It is a lash egg. It is caused by salpingitis or inflammation of the oviduct, a common reproductive disorder. Antibiotics may or may not help. The ones used are amoxicillin (Fish Mox, Aqua Mox,) 125-250 mg given orally every 12 hours for 10 days. Also, Baytril (enrofloxacin) 10% given 0.05 ml per pound (or 0.25 ml for a 5 pound chicken) every 12 hours for 5 days. Baytril has been banned in chickens for 16 years, but is still sometimes prescribed for certain infections by vets. It may be available from jedds.com in 10% liquid. Here are 2 articles to read about it: https://www.thepoultrysite.com/disease-guide/salpingitis
