Inbreeding -- the breeding of related birds is the worst of bad ideas! This includes breeding sibling pairs, parent-child pairs, half-sibling pairs, Uncle-neice or aunt-nephew pairs, or first cousin pairs. Inbreeding leads to many genetic problems including deformities, stunting, lack of resistance to illness, and others. The problem of baldness in lutinos is the result of inbreeding in the 1950's and still hasn't been eradicated after 40 some years of effort by responsible breeders.
If you feel that you cannot bear to destroy this nest unhatched and the babies survive, count your blessings and sell them with the clear understanding that these are inbred birds who should never be used in a breeding situation. Then immediately separate the parents so that they cannot breed again. It is true that experienced breeders will use a limited amount of close line breeding to fix new mutations and especially desirable traits but this is done according to a careful plan which includes immediate out crossing to maintain genetic health and with the willingness to cull unhealthy chicks and redirect the program when problems occur. This should be left to experts who know exactly what they are doing.
If these babies survive and appear OK you may wonder why I'm making such a big deal of the possible problems. However, even if these babies appear OK there has been damage done in the specie's genetic diversity. We cannot import new stock from Australia and are dealing with a captive and closed breeding population. Once diversity is lost it is lost permanently and cannot be restored. By breeding our birds we take responsibility for the future of the species as a whole and we must realize that our actions today affect what our cockatiel population will be like not only in the next few years but in the next century. Inbreeding leads to weakness.
I have mentioned this before when this topic came up a short while ago but I will repeat myself for the benefit of those who were not reading the NCS Tiel Talk message board at that time. I learned the lesson on inbreeding the hard way. As a child I had guppies. I kept them well and being guppies they bred. Since I was a child with a child's ignorance I did not know that I should take any measures to prevent inbreeding and ensure diversity. Within a year I noticed that I had mutations -- one a very pretty swordtail, but the others not so nice. My guppies were undersized, eventually only half the size of the ones in the pet store. They were lacking in color. There were a number of them with a dreadful spinal deformation. Even then I couldn't bear the idea of euthanising these fish so I left them in the tank and they too bred. What I could not see was a steady weakening of my guppies resistance to disease. I kept mixed species tank and the day came when every one of my guppies died -- of a disease that did not even show symptoms in the healthy swordtails, catfish, algae eaters, tetras and mollies that were not the result of my ignorance.
The generation time in cockatiels is longer than that of guppies but over the generations of inbreeding the same thing will occur. The birds will be weakened, genetic diversity will be lost, and tragedy will occur. The only way to prevent it is to stop it before it starts. If you don't believe that my fish example is relevant ask a dog breeder about the health problems of purebred dogs from the infamous "puppy mills". "Just this once" is once too many times where inbreeding is involved.
