According to the present invention, a method and an apparatus are provided for creating an opening in the calcified shell of an incubated bird egg containing an embryo, in the region of the blunt end of the incubated bird egg. Within the region of the blunt end there is an outer membrane and an inner membrane with an air cell located therebetween. The apparatus comprises a holder on which the incubated bird eggs are stored with their pointed ends facing downwards, wherein the embryo adjoins the inner membrane, a first detection device configured to detect the position and geometry of the air cell, and an opening device configured to create an opening in the calcified shell at the blunt end of the incubated bird egg above the taut inner membrane. Due to the open calcified shell, the sex of the embryo may be determined in a reliable way.
