Tuesday, June 16,
2009
Why aren't birds larger? Fifteen-kilogram swans hold the current
upper size record for flying birds, although the extinct Argentavis
of the Miocene Epoch in Argentina is estimated to have weighed 70
kilograms, the size of an average human. In a forthcoming article
in PLoS Biology, Sievert Rohwer, and his colleagues at the Burke
Museum at the University of Washington, provide evidence that
maximum body size in birds is constrained by the amount of time it
takes to replace the flight feathers during molt. As bird size
increases, feather growth rate fails to keep up with feather length
until, eventually; feathers wear out before they can be replaced.
This fundamental relationship requires basic changes in the molt
strategy as size increases, ultimately limiting the size of flying
birds.
Feathers deteriorate with continued exposure to ultra-violet light
and bacterial decomposition, and must be replaced periodically to
maintain adequate aerodynamic support for flight. Small birds
accomplish this in an annual or twice-annual molt, during which the
9 or 10 primary flight feathers are replaced sequentially, taking
about three weeks for each feather. Large species of birds need
different approaches to feather replacement. These involve several
alternative strategies: prolonging the total molt to two or even
three years; simultaneously replacing multiple feathers from
different molt-origination points in the feather sequence; and, in
species that do not require flight to feed or escape enemies (ducks
and geese, for example), replacing all feathers
simultaneously.
With increasing body size, the length of the primary feathers
increases as the one-third power of mass, approximately doubling
with each 10-fold increase in mass. However, the rate of feather
growth increases only as the one-sixth power of mass, meaning that
the time required to replace each feather increases by a factor of
about 1.5 for each 10-fold increase in mass, until 56 days are
required to replace a single flight feather in a 10-kg bird. The
cause of this discrepancy is not known, but the authors speculate
that it probably depends on the geometry of producing a
two-dimensional feather structure from a one-dimensional growing
zone in the feather shaft.
The avian feather is one of the most striking adaptations in the
animal world, and yet its growth dynamics are poorly understood. It
might be possible to achieve more rapid feather growth with a
larger growth zone, but this could also weaken the structure of the
growing feather, resulting in frequent breakage in large birds.
Understanding the engineering complexities of the growing feather
will require further study of the dynamics and structure of the
growing zone. And what about Argentavis? The authors speculate that
this giant bird most likely molted all its feathers simultaneously
during a long fast, fueled by accumulated fat deposits much in the
same way as emperor penguins do today.
###
Public Library of Science
Thanks to Public Library of
Science for this article.
