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Spread-Wing Postures and the Water Repellency of Feathers: a Test of Rijke's Hypothesis

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Spread-Wing Postures and the Water Repellency of Feathers: a Test of Rijke's Hypothesis SPREAD-WING POSTURES AND THE WATER REPELLENCY OF FEATHERS: A TEST OF RIJKE'S HYPOTHESIS A. M. ELOWSON Departmentof Zoology,University of Wisconsin,Madison, Wisconsin 53706 USA ABSTRACT.--Rijke(1967, 1968) proposed that the water repellency of feathers and the presenceor absenceof spread-wingpostures in water birds could be explainedby a structural mechanismfirst describedfor textiles.The textile model predictsthat the tendencyof water dropletsto bead up on grid-like surfacesis a mathematicalfunction in which the primary independent variable is an index, (r + d)/r, where r is the radius of and d is one-half the distancebetween cylinders in the grid. Largerindices indicate more water-repellent surfaces. Rijke found larger indices in the feathers of ducks than in the feathers of cormorantsand anhingas;hence, he concludedthat the latter birds mustspread-wing to dry their wettable feathers. However, there were mathematical inconsistencies, undefined variables and con- cepts,and inadequatedata in Rijke'spapers. Despite these flaws, Rijke's hypothesis has been frequentlycited in the ornithologicalliterature. I report here my evaluationof the applica- bility of the textile model to feathersand my test of Rijke's prediction that speciesthat assumespread-wing postures when wet have smaller (r + d)/r values for their ramus and barbulestructure than speciesthat do not. I usedscanning electron microscopy to measure the featherstructure of 14 speciesof water birds in 6 differentcategories (breast, back, and four regionsof a remex). I found that the textile-featheranalogy is not realistic,because feather structureis considerablymore complexand variable than the geometricmodel that is fundamentalto the textileequations. My (r + d)/r valuesshow considerable overlap among three behaviorally distinct groups of water birds: those that predictably,occasionally, or never assumespread-wing postures.Statistically, the (r + d)/r values of the rami in some feather categoriesof the group of speciesthat showsspread-wing behavior were smaller than thoseof the other two groupsof birds (which did not differ). Index valuesof the barbule structure,which constitutesmost of the feathersurface, however, do not differ significantly amongthe three groupsof birds. I alsomeasured the shapeof water droplets(by contact angles) on the breast and remex feathersof a Mallard (Anasplatyrhynchos) and a Reed Cor- morant (Phalacrocoraxafricanus) and comparedthese values between the two speciesas well as with thosemathematically predicted by the textile model. In general,the observedwater dropletshave a shapemore like that predictedby the (r + d)/r values of barbulesthan of rami. Droplets on the feathersof the Reed Cormorantwere more bead-shapedthan those on Mallard feathers, although the reverse should be true if the textile model holds for feathers.I concludethat Rijke's hypothesisis invalid for two reasons:the textile model cannotbe appliedreliably to feathers,and it doesnot accountfor the spread-wingbehavioral differencesamong water birds. Received12 April 1983,accepted 12 October1983. RIJKE(1967, 1968)published two nearly iden- however, reviewed Rijke's papers (1967, 1968; tical papers in which he proposed that a math- hereinafter reference to Rijke refers to these ematical model derived to explain the water years unless other datesare given) and pointed repellency of textiles could also resolve three out several problems, most notably, several ornithological issues:(1) the water repellency mathematical inconsistencies, an undefined of feathers;(2) the absenceof spread-wingpos- variable, and an unsupported assumption. De- tures in most speciesof water birds; and (3) the spite thesedrawbacks, Rijke's conclusionshave function of this behavior in cormorants and an- been generally accepted in the ornithological hingas. Rijke's model provided a plausible an- literature (see below). swer to the paradox that Townsend (in Bent Hailman recommendedthat I look into Rijke's 1922: 241) pointed out: if spread-wing postures paperswhen I was preparing a note on my field are necessaryto dry the wings, why is the be- observation of an Osprey (Pandionhaliaetus) havior not shown by water birds other than performing spread-wing behavior (Elowson- cormorantsand anhingas?Hailman (1969a, b), Haley 1982). In doing so, I found several more 371 The Auk 101: 371-383. April 1984 372 A.M. ELOW$ON [Auk,Vol. 101 inconsistencies(detailed below) that prompted me to reevaluate the hypothesis with respect to issues(1) and(2) above. I present the results of that study in this paper. I have not consid- ered here the function of spread-wing pos- tures,which is a complexquestion that