many features — such as feathers, wishbones, egg brooding, and perhaps even flight — that are seen only in birds among living animals first evolved in the dinosaurian ancestors of birds (Figures 4 and 5). Other features, such as rapid growth, a keeled sternum, pygostyle, and beak, are absent in the earliest birds and evolved, often multiple times, in more derived birds during the Cretaceous. Therefore, what we think of as the bird ‘blueprint’ was pieced together gradually over many tens of millions of years of evolution, not during one fell swoop.
Other classic avian features, such as the keeled breastbone to support flight muscles and highly reduced tail, evolved after the origin of birds, meaning that the earliest birds looked more like dinosaurs in lacking these features.
The evolution of feathers likely began in the earliest dinosaurs, or perhaps even in the closest relatives of dinosaurs. A variety of primitive theropods (including the tyrannosaurs Dilong and Yutyrannus, as well as some plant-eating dinosaurs) are now known from spectacularly preserved fossils covered in simple, hair-like filaments called ‘protofeathers’ that are widely considered to be the earliest stage of feather evolution
Some non-bird dinosaurs like Microraptor possess feathers basically indistinguishable from the flight feathers of living birds. The story of feather evolution is becoming increasingly clear: the earliest feathers evolved in non-flying dinosaurs, likely for display and/or thermoregulation, and only later were they co-opted into flight structures in the earliest birds and their very closest dinosaurian relatives. And not just feathers – layered feathers on arms and tails (ie wings) were also seen on non-avian dinosaurs. Their function is of course unknown but microraptor could almost certainly glide, at least.
Tail feathers tend to be symmetrical which goes against their use for flight (poor resistance to air stream) – that plus the diversification of melanomes (ie colours) suggests non flight uses eg sexual display.
Proper flight only really possible with later developments eg keeled sternum, triosseal canal in the shoulder (wishbone system – ends move apart to absorb some energy that then automates the upstroke).
Birds exhibit unidirectional breathing – air sacs in the front and back of the body mean air passes over gas exchange tissues (lungs) in both inspiration and expiration.