Extensive research on avian adaptive radiations has led to a presumption that beak morphology predicts feeding ecology in birds. However, this ecomorphological relationship has only been quantified in a handful of avian lineages, where associations are of variable strength, and never at a broad macroevolutionary scale. Here, we used shape analysis and phylogenetic comparative methods to quantify the relationships between beak shape, mechanical advantage, and two measures of feeding ecology (feeding behaviour and semi-quantitative dietary preferences) in a broad sample of modern birds, comprising most living orders. We found a complex relationship, with most variables showing a significant relationship with feeding ecology but little explanatory power, for example, diet accounts for less than 12% of beak shape variation. Similar beak shapes are associated with disparate dietary regimes, even when accounting for diet-feeding behaviour relationships and phylogeny. Very few lineages optimize for stronger bite forces, with most birds exhibiting relatively fast, weak bites, even in large predatory taxa. The extreme morphological and behavioural flexibility of the beak in birds suggests that, far from being an exemplary feeding adaptation, avian beak diversification may have been largely contingent on trade-offs and constraints.