From Opus

Ostrich, the largest but flightless bird
Photo by Leon

Vervain Hummingbird, one of the smallest birds on earth
Photo by Juan Estevez

Aves (all birds)

[edit] Description

All birds are placed in the zoological class Aves. They are vertebrate, warm-blooded animals with wings, two feet and they lay eggs. They are characterised by some unique features:

• Feathers
• Laying of hard-shelled eggs
• Beak without teeth
• High metabolic rate
• Strong but lightweight skeleton
• Heart with four chambers
• Unique respiratory and digestive systems

The accepted number of species varies in the different sources, but there are about 10'000 species worldwide, found on all continents.

[edit] Taxonomy

Recent fossil finds make it highly likely that the closest relatives of birds were the dinosaurs.

Below is a list of all the orders in the class Aves that are recognized by Opus, which at this level of taxonomy follows the Clements Checklist of Birds of the World. If you are looking for an order that you cannot find in this list, there may be a link to it at the end of this page; in the linked page you should find the families associated with that order as well as a link to the order under which those families are currently listed in Opus.