<p>Functional traits are those whose variation affects fitness. We compared the interdependence of beak and postcranial functional morphological traits across birds with different lifestyle strategies and trophic niches to identify those traits that drive major phenotypic changes, and tested for differences in modular organization and integration among groups. Body mass consistently emerged as influencing all other traits. We found a lower integration in aquatic birds, although this may mask the presence of distinct morphotypes. Integration is highest in vertivore birds, revealing constraints imposed by the feeding habit. Overall, phenotypes were more modular according to their trophic niches, and beak and postcranial traits were distributed in unexpected patterns among modules. The ability of functional traits to evolve independently appears to be a necessary condition for specialization.</p>