It is often assumed that there is a link between diet and the dimensions of the avian intestinal tract. Typically, species that feed on hard-to-digest items have longer intestines, the opposite being true for species that consume highly digestible diets. However, few studies tested this hypothesis. Here, we collated data on the length of intestinal sections and body mass of 390 bird species and tested various relationships with diet, climate, and locomotion proxies. There was a strong phylogenetic signal in all datasets. The total and the small intestine scaled more than geometrically (95%CI of the scaling exponent excluding 0.33). The traditional dietary classification (faunivore, omnivore, herbivore) had no significant effect on total intestine length. Significant dietary proxies included %folivory, %frugi-nectarivory, and categories (frugi-nectarivory, granivory, folivory, omnivory, insectivory, and vertivory). Individual intestinal sections were affected by different dietary proxies. The model that best explained avian total intestine length included %frugi-nectarivory, habitat aridity, and degree of flight. Higher consumption of fruit and nectar, drier habitats, and a high degree of flightedness were linked to shorter total intestine length. However, the large intestine length was longer in species from drier habitats. This study corroborates the correlation between trophic niche and intestinal length only when using more detailed dietary proxies than those conventionally used in mammals. Notably, the length of the avian intestine depends on other biological factors as much as on diet. Given the weak dietary signal in our datasets, the diet-intestinal length relationships lend themselves rather to a narrative of flexibility (‘morphology is not destiny’) than of distinct adaptations that facilitate using one character (intestine length) as a proxy for another (diet). Generally, compared to mammals, birds have shorter intestinal sections, with a total intestine about 85% that of mammals of similar size.