Birds, members of the class Aves, include some of the Earth's most intelligent animal species.

They can be found in the wild on all seven continents.

All birds are vertebrates (have backbones) and are endothermic (warm-blooded).

Birds are tetrapods.

They are amniotes, and lay eggs with hard shells.

Birds are well adapted for flight. Most, but not all, birds can fly.

All birds have feathers, and their front limbs have evolved to become wings.

MacawsThey have strong, lightweight skeletons.

All birds have four-chambered hearts.

Birds have specialized circulatory and respiratory systems that enable them to maintain the high metabolic rates that are needed for flying.

Many birds migrate to take advantage of seasonal changes in different parts of the world.


A gizzard, also known as a ventriculus, is an organ, found in all birds, that mechanically digests food.

Other animals, such as alligators and crocodiles, may have organs like gizzards.

Animals, such as birds, that do not have teeth – as well as animals that use their teeth for biting and tearing but not for chewing – may have gizzards, or similar organs, for breaking food into smaller pieces.

A gizzard may be considered a second stomach.

When a bird swallows food, it first passes through a glandular stomach, which is also known as the proventriculus, or true stomach.

PenguinsAcids that aid digestion are secreted in the true stomach.

After the food has been broken down chemically by stomach acid, it moves on to the gizzard.

The gizzard has a strong inner membrane that is surrounded by thick, muscular walls that grind up food.

It may also contain gravel or stones, known as gizzard stones, or gastroliths, which have been swallowed by the bird.

Gastroliths are usually round and smooth, because they action of the stomach polishes them.

The stones and gravel in the gizzard help to break up the food.

Food may pass back and forth between the glandular stomach and the gizzard, alternating between chemical and mechanical digestion.

Stones resembling gastroliths have been found near dinosaur fossils. This has led scientists to believe that some dinosaurs may have had gizzards or gizzard-like digestive organs.


Birds and some carnivorous bipedal dinosaurs, including tyrannosaurs and velociraptors, make up the clade Coelurosauria.

A clade is a group of organisms that share a common ancestor. All of the descendents of this common ancestor are members of the clade.

The coelurosaurs, including birds and other bipedal predatory dinosaurs make up the clade Theropoda (the theropods).

All of the dinosaurs, as well as birds, together form the clade Dinosauria.

The dinosauria (birds and dinosaurs) and the crocodilians (alligators, crocodiles, caimans, gharials and false gharials) belong to the clade Archosauria.

Archosaurs, along with squamates (snakes and lizards) and tuataras are diapsids.

A diapsid is an amniote that has two temporal fenestrae on each side of its skull or is descended from an amniote with two temporal fenestrae on each side of its skull.

Temporal fenestrae are holes in the skull, above or behind the eyes.

1 Anseriformes - Ducks, Geese, Swans 2408
2 Baltimore Oriole 1184
3 Barbets 1815
4 Bee-eaters 1181
5 Blue-Naped Mousebird 1557
6 Bowerbirds 3089
7 Brush Turkey 1292
8 Cacique 1279
9 Ducks 2306
10 Eurasian Jay 2030
11 Eurasian Nuthatch 1344
12 Eurasian Treecreeper 1201
13 Eurasian Woodcock 1499
14 European Pied Flycatcher 1144
15 Flamingo 1705
16 Flowerpeckers 1503
17 Galliformes - Chickens and Turkeys 2444
18 Geese 2272
19 Gray Heron 1598
20 Hammerkop 3322
21 Honeyguides 2123
22 Hornbills 2903
23 Horned Coot 1487
24 Hummingbirds 1335
25 Jacamars 1445
26 Kingfishers 1788
27 Leaf Warblers 1129
28 Magpie Goose 1893
29 Magpie Lark 1511
30 Malleefowl 1109
31 Maroon Oriole 1258
32 Marsh Tit and Willow Tit 1365
33 Migration of Birds 1379
34 Nightingale 2059
35 Orioles 1081
36 Oropendola 1835
37 Ovenbird of North America 1264
38 Ovenbirds of South America 2323
39 Passerines - Perching Birds 1840
40 Penduline Tits 1681
41 Piciformes - Woodpeckers and Toucans 2278
42 Pittas 1345
43 Puffbirds 1852
44 Rallidae - Rails, Crakes, Coots 1252
45 Red-and-Yellow Barbet 1934
46 Ruby-Throated Hummingbird 1185
47 Screamers 2623
48 Sunbirds 1662
49 Swallows 1533
50 Swans 1242
51 Swifts 1353
52 Tawny Owl 1547
53 Toucans 2378
54 Troupial 2035
55 Trumpeter Swan 2628
56 Warblers 1771
57 Weaver Birds 20233
58 Woodpeckers 1870
59 Woodpeckers - Great Spotted and Lesser Spotted 2586
60 Woodpeckers - Green 1498