Skeletal correlates of flight specialization in large birds and their application to extinct giants
Authors: Asif Aamna , Habib Michael 
Published: April 2020

This study addresses questions related to the size limits of flying birds and tradeoffs associated with the evolution of large size. Flight imposes unique limits on size in animals, and as such, form and function in large flyers can provide unique insights regarding the mechanical and physiological limits of vertebrate animals. We focused on specializations in skeletal morphology associated with the large size of flying birds. We developed a large comparative dataset of axial and appendicular skeletal proportions for living birds, using the collections of the Natural History Museum of Los Angeles County. We examined skeletal proportions because they provide mechanically relevant data and because of their applicability to fossil taxa. Some extinct, flight‐capable birds achieved sizes far exceeding that of any living flying birds. The ecologies and flight performance profiles of these extinct forms are a matter of debate. Our dataset and methodology address these questions related to flight capacity.

We consider the implications of our results for three different extinct giant flying birds: pelagornithids, teratorns, and giant storks. The largest pelagornithids had wingspans of up to 6 meters, twice that of the largest span in living birds. The teratorns contain the largest flying birds in the world with wingspans potentially reaching 6.5 m, and masses up to 75 kg. The flight capability of Leptoptilos robustus, a giant stork from the Indonesian island of Flores, is largely debated. This animal was nearly 1.8 meters tall and likely massed over 16 kg.

We took mechanically relevant skeletal measurements from over 300 living birds, focusing on the forelimb, hind limb, and sternal elements. Analysis of residuals from a series of linear regressions showed that large soaring specialists have significantly longer humeri compared to their sterna than other birds. This analysis also revealed that wing propelled swimmers and burst flyers tend to possess significantly shortened humeri. Burst adapted flyers, especially the largest burst specialists (wild turkeys), possess sterna that are significantly longer and deeper, relative to sternal width, than those of other birds. Interestingly, large wing‐propelled swimmers also have higher sternal length to width ratios than other birds. By contrast, soaring specialists have relatively “typical” sternal proportions, as indicated by small residuals in sternal length when regressed against sternal width.

These results, taken together, indicate that relatively simple measurements of limb bone and sternum dimensions can be a powerful tool for distinguishing between burst adapted and soaring adapted large flyers. Since large flying animals rely on high fractions of anaerobic flight muscle to maintain sufficient mass‐specific power for flight, most large flyers specialize in either burst takeoff or soaring flight. Our methodology distinguishes between these specializations with a high degree of accuracy in living taxa. We propose the same approach can be used for fossil taxa. Preliminary application of our comparative metrics suggest that teratorns were less condor‐like in their flight performance profile than previously suggested and pelagornithids, while clearly soaring specialized, may have been marine thermal users, rather than dynamic soaring specialists.