Re: undulating flight

Vance Tucker at Duke University showed that at lower flight speeds
ascending flight and descending flight were each more energy efficient than
level flight, however counterintuitive that might seem. (At higher flight
speeds, level flight was more energy efficient.) This was not determined
by theory but by actual measurements of flight speeds, flight direction
(up, down or level), and oxygen consumption. It was also found that at
slower speeds ascending flight was more energy efficient than decending
flight! So at slow speeds a bird that ascends in flight, then glides (no
wing strokes), thus an undulating flight pattern, is most energy efficient.
The upshot of all this is that for long distance migrants, where covering
distance is somewhat of a priority, higher speeds were probably
advantageous, and if so, then level flight is most efficient. But for birds
simply moving from one grove of trees to another in patchy habitat, or from
one set of trees in a forest with abundant cones or bark insects to another
such set, like finches and woodpeckers do, speed is of less priority and in
any event less apt to be attainable in short flights, so flight is slower
and the undulating pattern is more energy efficient.

This doesn't mean that other factors aren't also important in determining
which species and lifestyles are best served by undulating flight. As I
recall, somebody on tweeters (forgive me, I cleared most of my messages
before I should have, so have forgotten who said what) suggested undulating
flight might be useful as evasive behavior. Perhaps it IS part of the mix
too. Perhaps, all other factors being equal, birds in undulating flight can
make evasive maneuvers better than birds in level flight. I have certainly
seen finches change direction on a dime while in undulating flight. And I
have seen them lose a pursuing predator that way. An accipiter that is
right on the tail of a finch sometimes suddenly finds itself flying all
alone, the finch having turned at a 90 (or more) degree angle. It can be
quite impressive.

I have also seen it suggested in print somewhere that undulating flight is
adaptive for woodpeckers because it positions them properly for landing on
vertical tree trunks.


Richard (Dick) E. Johnson
Conner Museum
Washington State University
Pullman, WA 99164-4236
johnsonre at wsu.edu