Birds are some of the most diverse organisms on Earth, with species inhabiting nearly every conceivable niche in every major biome. As such, birds are vital to our understanding of modern ecosystems. Unfortunately, this is hampered by knowledge gaps relating to the origin of this modern diversity and its role in ecosystems. A crucial part of addressing these shortcomings is improving our understanding of the earliest birds, the non-avian avialans i.e. non-crown birds. The diet of non-avian avialans has been a matter of substantial debate, partly related to some of the ambiguous qualitative approaches that have been used to reconstruct it. Here we review the methods of determining diet in both modern avians and fossil avian and non-avian theropods, and comment on their usefulness when applied to non-avian avialans. We use this to propose a set of comparable, quantitative approaches to ascertain fossil bird diet and on this basis provide a consensus of what we currently know about fossil bird diet. While no single approach can precisely predict diet in birds, each can exclude some diets and narrow the dietary possibilities. We recommend combining [1] dental microwear, [2] landmark-based muscular reconstruction, [3] stable isotope geochemistry, [4] body mass estimations, [5] traditional and/or geometric morphometric analysis, and [6] finite element analysis to accurately reconstruct fossil bird diet. Our review provides specific methodologies to implement each approach and discusses complications future researchers should keep in mind. On this basis we report the current state of knowledge of non-avian avialan diet which remains very incomplete. The ancestral dietary condition in non-avian avialans remains unclear due to a scarcity of data and contradictory evidence in Archaeopteryx. Among early non-avian pygostylians, Confuciusornis has finite element analysis and mechanical advantage evidence pointing to herbivory, whilst Sapeornis only has mechanical advantage evidence indicating granivory, which agrees with fossilised ingested material known for this taxon. The enantiornithine ornithothoracine Shenqiornis has mechanical advantage and pedal morphometric evidence pointing to carnivory. In the hongshanornithid ornithuromorph Hongshanornis, only mechanical advantage evidence indicates granivory, but this is congruent with evidence of fossilised ingested material in this taxon. The same is true for the songlingornithid ornithuromorph Yanornis and its inferred carnivorous diet. Due to the sparsity of robust dietary assignments, no clear trends in non-avian avialan dietary evolution have yet emerged. Dietary diversity may seem to increase through time, but this is a preservational bias associated with a predominance of data from the Early Cretaceous Jehol Lagerstatte. With this new framework and our current synthesis of current knowledge of non-avian non-avialan diet, we expect dietary knowledge and evolutionary trends to become much clearer[…]