Re: Cormorants, anhingas & soaked feathers

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

There is a paper on this very topic in a recent issue of *Journal of Avian 
Biology* :

David Grémillet, Christophe Chauvin, Rory P. Wilson, Yvon Le Maho, Sarah 
Wanless
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.

Tommy Tyrberg