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Re: Cormorants, anhingas & soaked feathers
There is a paper on this very topic in a recent issue of *Journal of Avian
David Grémillet, Christophe Chauvin, Rory P. Wilson, Yvon Le Maho, Sarah
Unusual feather structure allows partial plumage wettability in diving
great cormorants Phalacrocorax carbo
Journal of Avian Biology 36:57-63.
"The great cormorant Phalacrocorax carbo is thought to have a wettable
plumage, providing low body insulation during foraging. Great cormorants
should thus be constrained by water temperatures, and show high energy
requirements. Surprisingly, this species has one of the widest breeding
distributions of all diving birds, and does not require more food than
these other species. We explored this apparent paradox by comparing the
insulative properties of body plumage in four subspecies of great
cormorants ranging from tropical to polar regions. We found that all
subspecies retained an insulating air layer in their plumage, which was,
however, much thinner than for other species of diving birds. Detailed
examination of the plumage showed that each cormorant body feather has a
loose, instantaneously wet, outer section and a highly waterproof central
portion. This indicates that the plumage of great cormorants is only partly
wettable, and that birds maintain a thin layer of air in their plumage. Our
findings suggest an unusual morphological-functional adaptation to diving
which balances the antagonist constraints of thermoregulation and buoyancy."
In brief, cormorants sacrifice some insulation but gain better
maneuvrability, important in a group of birds that catch fish mostly by
active hunting near the bottom.
Probably something similar applies to anhingas, they fish quite differently
from cormorants (spearing) but have a unique ability to regulate their
buoyancy with high precision (an anhinga can sink until just the bill
protrudes and then "re-float" itself, both without moving). This would
probably be impossible with a large and probably not completely
controllable amount of air in the plumage.