Data from: Diet, habitat, and flight characteristics correlate with intestine length in birds

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.