Most ornithologists believe that birds have evolved from the reptiles that lived millions of years ago - the dinosaurs. As unlikely as this relationship may seem there is compelling evidence, in both living birds and fossils, to support this theory.
Modern birds share some characteristics with reptiles (dinosaurs), in particular:
Beyond these simple observations, however, it does seem outrageous to link birds with dinosaurs, because there are no intermediate animals, e.g. reptiles with a few feathers. Or is it?
In 1861, a German palaeontologist, Hermann von Meyer, found a fossil in a quarry of an animal that had lived around 150 million years earlier, at the end of the Jurassic period.
The fossil showed a feathered creature, about the size of a pigeon and:
Clearly, this animal was part reptile and part bird, and provided a link between dinosaurs and birds. This discovery was named Archaeopteryx lithographica.
Today, only a few species of birds have clawed digits on the forewing, the best example is the Hoatzin of South America. The young Hoatzin uses the claws to climb trees in the mangrove swamps before they can fly properly. This observation suggests that Archaeopteryx was probably not a very good flyer, but climbed trees and then glided to another tree. Indeed, this idea is supported further by the Archaeopteryx not having a keel (sternum) to which the strong breast muscles needed for flight are anchored.
The Archaeopteryx had reptilian scales and, as already mentioned, sharp teeth. All modern species have scales on the legs and feet, but only a few have teeth (but not bony jaws); the sawbill ducks, such as Goosander and Merganser.
The fossil also revealed a "wish-bone" (furcula) formed from the joining of the collarbones (clavicles). The furcula is unique to today's birds and some dinosaurs called pseudosuchians.
Archaeopteryx was quite well developed as a bird, which in terms of evolution must have taken millions of years, and so there must have been earlier creatures that were less well developed. There are two main theories as to the origin of birds, namely: they evolved from thecodonts, the reptile ancestors of dinosaurs, pterosaurs, and crocodiles; or they were direct descendants of agile, predatory dinosaurs called coelurosaurs (like featherless ostriches).
Similarly, there have to be ancient birds that are better developed.
Between the disappearance of the Archaeopteryx and modern birds there are relatively few fossils to show the development of birds. One reason for this is that the hollow bones of birds are easily crushed and disintegrate before becoming fossilised.
In 1995, fossils were unearthed in China of an animal with downy feathers on its head, neck and spine, and a sternum capable of supporting the strong breast muscles needed for flight. This bird has been called Sinosauropteryx and is about 120 million years old, from the Cretaceous period. Another species, Confusciusornis, seems more closely related to Archaeopteryx in that it has clawed wings, but the tail is shorter and the jaws are replaced with a horny beak.
Thus, it seems that the birds of the early Cretaceous period resembled today's birds much more than Archaeopteryx, and even more so later in the period. For example, about 70 million years ago there were flightless, fish-eating birds called Hesperornis, similar to today's cormorants, with teeth that were probably used to grip fish, and gull-like birds called Ichthyornis.
During the Eocene period (38 to 54 million years ago) there were many different birds: flightless game birds like Ostriches, gliding sea birds like Albatrosses, penguins that could "fly" underwater, and so on.
Between 10 000 and 1.6 million years ago (Pleistocene period) it is estimated that there were up to 500 000 species, but many species were in decline and became extinct. Today, there are about 13 000 species and their numbers are still declining.
Unfortunately, until more fossils are discovered the origin of birds will remain a mystery. In the meantime, there is some appeal in thinking that dinosaurs are alive and well, and are visiting our gardens!