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 Marcia Malory 2249
2 Baltimore Oriole Marcia Malory 1080
3 Barbets Marcia Malory 1703
4 Bee-eaters Marcia Malory 1069
5 Blue-Naped Mousebird Marcia Malory 1421
6 Bowerbirds Marcia Malory 2902
7 Brush Turkey Marcia Malory 1201
8 Cacique Marcia Malory 1183
9 Ducks Marcia Malory 2161
10 Eurasian Jay Marcia Malory 1881
11 Eurasian Nuthatch Marcia Malory 1258
12 Eurasian Treecreeper Marcia Malory 1118
13 Eurasian Woodcock Marcia Malory 1392
14 European Pied Flycatcher Marcia Malory 1049
15 Flamingo Marcia Malory 1577
16 Flowerpeckers Marcia Malory 1388
17 Galliformes - Chickens and Turkeys Marcia Malory 2298
18 Geese Marcia Malory 2100
19 Gray Heron Marcia Malory 1498
20 Hammerkop Marcia Malory 3055
21 Honeyguides Marcia Malory 1906
22 Hornbills Marcia Malory 2576
23 Horned Coot Marcia Malory 1370
24 Hummingbirds Marcia Malory 1200
25 Jacamars Marcia Malory 1333
26 Kingfishers Marcia Malory 1651
27 Leaf Warblers Marcia Malory 1039
28 Magpie Goose Marcia Malory 1772
29 Magpie Lark Marcia Malory 1394
30 Malleefowl Marcia Malory 1008
31 Maroon Oriole Marcia Malory 1139
32 Marsh Tit and Willow Tit Marcia Malory 1259
33 Migration of Birds Marcia Malory 1285
34 Nightingale Marcia Malory 1894
35 Orioles Marcia Malory 971
36 Oropendola Marcia Malory 1686
37 Ovenbird of North America Marcia Malory 1162
38 Ovenbirds of South America Marcia Malory 2187
39 Passerines - Perching Birds Marcia Malory 1709
40 Penduline Tits Marcia Malory 1564
41 Piciformes - Woodpeckers and Toucans Marcia Malory 2133
42 Pittas Marcia Malory 1188
43 Puffbirds Marcia Malory 1671
44 Rallidae - Rails, Crakes, Coots Marcia Malory 1152
45 Red-and-Yellow Barbet Marcia Malory 1774
46 Ruby-Throated Hummingbird Marcia Malory 1098
47 Screamers Marcia Malory 2464
48 Sunbirds Marcia Malory 1502
49 Swallows Marcia Malory 1388
50 Swans Marcia Malory 1131
51 Swifts Marcia Malory 1247
52 Tawny Owl Marcia Malory 1440
53 Toucans Marcia Malory 2143
54 Troupial Marcia Malory 1877
55 Trumpeter Swan Marcia Malory 2508
56 Warblers Marcia Malory 1659
57 Weaver Birds Marcia Malory 18174
58 Woodpeckers Marcia Malory 1733
59 Woodpeckers - Great Spotted and Lesser Spotted Marcia Malory 2458
60 Woodpeckers - Green Marcia Malory 1390