That certainly seems to be what's happening. It's like her cycle is a bit shorter than the 25 hours it is supposed to be, and the next yolk is dropping too soon and moving through the system a little too early. That would also explain this mornings semi hard shelled egg, after she dropped the soft shell sometime during the night. Seems as if that one spent a little more time in the shell stage, but not enough before this one tonight started moving through.
I really want to believe it is not IB, especially since she isn't exhibiting any other symptoms, nor are any of the other birds.
As the article stated there can be other causes of wrinkled eggs, such as a defective shell gland in the oviduct, stress, and I have seen problems with shells after an injury. Usually if only one chicken in a flock is exhibiting a problem, it is something going on within that particular chicken. Many chickens that I have seen over the years that have had problems, they soon will stop laying all together. They may carry on as normal chickens for years, jut not laying. Some may have an acute illness and pass away from a reproductive or crop problem. It helps to do a necropsy whenever you lose one, just to look for reproductive disorders, such as salpingitis and lash eggs, cancer, ascites, and infection (peritonitis.)
