Data from: Longer wing bones in warmer climates suggest a role of thermoregulation in bird wing evolution

The tendency for animals in warmer climates to be longer-limbed (Allen’s
Rule) is widely attributed to the demands of thermoregulation. The role
played by thermoregulation in structuring bird wings, however, has been
overshadowed by the selective demands placed on wings by
flight. Using computer vision, we measure wing bone length from
photographs of museum skeletal specimens for 1,520 species of passerine
birds. We then model the relationship between wing bone length and
temperature, accounting for allometry, the demands of flight efficiency
and maneuverability, and a range of ecological and environmental
variables.  We find that wing bones are longer in warmer
climates. Our models, largely as a result of allometric effects, explain
nearly all the variation in wing bone length in our data, with a marginal
R2 = 0.80 and a conditional R2 > 0.99.  Thus, across 1,520
species of birds, higher temperatures are associated with longer wing
bones, as predicted by Allen’s Rule. The vascularized musculature along
these bones is maximally uncovered when birds actively hold their wings
away from their bodies to aid in cooling or during flight. Conversely, the
musculature along the wing bones is insulated by feathering when at rest,
such that wings play a minor role in heat exchange when individuals are
less active and may need to retain heat. While our analyses do not
directly establish the mechanistic basis underlying the pattern we
recover, given the asymmetry in the role of wings in thermoregulation, we
interpret the positive relationship between temperature and wing bone
length to reflect increased demand for heat dissipation in warmer
climates. Our findings highlight the role of thermoregulation in shaping
even the most critical features of vertebrate anatomy.