Musselshell

People often associate birds with the places they come to rest or feed. But really that’s just a small part of many birds lives, with most of their time spent on the wing. Current, winds, and eddies mean that the aerial environment through which birds maneuver is a constantly shifting jungle gym. Sometimes the changes to the environment are predictable, sometimes erratic. But as far as we can tell, birds are responding to the changes simply by feel – the acrobatic aeroscape is as invisible to birds as it is to us.

Learning about Earth’s dynamic atmosphere and how it affects bird flight led University of Montana graduate student Erin Keller to ask, “When birds are in the air, how do they deal with disturbance?” Gusts of unexpected turbulence, for example, could throw birds for a loop. Keller hypothesized that if birds need to be prepared for disturbances that they can’t see, then they should have strategies to deal with those disturbances when they occur. Keller considered different types of possible strategies and divided them into two groups. Active strategies are when birds flex their muscles, spending energy to change their “steady state” in flight. For example, wings can be flapped asymmetrically to counteract off-centered wind vortices. Passive strategies are when things that the birds are already doing with their bodies and wings dampen disturbances. For example, wings held in a high dihedral position counteract the roll forces created by crosswinds.

Keller is especially interested in raptors because their flight styles include both active wing-flapping and passive soaring. To analyze movement mechanics, Keller spent the summer of 2020 recording wind data at osprey nest sites while also recording videos of parent osprey landing and taking off. Keller was fortunate that her study area, near Missoula, generally has calm air during summer mornings and increasing wind in the afternoons. This allowed Keller to compare flight in calm versus turbulent conditions. Through analyzing video, she was able to examine active 3D wing- and tail-movements in response to different wind patterns. As an added bonus, the weather allowed Keller to observe these movements and behaviors outside in a natural setting, rather than solely under controlled laboratory conditions, where much of biomechanics research takes place.

When she’s not in the field, Keller has been working with a team of researchers, including Dr. Bret Tobalske at the University of Montana and Dr. Daniel Inman at the University of Michigan, with whom she is investigating the capacity of primary feathers to mitigate air-current disturbances. Emarginate primary feathers (the “finger feathers”) in soaring birds have a particular slotted shape that makes them very flexible. This flexibility may enable slight changes to the shape of the wing and, consequently, the forces acting upon it. Such passive deformation can help to dampen disturbances along the wing in flight. To test this, Keller and her team are analyzing 3D feather movements and air-current forces along spread wings of large raptors. This work takes place in a wind tunnel, using raptor specimens on-loan from the UM Zoological Museum.

Keller’s love of the natural world and of birding push her to understand how birds interact with their aerial environment. By sharing her work through science communication and outreach, Keller aspires to draw more curious minds to learn about the marvel of avian flight.

Erin Keller is an MS student in Dr. Bret Tobalske’s lab at the University of Montana, in Missoula. Photos by Sam Getty.