The Flight of the Hummingbird

Undergraduate Olivier Boissier explains a new study showing how hummers hover

There are four different ways to hover. Gulls and Red-tailed Hawks, for example, stay aloft by facing the wind, wings still, as the wind holds them aloft. Terns, kingfishers, and kestrels flap their wings forcefully to fight gravity. Hummingbirds and nectar-loving insects hover at flowers while they feed, their wings a blur as they beat rapidly back and forth.

When hawkmoths hover, they generate lift equally on downstrokes and upstrokes.

Hawkmoth by Mark Chao

Because of outward similarities in the wing shape and motion of hummingbirds and hovering insects of similar size, some studies have predicted that hummers and insects use the same aerodynamic mechanism to stay in the air. Hawkmoths and other large hovering insects use downstrokes and upstrokes equally to generate lift, an assumption widely applied to explain hummingbird flight as well.

However, a recent study published in the journal Nature (June 23, 2005) shows that hummingbirds actually generate lift in a unique way. A team led by Douglas Warrick from Oregon State University trained Rufous Hummingbirds to hover for food in an idle wind tunnel--the kind engineers use to test the aerodynamics of model cars and airplanes. They made the airflows generated by the birds' wings visible by seeding the air with tiny olive oil droplets illuminated with laser lights.

A hummingbird's hovering differs from that of insects as well as from other birds.

Ruby-throated Hummingbird by Harold B. Key

The team found that hummingbirds produce 25 percent of their weight support during the upstroke and the remainder during the downstroke. This differs from all other birds, which rely on the downstroke for 100 percent of weight support while hovering. It also differs from the flight style of insects, which generate lift equally on the upstroke and downstroke, thanks to thin, elastic wings that can curve with motion.

Although hummingbirds can rotate their wings around their shoulder joints to a greater extent than other birds can, their musculature prevents the wings from inverting completely during the upstroke, as required to hover in the way that large insects do. Thus, birds and insects have evolved hovering flight using different methods, influenced by the constraints of anatomy.

Oh, and by the way, what is the fourth way to hover? Take a helicopter.

Olivier Boissier was a visiting student from France this summer. In the Lab of Ornithology's Evolutionary Biology Program, he used DNA sequencing to explore how different warbler species are related to one another.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu