Some of the general characteristics of class Aves:- They are genuinely flying vertebrate having feathered bipeds; 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; they have pneumatic (hollow) bones; they are uricotelic; minute, oval and nucleated erythrocytes are present; they are warm-blooded (homeothermic), and separate sexes are present, and fertilization is internal with direct development; they are oviparous. Ostrich is the largest bird, and it has two toes, while Hummingbird is the smallest bird.