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Spread-Wing Postures and the Water Repellency of Feathers: A Test of Rijke's Hypothesis
A. M. Elowson
Vol. 101, No. 2 (Apr., 1984), pp. 371-383
Published by: American Ornithological Society
Stable URL: http://www.jstor.org/stable/4086376
Page Count: 13
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Rijke (1967, 1968) proposed that the water repellency of feathers and the presence or absence of spread-wing postures in water birds could be explained by structural mechanism first described for textiles. The textile model predicts that the tendency of water droplets to bead up on grid-like surfaces is a mathematical function in which the primary independent variable is an index, (r + d)/r, where r is the radius of and d is one-half the distance between cylinders in the grid. Larger indices indicate more water-repellent surfaces. Rijke found larger indices in the feathers of ducks than in the feathers of cormorants and anhingas; hence, he concluded that the latter birds must spread-wing to dry their wettable feathers. However, there were mathematical inconsistencies, undefined variables and concepts, and inadequate data in Rijke's papers. Despite these flaws, Rijke's hypothesis has been frequently cited in the ornithological literature. I report here my evaluation of the applicability of the textile model to feathers and my test of Rijke's prediction that species that assume spread-wing postures when wet have smaller (r + d)/r values for their ramus and barbule structure than species that do not. I used scanning electron microscopy to measure the feather structure of 14 species of water birds in 6 different categories (breast, back, and four regions of a remex). I found that the textile-feather analogy is not realistic, because feather structure is considerably more complex and variable than the geometric model that is fundamental to the textile equations. My (r + d)/r values show considerable overlap among three behaviorally distinct groups of water birds: those that predictably, occasionally, or never assume spread-wing postures. Statistically, the (r + d)/r values of the rami in some feather categories of the group of species that shows spread-wing behavior were smaller than those of the other two groups of birds (which did not differ). Index values of the barbule structure, which constitutes most of the feather surface, however, do not differ significantly among the three groups of birds. I also measured the shape of water droplets (by contact angles) on the breast and remex feathers of a Mallard (Anas platyrhynchos) and a Reed Cormorant (Phalacrocorax africanus) and compared these values between the two species as well as with those mathematically predicted by the textile model. In general, the observed water droplets have a shape more like that predicted by the (r + d)/r values of barbules than of rami. Droplets on the feathers of the Reed Cormorant were more bead-shaped than those on Mallard feathers, although the reverse should be true if the textile model holds for feathers. I conclude that Rijke's hypothesis is invalid for two reasons: the textile model cannot be applied reliably to feathers, and it does not account for the spread-wing behavioral differences among water birds.
The Auk © 1984 American Ornithologists' Union