A method for treating a fertilized egg of a bird, a method for increasing the body weight of a chick at hatch, and a method for increasing the hatchability of a fertilized egg of a bird, comprising injecting, into the fertilized egg after start of incubation, a solution containing amino acids which determine the nutritive value of proteins in the fertilized egg; a method for hatching a fertilized egg of a bird, comprising hatching the fertilized egg after start of incubation treated according to the treatment method; a fertilized egg of a bird after start of incubation, the egg being treated according to the treatment method; and a bird hatched from the fertilized egg.