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 2399
2 Baltimore Oriole 1177
3 Barbets 1809
4 Bee-eaters 1174
5 Blue-Naped Mousebird 1546
6 Bowerbirds 3077
7 Brush Turkey 1285
8 Cacique 1268
9 Ducks 2292
10 Eurasian Jay 2023
11 Eurasian Nuthatch 1339
12 Eurasian Treecreeper 1194
13 Eurasian Woodcock 1494
14 European Pied Flycatcher 1139
15 Flamingo 1700
16 Flowerpeckers 1494
17 Galliformes - Chickens and Turkeys 2439
18 Geese 2255
19 Gray Heron 1593
20 Hammerkop 3305
21 Honeyguides 2113
22 Hornbills 2887
23 Horned Coot 1479
24 Hummingbirds 1329
25 Jacamars 1440
26 Kingfishers 1781
27 Leaf Warblers 1125
28 Magpie Goose 1885
29 Magpie Lark 1503
30 Malleefowl 1102
31 Maroon Oriole 1247
32 Marsh Tit and Willow Tit 1359
33 Migration of Birds 1373
34 Nightingale 2050
35 Orioles 1072
36 Oropendola 1827
37 Ovenbird of North America 1257
38 Ovenbirds of South America 2314
39 Passerines - Perching Birds 1835
40 Penduline Tits 1673
41 Piciformes - Woodpeckers and Toucans 2270
42 Pittas 1337
43 Puffbirds 1845
44 Rallidae - Rails, Crakes, Coots 1241
45 Red-and-Yellow Barbet 1914
46 Ruby-Throated Hummingbird 1180
47 Screamers 2617
48 Sunbirds 1651
49 Swallows 1526
50 Swans 1236
51 Swifts 1344
52 Tawny Owl 1541
53 Toucans 2369
54 Troupial 2027
55 Trumpeter Swan 2624
56 Warblers 1766
57 Weaver Birds 20114
58 Woodpeckers 1861
59 Woodpeckers - Great Spotted and Lesser Spotted 2582
60 Woodpeckers - Green 1495