The study of the evolution of birds began about 150 years ago with the finding of Archaeopteryx. Yet, many unsolved questions about avian evolution remain to be answered. This thesis aims at addressing some of these questions.

The Early Cretaceous Confusiusornis is the most well-represented Mesozoic bird in the fossil record. The abundance of fossils facilitates a study of the preservation of specimens in the two geological formations in which this taxon is found. It was demonstrated that specimens in the Yixiang Formation always are represented by complete, articulated skeletons, while those in the Jiofutang Formation often lack the pectoral girdle and the wings.

Despite the many specimens available of Confusiusornis few clues to the diet of this taxon have been found. We describe a Confusiusornis specimen with a pellet of fish remains preserved in the throat region.

The enantiornithid birds probably constituted the most species-rich and diverse bird group during the Cretaceous. Several well-preserved specimens have been found in China, e.g. Grabauornis lingyuanensis described herein.

The mass extinction at the end of the Cretaceous probably gave the only surviving group of birds,Neornithes,chance to radiate and evolve into new niches. One such group is the Strigiformes (owls). We describe a new species from the Eocene Green River Formation in USAthat we suggest is closely related to the contemporary European Prosybris antique and P. medius.

Although birds are known from several Miocene localities in Europe, the discovery of vertebrate fossils in the Hambach opencast lignite mine was thus unexpected and remarkable. The most significant bird found in Hambach is a specimen of darter, genus Anhinga. It agrees in size, proportions and morphology the fossil species Anhinga pannonica to which we refer the Hambach specimen. Fossils of ducks and galliforms have also been found in deposits at Hambach dated to the Pliocene.