鳥類は恐竜の生き残りとして大空を支配している

鳥類は9000以上現生種を数える高度に繁栄した飛行性脊椎動物である。

最古の鳥類、中生代ジュラ紀後期の始祖鳥（アーケオプテリクス）は、保存状態の良いいくつかの標本が知られ、古くから詳細に研究されてきたのにもかかわらず、その祖先と現生鳥類との進化のギャップは十分に解明されているとは言えなかった。しかし、中国の遼寧省での羽毛恐竜の発見によって、恐竜から原始的な鳥類への進化の過程はかなり詳細に解明されつつある。

鳥類に最も近い恐竜はドロマエオサウルス類やトロオドン類の恐竜だと考えられている。鳥類の特徴である羽毛は、既に恐竜の段階で多様なグループで発達しており、鳥類に近い恐竜のグループでは前肢と後肢に風切り羽（飛行に適した羽）を持つものも現れた。一部の恐竜や初期の鳥類は手足に備わった４つの翼で空を滑空していたが、やがて前肢だけで力強く羽ばたく飛翔方法を獲得していった。

Birds have, as dinosaur survivor, colonized the sky.

Birds are a highly prosperous vertebrate group counting more than 9000 extent species. The late Jurassic earliest bird called Archeopteryx, known for the exceptional preservation of its fossils, has been studied in detail since its discovery in 1861. Despite all these studies, the evolutionary gap between birds and their ancestors has not been fully clarified. However, the discovery of a feathered dinosaur in the chinese province of Liaoning, considerably helped us to elucidate the process of evolution that occurred from the dinosaurs to the primitives birds.

It is thought that the closest bird's relatives among the dinosaurs are the Troodontidae and the Dromaesauridae. Feathers, which are characteristics of birds, were already developed in diverse group among dinosaurs and some dinosaurs belonging to birds closest groups had wind-cutting feathers (feathers suitable for flight) on their forelimbs and hindlimbs. Although some dinosaurs and early birds glided through the sky with their 4 wings on their limbs, some eventually developed flight method by only flapping strongly their forelimbs.