Double Egg Shell ?
This picture shows what can happen when one egg hatches & the parents have not removed it. This is rare, but it can occur. The egg (left side) is inside the hatched egg shell (right side). In this scenario an infertile egg is slightly smaller; allowing the hatched egg shell to slip over one end. This in no way caused the infertility of the egg. Many times the hatched egg shell is not removed right away by the parents & their moving & re-adjusting of the other egg & baby causes the larger end of the hatched egg shell to slip over one end of the other egg in the nest. If the other egg was fertile the baby could have trouble trying to hatch; would not have the strength to pip thru both egg shells (if second egg shell was covering the hatching end).
Now, do not get over zealous & run & help remove the hatched egg shells from under your birds. In over 30 years I have seen this occur 3 times & all the other eggs in the clutch were infertile. Constantly interfering or checking can also cause the parents to set tighter & possibly when they see you feel it is not a good time to remove any hatched egg shells & with their setting tighter can cause the shell to slip over the other egg. By keeping records you can do your checking & if you happen to see this & the other egg is fertile just remove the egg shell & replace the fertile egg back under the setting parent.
Note: many times leaving the infertile egg or hatched egg shell in the nest with a single youngster is ideal. it keeps the parents from squishing the newly hatched young (for any number of reasons) & also the youngster has something to rest it head upon.
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