While fever is known to occur in invertebrates and vertebrates, the mechanisms of fever in animals other than mammals have received scant attention. We look initially at the recognition, by the avian immune system, of pathogen associated molecular patterns and the likely role of toll-like receptors in signaling the presence of bacteria and viruses. Several mediators of fever are subsequently released by immune cells, including interleukin-6 and interleukin-1β, that eventually reach the brain and alter thermoregulatory function. As is the case in mammals, prostaglandins appear to be the ultimate mediators of fever in birds, since the febrile response is attenuated when prostaglandin synthesis is inhibited. Ambient temperature modulates the fever response, with larger fevers at higher, and smaller fevers at lower ambient temperatures. Glucocorticoid levels are increased during fever and seem to play an important role by modulating the extent of fever generation, possibly playing a role in the attenuation of fever after repeated exposure to a pathogen in a process termed tolerance, suggesting that the fever process can be phenotypically adapted to likely future conditions. While fever has an ancient phylogenetic history and many of the underling mechanisms in birds appear similar to mammals, there are several important differences that suggest fever has evolved quite differently in these two homeothermic classes.