very infrequently. Mutants transmit the altered character to their offspring and are usually recessive. Mutations are seldom beneficial to the species and are in fact usually harmful, sometimes even resulting in the death of the individual, in which case of course the mutation is not perpetuated. Mutations have been responsible for the development of some unusual breeds or colors of canaries and budgerigars. A well known example is the lutino budgerigar which has a pure yellow plumage and pink eyes. When it is remembered what a vast number of characters are inherited in addition to color (chosen here for simplicity), it will be appreciated that the study of genetics is extremely complicated and beyond the scope of these articles.
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Hamilton & District Budgerigar Society Inc.