<P> Birds have two sexes: either female or male . The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals . Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ). </P> <P> In nearly all species of birds, an individual's sex is determined at fertilisation . However, one recent study demonstrated temperature - dependent sex determination among the Australian brushturkey, for which higher temperatures during incubation resulted in a higher female - to - male sex ratio . This, however, was later proven to not be the case . These birds do not exhibit temperature - dependent sex determination, but temperature - dependent sex mortality . </P> <P> Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signalling . There are several types of feathers, each serving its own set of purposes . Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae . The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics . The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex . </P> <P> Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" plumage, or--in the Humphrey - Parkes terminology--"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey - Parkes system as "alternate" plumages . Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years . Moulting patterns vary across species . In passerines, flight feathers are replaced one at a time with the innermost primary being the first . When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop . After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap . </P>

What does a bird have that no other animal has