The Z chromosome contains genes for both male and female characteristics, while the W chromosome is smaller and contains only a few genes, including the gene that determines sex. The sex of a bird is determined at fertilization. If the egg is fertilized by a sperm carrying a Z chromosome, the offspring will be male. If the egg is fertilized by a sperm carrying a W chromosome, the offspring will be female.
*This sex determination system is similar to that found in some other reptiles and insects.*
In general, male birds have two copies of the Z chromosome, while female birds have one copy of the Z chromosome and one copy of the W chromosome.
The sex chromosomes also contain genes for other traits, such as feather color and body size. This means that the sex of a bird can also be influenced by the genes on the sex chromosomes.
In some rare cases, birds can have atypical sex chromosome arrangements, such as ZZ females or ZW males. These birds are usually sterile and cannot produce offspring.