Infertile Purple Martin Eggs

There are times when purple martins lay infertile eggs and some how the female knows which one out of the clutch is infertile and will push it out of the nest. Some believe that they can somehow "feel" which ones are infertile. Years back a friend had built for me a wooden T-14 house which had removable trays allowing for an easier nest check. I had found that one egg out of a clutch of four had been rolled out of the nest to the front of the tray, so I decided to test this theory. I took the egg and made a slight mark on it with a pencil and placed it back into the clutch. Five days later when I performed another nest check I once again found an egg that had been rolled out of the nest- in fact it was the egg that I had marked. So whether it is by feel or some other means they do know if the egg is infertile. The one in the photo is one such egg that I found today while checking my gourds. BTW, the egg count for today was 62!