Mammals are the highest class of vertebrates and the female has mammary glands which secrete milk for the nourishment of young after birth. Mammals are warm-blooded and the majority of their body is covered with hair. Mammals give birth to live young (except the platypus and echidna, who lay eggs). They also have three middle ear bones.
Most mammals live solely on land, but there are several groups that have specifically evolved to exist solely in aquatic environments (dolphins and whales) one of which is the largest of all animals. Only one group has mastered the art of true flight (bats) although several have developed the ability to glide through tree tops.
Birds are warm-blooded, egg-laying, vertebrate animals. They are covered in feathers and their forelimbs are modified into wings which are used for flight. Their feathers are highly protective against cold and also against water. Birds have adapted for flight with their bones being made up of a honeycomb structure so as to reduce their structural weight. They also have acute eyesight and hearing but little sense of smell. Most birds are diurnal (only active during the day) although some have become specifically adapted to a nocturnal lifestyle (only active during the night).
Depending upon the species, birds feed on a variety of food sources; nectar, seeds, insects, rodents, fish and even on other birds however all have specific adaptations to aid in their individual feeding technique. Most birds build a nest in which they lay their eggs, although there are a few that actually lay their eggs in other birds' nests so as to 'trick' the other birds into raising their young (Cuckoos)! Eggs, like birds, vary in colour, size and shape.
Animals that have dry scaly skin and are ectothermic (cold-blooded) vertebrates are called reptiles. The major way reptiles differ from mammals and birds is that they are ectothermic (cold-blooded). This does not mean that they necessarily have cold blood but rather that they cannot regulate their own body temperature through any means other than modifying their behavior. To heat up they use an external heat source within their environment such as basking in the sun or sitting on a hot rock (most mammals would shiver). To cool down they choose a cooler area to rest (most mammals would sweat). Due to this they are generally found through out the warmer regions of the world although a few have adapted to quite cold areas (even where it snows)! Most reptiles though are found in the tropics. The majority of reptile groups also live on land, although there are a few that have specialized to live in water. Reptiles can only breathe air and do not possess gills. They have thick waterproof skins to retain their body moisture.
Most reptiles lay eggs (oviparous), however there are a few species that incubate and hatch their eggs internally (ovoviviparous). Reptile eggs contain a large quantity of yolk to nourish the embryo, and they have a porous shell. Most reptiles are carnivorous.