|
Inbreeding, line breeding, back crossing, what-have-you.
Mark Nissen. 30/6/2004. It doesn't matter what you call it, the process of improving livestock through selective breeding has been with us for a long time, many hundreds of years in the case of cattle. Man has, with great success, turned many species of animals, birds and plants into forms that are completely different to the original ancestral stock. In the cage bird fancy, selective breeding is a process that has been used to develop birds to conform to a standard and to create new varieties from sports. This article is a brief but hopefully clear look, at this science or art. And whilst I am no expert on these matters I hope to illustrate the process and encourage it's use. The aim is to improve one's stock, and it does not matter what species we apply the process to, the principles are simple and valid. To begin in the beginning, we may wish to improve our stock in only one aspect, say, large size. Even though the characteristics that make up a bird are many and when combined with the same characteristics of the potential mate, offer a potentially confusing soup of possible outcomes, let's just concentrate on size at first. And for size you can substitute any other single characteristic, like colour or feather quality. If we were to take 100 pairs of birds and just choose the potential mates on the basis of size and size alone, after many generations the overall size of the birds would improve. At the end of the process there would be large birds, but we would still get small birds amongst the young, as there would have been no "weeding" of the genetics of the birds. even though big birds would be more common, the genetics still would exist for the smaller birds. So this is where we should think about selective breeding that concentrates on removing the unwanted characteristics and reinforcing the positive attributes. By simply choosing big birds, we are not refining the genome (a bird's genetic makeup). We could be mating big birds that are genetically half small bird, together and not removing the small bird influence. Inbreeding can solve this problem. |
In diagram 1, you can see that we'd like to produce more birds that are like the cock bird (black) and fewer birds that are like the hen (grey). By mating a young hen of each generation back to the original cock, we can eventually remove the influence of the first hen in the stock. If the black colour represents a large bird and the grey colour represents a small bird, you can see that before too long the small bird genes will be gone, or at least so diluted as to be unimportant. But if we use the black and grey colours to represent other characteristics such as feather quality, you can see how this can be improved as well.
There are factors that complicate the above, simple view, of this process. These can be: sex linked characteristics (where a factor behaves differently depending on the sex of the bird), pre-potency (where one bird's factors seems to have an unexpectedly strong effect on the young), the compounding of 'bad' factors along with the 'good', and mutations and throwbacks (where some long forgotten, or completely new, factor makes an appearance). And not least, the reality that each bird is made up of many factors, (most species of birds have 29 pairs of chromosomes) the combination of which requires real skill to manage in the breeding process in order to achieve a satisfactory result. But if it was really simple it wouldn't be interesting or fun!
Charles Darwin, in the book 'The Origin of Species' wrote, 'we follow pigeons on down to the days of that most skillful breeder Sir John Sebright, who used to say, with respect to pigeons, that - he would produce any given feather in three years, but it would take him six years to produce beak and head'.
And this I think, is what we aim to do, by the processes of selection and by being able to concentrate good factors and remove the bad ones. Inbreeding is essential to accomplish these aims in a realistic time frame.
In the budgerigar fancy, there has been some talk about the gene pool being too small, or that good birds from elsewhere are required to improve the type. With the techniques described above, there should be no issue with breeding the required bird one's self. And if we take Sir John Sebright's words at face value, six years would see the improvement required.
Also from Charles Darwin, in the book 'The Origin of Species' the following passage describes the selection of sheep, very much what we do with our birds today. 'In Saxony the importance of the principle of selection in regard to merino sheep is so fully recognized, that men follow it as a trade: the sheep are placed on a table and are studied, like a picture by a connoisseur; this is done three times at intervals of months, and the sheep are each time marked and classed, so that the very best may ultimately be selected for breeding.'
One of the disadvantages of inbreeding that may become apparent, is that the birds become less fecund and more difficult to breed. This could be because the selection process that the stud is under, has a too narrow focus on the physical aspects of the birds and takes no account of virility. When selecting one's birds, the whole of the bird must be taken into account, factors such as fertility, feather plucking, steadiness and attitude must be judged along with the more obvious attributes of type.
Eventually, for any number of reasons, an out cross will be considered desirable. It is important to take the progeny of this out cross and inbreed them back to the original line, retaining the factors for which the out cross was brought in. Otherwise you may dilute your stock with factors about which you know little and that may be detrimental to your stud.
A further enhancement to the process of inbreeding, is to allow a small number of good birds to dominate the breeding season by taking say, five of the best cocks and letting them mate with two or more hens, indeed four or five hens may be used for each cock and two rounds may be taken from each hen (foster the first and also the second if needed). This does mean that you could have twenty hens, twenty or more foster pairs and forty clutches to keep track of, so accurate records that track lineage and progeny are essential.
Once again, I am no expert in these matters, but I hope that others may benefit from the research I have done and written about above. Happy inbreeding!