and a lightweight skeleton. They also have a wide variety of reproductive strategies among
the different types of birds. Let us examine some of their principle traits so we can get a
better appreciation of what birds can do.
Characteristics of Birds
Birds (class Aves) are warm-blooded, vertebrate animals with two legs (bipedal), who lay
eggs. They range in size from the tiny 2 in (5 cm) Bee Hummingbird to the 9 ft (2.7
m) ostrich (Figure14.1). With approximately 10,000 living species, birds are the most
numerous vertebrates with four limbs (tetrapod). They occur in diverse habitats across the
globe, ranging from the Arctic to the Antarctic.
Figure 14.1: The ostrich can reach a height of 9 feet! Pictured here are ostriches with young
in Namibia, Africa. ( 4 )
Defining characteristics of modern birds include:
- feathers
- high metabolism
- a four-chambered heart
- a beak with no teeth
- a lightweight but strong skeleton
- production of hard-shelled eggs
The digestive system of birds is unique, with a crop for storage and a gizzard that contains
swallowed stones for grinding food to compensate for the lack of teeth. Birds have forelimbs