This project aims to substantially increase our knowledge of the interplay between morphological evolution and organismal diversity. The radiation of birds will be used as the model system for reaching this goal. Birds are an important group of vertebrates, a very species-rich clade that has had a profound impact on Earth’s ecosystems. Specific questions that will be addressed include: was avian diversification driven by the origin of flight or by another morpho-functional complex of characters? Was the increasing diversification of the bird clade a single event of huge speciation burst or did it occur steadily? How did the rise of birds affect potential competitors (pterosaurs) and their sister theropod group? Do successful clades diversify equally or is the diversification driven by particular subclades? How did the Cretaceous-Tertiary mass extinction affect the evolution of birds? And more generally, how do diversity and disparity relate to each other? These questions lie at the core of our understanding of large-scale evolution and will be tackled via the implementation of a diverse array of well-established numerical techniques. These will include maximum likelihood approaches for assessing character dependence, new methods for quantifying disparity and amount of morphospace occupation, rarefaction analysis for assessing differences in morphological disparity, statistical methods for estimating rates of character change, and sensitivity analyses. These techniques document the apomorphies that distinguish the clades, and thus, they can be used to test hypotheses of character evolution, functional complexes of characters, adaptation, and competition. This research will help understand significant themes of current evolutionary research, such as how modern diversity occurred, whether diversification is typically driven by internal dynamics of the system or by external physical environmental stimuli, and how do novelties arise.