Birds have a ubiquitous, female heterogametic, ZW sex chromosome system. The current model suggests that the Z chromosome and its degraded partner, the W chromosome, evolved from an ancestral pair of autosomes independently from the mammalian XY male heteromorphic sex chromosomes--which are similar in size, but not gene content (Graves, 1995; Fridolfsson et al., 1998). Furthermore the degradation of the W has been proposed to be progressive, with the basal clade of birds (the ratites) possessing virtually homomorphic sex chromosomes and the more recently derived birds (the carinates) possessing highly heteromorphic sex chromosomes (Ohno, 1967; Solari, 1993). Recent findings have suggested an alternative to independent evolution of bird and mammal chromosomes, in which an XY system took over directly from an ancestral ZW system. Here we examine recent research into avian sex chromosomes and offer alternative suggestions as to their evolution.