Sometimes they just occur b/c that's the way nature is. However, if you get a high percentage of defects it could mean your temp or humidity is off. I suppose if you inbred them heavily enough that could also cause defects, but it's unusual in chickens.
The cells just didnt develope right happens all the time in people and other animals. I had one last week that hatch with one eye and crooked beak... She has one good eye but not one on the other side. Waited to see if she would be able to eat as her one eye was sufficient and she is doing fine. I have a handicap pen so she will have a happy life. Its just life I guess as hard as it is.
ewww selling it on ebay seems a little to circus-freakish... wait wasnt i just talking about putting up billboards and building a truck stop.... hmm i think my views and limitations to getting rich quick (21 days quick) are crippling me lol
That is definately a genetic defect, which could be environmental in a sense, but not due to conditions in the egg. More like pesticide or some other toxin.