Andrew Moore has long been captivated by the mysteries of avian flight.
An assistant professor in the Stony Brook University Renaissance School of Medicine's Department of Anatomical Sciences, Moore recently collaborated with an international team of researchers — led by University of Florida evolutionary biologist Emma Schachner — that has resulted in findings that challenge long-held assumptions and reveal astonishing new insights into the flight mechanics of birds.
The team's groundbreaking study is published in the journal Nature. The investigation centered on the subpectoral diverticulum (SPD), a large air-filled sac branching off the lung that weaves between the primary flight muscles of various birds such as the red-tailed hawk, which raised the question: What role does the SPD play?
"Bird lungs are very different from those of humans and other mammals, and have been hypothesized to play all sorts of non-breathing roles, including controlling body temperature and acting like a protective crash suit for certain plunge diving birds," said Moore. "But few of these hypotheses have actually been tested. This discovery is important because we demonstrate a totally novel role for the lung in improving how flight works in soaring birds like eagles and pelicans. Our team is excited to continue testing hypotheses about the diverse functions of the avian respiratory system."
The team surveyed 68 species of extant birds to assess the presence or absence of the SPD, and relied on computed tomography (CT) scans of both living and deceased specimens in order to visualize the internal structures without invasive procedures. In the dataset, 57 species were examined using CT or microCT scans.
Moore and Schachner then compiled extensive data on the flight styles of these bird species to test the hypothesis that there is a correlation between soaring flight and the presence of the SPD. By integrating evolutionary history into their analysis, they discovered a striking pattern: the SPD had independently evolved at least seven different times exclusively in soaring lineages, suggesting a strong functional link between the SPD and the mechanics of soaring flight.
To answer the question of how the SPD enhances soaring capabilities, the team relied on the biomechanical expertise of co-author Karl Bates of the University of Liverpool, who applied sophisticated computer modeling techniques to simulate the impact of an inflated SPD on the pectoralis muscle (the primary muscle responsible for the downstroke in bird flight).
The resulting analyses revealed that the SPD, along with specialized pectoralis muscle architecture, significantly improved the torque-generating capacity of the wing when held in the extended soaring position.
Further CT scans of live, sedated red-tailed and Swainson's hawks showed that the SPD did not play a critical role in breathing. The birds could voluntarily collapse the SPD and still breathe normally, even independently opening and closing it on one side. This discovery was pivotal to their findings because it indicates that the SPD's primary function is not respiratory, but mechanical, and shows that the respiratory system helps reduce the energy needed for muscle work.
The implications of this research are profound and demonstrate that in soaring birds, the respiratory system has evolved to mechanically enhance the flight muscles, a previously unknown role for avian air sacs.