[30] Feduccia's early work on flamingos and waterfowl contributed to the development of his hypothesis that there was an explosive Cenozoic adaptive radiation of neornithine birds following the extinction event at the end of the Cretaceous.
[34] Furthermore, Feduccia has suggested that this rapid adaptive radiation of modern birds, compressed into such a short period of geologic time, might obscure interordinal relationships and make elucidation of the phylogeny of modern birds particularly difficult, barring the isolation of conserved characters or mosaic fossils demonstrating transitional character states bridging extant orders.
[3] This reiterates an early theme from his research in the 1970s, in which Feduccia had repeatedly emphasized the importance of homoplasy in evolution, and its ability to confound the interpretation of phylogeny.
This has also been a theme in his study of flightlessness in birds, a phenomenon the pervasiveness of which has been stressed in his work, and the mechanisms by which flight is lost, including heterochrony and differential development.
Feduccia's first contribution relative to the origin and early evolution of birds, and their relationship with dinosaurs, was a critical review of the evidence then available for dinosaurian endothermy in 1973.
[37] In a 1979 paper, Feduccia and Tordoff argued, against the position taken by John Ostrom,[6] that Archaeopteryx was capable of powered flight, as indicated by the asymmetrical vanes of its primary feathers, a feature found only in flying birds.
[40] Feduccia nevertheless suggested that on the basis of closer stratigraphic fit, ancestry from basal archosaurs rather than from coelurosaurian theropods might prove a better phylogenetic hypothesis.
He argued that these hypotheses failed to account for the elaborate aerodynamic architecture of the feather vane and rachis, and that thermoregulatory functions would have been adequately served by hair, which is a developmentally simpler structure.
This emerged as a principal argument in Feduccia's research on the origin of birds, and was the subject of developmental studies of the ostrich definitively identifying first and fifth digital condensations in the embryonic hand, confirming a pentadactyl ground state for the avian manus with symmetrical reduction, unlike the situation indicated by paleontological evidence for theropods.
[51][52] From 2002, Feduccia has argued that the discovery of spectacular new fossils from the Cretaceous of China, like Microraptor, and other taxa with unambiguous feathers, like the oviraptorosaur Caudipteryx, suggest that there might have been an extensive, and hitherto unrecognized radiation of cryptic avian lineages, some of which rapidly lost flight and secondarily adopted a cursorial lifestyle.
Feduccia's principal academic work, The Origin and Early Evolution of Birds, was well received by some workers,[63] and was winner of the Association of American Publishers 1996 award for Excellence in Biology.
[66][67] In a 2002 paper in The Auk, the journal of the American Ornithologists' Union, Richard Prum presented a summary of the current state of the theropod hypothesis for the origin of birds, and urged its acceptance by and integration within ornithology.
[68] Feduccia responded by arguing that the origin of birds was a complex and as yet unresolved problem to which the theropod hypothesis as presently formulated was a simplistic answer, ignoring contrary evidence.
Overview of paravian phylogeny released in 2019 concludes that his theory about scansoriopterygid affinity "rest on weak evidence, and most authors do not consider them to be viable".
[82] Feduccia has appeared frequently on national TV and radio, including NPR, Voice of America, BBC, CNN, ABC (Australia), NHK (Japan) and MacNeil/Lehrer Report.