Birds have a system that works the other way round: It is called "ZW sex-determination system". The W chromosome has factors for female development. By default (if the chromosome is missing), the organism will be male. In this case, ZZ individuals are male and ZW are female. The majority of butterflies and moths also have a ZW sex-determination system. In both XY and ZW sex determination systems the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.