You guys are right. I was thinking mammals. Sorry about that! That you for correcting me. The ZZ:ZW system is correct for birds.I don't agree with the above in mamals yes, in chickens no.
In chickens gender is determined not by X and Y chromosones. But rather by ZZ:ZW chromosone system. That of which is highly affected by the estrogen. The chicken embryo resembles lower vertebrates in that estrogens play a central role in gonadal sex differentiation. Not to say she can choose but she does infact influence her offspring. In case your wondering ZZ is the boy and ZW the girl.
Sorry!