Thanks for reaching out to Ask Lafeber.
Usually, a pair of birds start to incubate their eggs after the second or third one is laid. Since I don’t know where their cage is located, I can only speculate. Sometimes if there’s too much going on in their environment, they will delay or not sit on them at all. If they are in a busy gathering area such as in the kitchen, playroom or living room, they may not feel their nest is secure enough to raise babies. When birds start the nesting process, it’s always best for them to have as much privacy as possible. I always like to tell bird owners to place their cage in a corner of the room and not next to a window or a doorway. They need a split second to decipher who’s entering the room, friend or foe. Also, some of the activities outside can be disrupting them, so a corner is always the best option. If this is not the situation with your birds, please write back with additional details and we can look at other reasons they are not sitting on their eggs.
As for the smaller egg, it’s doubtful it’s viable. My suggestion is to remove it from the others. There’s no reason to risk it getting broken and contaminate the other eggs.
Kind regards,
Debbie