Rest stop

Natural History, Sept, 2004 by Erin Espeli

Flying long distances can take the wind out of almost any traveler. Even members of the hummingbird family, champions of the continuous wing beat, can tire on their long seasonal migrations. The female ruby-throated hummingbird (Archilochus colubris) pictured here was probably leapfrogging its way south for the winter when it landed on a feeder in Michigan one sunny August morning and dropped into an energy-conserving state even deeper than sleep: torpor. In torpor, brain waves slow down, the body cools, and the metabolic rate plummets.

Sara Hiebert, a biologist at Swarthmore College in Pennsylvania, studies torpor and stress in a similar migratory "hummer," Selasphorus rufus. She recently found that the birds go into torpor most often just before they head south, when their energy reserves are greatest. In winter, torpor is less frequent, but more closely tied to the birds' day-to-day stress-hormone levels than it is in summer. Torpor is also known to happen almost exclusively at night in S. rufus, ending like clockwork about two hours before sunrise.

The hummer in our picture was literally at the lip of a liter of sugar water when photographer Roger Eriksson spotted it at the feeder. As Hiebert put it, the bird was probably "in energetically deep trouble." Eriksson took a few pictures, and then moved both feeder and lolling bird to a safe area. A few hours later the rubythroat revived and quickly flew off--renewed, one hopes, for its flight south.

Photograph by Roger Eriksson