Question

How is the sex-determination mechanism different in the birds?

Answer 1

Sex determination in birds is opposite of sex determination in mammals and it is determined by the female within in her ova. There is ZW/ZZ sex chromosomes which determine the sex.

Complete answer: The sex of the embryo is determined by the egg in birds, not the sperm as they have a ZW system of sex determination and in this system, sex is determined by the female within her ova, and the sex of the offspring is not affected at all by the sperm which the male contributes.
The sex of birds is determined by sex chromosomes which are ZZ and ZW. Birds having ZZ sex chromosomes are considered as male and birds having ZW chromosomes are considered as female. The determination of sex occurs at fertilization with the inheritance of sex chromosomes and the sex is cell autonomous in birds.
This is opposite of sex determination in humans as in human beings, sex is determined by the male within her sperm as individual having XX chromosomes is a female and individual having XY chromosome is male whereas in birds, sex is determined by the female within her ova.
Sex determining gene may operate not only in the gonads in order to produce testes and ovaries but also throughout cells of the body and the evidence is from gynadomorphic chickens i.e., male on one side, female on the other which points out the sex is determined directly in each cell of the body.
The development of sexual phenotype occurs during embryonic development which is regulated by genetic and hormonal pathways and in vertebrates, sex can be determined by some factors like environmental or genetic factors.

Note:
In birds, there is ZW/ZZ system which determines the sex and it is totally opposite of mammals as birds with ZW sex chromosome is female in and birds with ZZ sex chromosomes is male. It means that in birds, sex is determined by female within in her ova as ZW is considered as sex chromosome for females in birds.