has been investigated elsewhere (Bernstein and Maxson 1982, Hennemann 1982, Winkler 1983, and the references therein). Fig. 1. Contactangles of water dropletsresting Rijke basedhis hypothesison a resemblance on (a) wettableand (b) water-repellentsurfaces. of feather structure to textile structure. The tex- tile model (Cassie and Baxter 1944, Baxter and Cassie1945) statesthat the water repellency of by a bird at any given time). Despitethe fact porous surfaces increases with the size of an that Rijke did not evaluatethe wettability of index that is calculated from the dimensions of the feather coat as a whole, several authors have elements in the surface structure (i.e. the citedhim asproviding a mechanismthat makes threads). Cassie and Baxter (1944) even sug- this suggestionplausible (Clark 1969,Kennedy gestedthe applicability of their model to feath- 1971,Kahl 1971,George and Casler1972, Sieg- ers, which may have motivated Rijke to test the fried et al. 1975,Mahoney 1981,Bernstein and idea. Using light microscopy,he measured the Maxson1982, Hennemann 1982).Finally, Ken- feather structure of seven speciesand found nedy (1969) has cited Rijke'swork as having larger indices in ducks than in cormorants. establisheda drying function for the spread- Therefore, he concluded that cormorants, un- wing posturesof cormorantsand anhingas. like ducks,have wettable feathers that require Neither Rijke's hypothesisnor my test of it drying by spread-wing postures. is comprehensiblewithout someexplanation of Rijke's work has been noted in the literature the textilemodel itself. In what follows,I pre- in two somewhat different contexts: feather sent that model followed by Rijke's data and structure determines its water repellency and finally the predictions that should hold if the the feathers of cormorantsand anhingas are hypothesis were true. wettable. Someauthors have cited the original The textilemodel and feathers.--Thedistinc- textile papers (Cassie and Baxter 1944, Baxter tion between "water-repellent" and "water- and Cassie1945) as having establishedthe first proof" surfacesneeds clarification, as authors concept (Thompson 1953; Kennedy 1970a, b, (Rowen and Gagliardi 1947, Crisp 1963) have 1972; Rutschke 1976). Rutschke (1960) took mea- pointed out multiple uses of the latter term. surements from mallard feathers and con- Truly waterproof materials,such as a yellow curred with the estimates Cassie and Baxter rain slicker, are almost impermeable to water. (1944) gave.Many more authors,however, have Water-repellent surfaces, by contrast, cause cited Rijke's conclusionswith respectto feather water to bead up and roll off under brief ex- structure and water repellency (Clark 1969; posuresin ordinary atmosphericconditions, but Kennedy 1970b; Stettenheim 1972, 1976; suchsurfaces will becomewetted upon extend- Rutschke1976; Rhijn 1977;Schreiber 1977; Jones ed exposureor under increasedpressure. Den- 1978; Winkler 1983). Bernstein and Maxson im fabricsand feathersare water-repellentsur- (1982) discussed Rijke's conclusions in both faces,and Rijke'smodel deals with repellency contexts.Although they measuredthe feather per se. elementsof the Antarctic Blue-eyedShag (PhaI- The textile model (Cassie and Baxter 1944, acrocoraxatriceps) to contrast with Rijke's data Baxter and Cassie 1945) is considered to be the for other speciesof cormorants,they noted stateof the art for the water repellencyof po- Hailman's (1969a, b) objections to the model. roussurfaces (Crisp 1963).The water repellen- Without reference to the underlying process, cy of a surfaceis determinedby whether water McAtee and Stoddard (1945) and Owre (1967) dropletson it beadup (repellent) or flatten and proposed that cormorantsand anhingas have a spread out (wettable) (Fig. 1). Thus, surface wettable plumage or feather coat (nomencla- wettability can be operationally expressedas ture from Humphrey and Parkes 1959; the the angle made by the surfaceand a tangent to feather coat is the aggregateof feathers worn a droplet's curvature at the point of contact, April 1984] WaterRepellency ofFeathers 373 (see Fig. 2). Cassieand Baxter's (1944) mathe- matical statementfor water repellency is: cos0'u = f• cos0• - f2, (1) where O'•is the effective receding contactangle on the porous surface; 0R is the intrinsic receding contact angle on the same solid ma- terial as is in the porous surface, but smooth Fig. 2. Crosssection of a pore coveredby a water (i.e. not porous);and f• is an expressionfor the droplet many times larger in size; r is the radius of ratio of solid/watercontact and f2 for air/water the cylindersand d is •,5the distancebetween adja- contact. Thus, equation (1) is an expressionfor •centcylinders. 0 is an angleused to derivef, andf2. the shape of a water droplet on a porous sur- (The meniscusof water is ignored.) facecaused by the affinity of the solid material for water (0•) and
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