Relative forelimb-hindlimb investment is associated with flight style, foraging strategy, and nestling period, but not nest type

We investigated Dial’s 2003 hypothesis that birds that rely more heavily upon flight as their primary mode of locomotion and thus invest more in their forelimbs than hindlimbs will experience selection for smaller body sizes, greater altriciality, and more complex nests. To test this hypothesis, we examined the skeletons of over 2,000 individuals from 313 species representing the majority of avian families and all major branches of the avian tree. We used the lengths of the sternal keel and long bones of the wing relative to the lengths of the leg long bones as an index of relative locomotor investment. We found that locomotor investment was predicted by flight style, foraging method, and length of nestling period, supporting Dial’s hypothesis. Soaring birds and birds with more acrobatic flight styles, birds whose foraging methods were heavily reliant upon flight, and whose young spent more time in the nest tended to invest more in their forelimbs relative to hindlimbs. Nest type and body size were not significant predictors of relative forelimb-hindlimb investment, however, suggesting that the relationships among flight style, locomotor investment, and life history are not as tightly intertwined as Dial originally hypothesized.