Some of the general characteristics of class Aves :- They are genuinely flying vertebrate having feathered bipeds ; except some flightless bird like ostrich, penguin and kiwi, most of them can fly ; wings are modified forelimbs ; have streamlined body and possesses a beak ; for walking, swimming or clasping the tree branches, modified hindlimbs are present which generally have scales ; the respiration is through spongy and inelastic lungs ; to supplement respiration air sacs are connected to lungs and these sacs help to lessen the weight ; they have pneumatic (hollow) bones which support in weight balance and in flight ; they are uricotelic ; minute, oval and nucleated erythrocytes are present ; Lagena is an outpocketing of the saccule of birds and it corresponds to the cochlear duct of mammals; they are warm blooded (homeothermous) and seperate sexes are present and fertilization is internal with direct development ; Only three percent of all species of birds have penis. These include ducks, geese, swans, emus and ostriches which penis is wholly lost in eagles, flamingos, penguins, cranes, owls etc. ; they are oviparous. The common Rheas are the largest bird of the new world.