Each egg weighs about the same as 10% of the female’s body weight; a clutch of eight eggs is an enormous drain on her body. She eats the eggshells in an effort to recover some of the calcium and other essential minerals that she put into making the eggshell in the first place.
Southwick Country Park Nature Reserve 
Why does the mother bird eat the eggshells. Is she just clearing up the nest?
Each egg weighs about the same as 10% of the female’s body weight; a clutch of eight eggs is an enormous drain on her body. She eats the eggshells in an effort to recover some of the calcium and other essential minerals that she put into making the eggshell in the first place.