Introduction
The contour feathers of birds serve a variety of functions that range from intraspecific signaling to such physical qualities as thermal insulation, water repellency, and resistance to impact. It is no surprise, therefore, that they are composed of an array of elements that confer these qualities to the optimal benefit of their avian bearer.
The structural details of contour feathers have been well described in the ornithological literature (Thomson 1964; Stettenheim 1972). The downy (plumulaceous) parts alongside the proximal two-third of the rachis are thought to function as a means to regulate body temperature by entrapping air (King and Farner 1961; Lucas and Stettenheim 1972; Stettenheim 2000; Lei et al. 2002). The distal one-thirds have a patterned, pennaceous structure with barbs extending from the rachis, each sprouting barbules of which the distal ones have hooks that catch upon the curled, proximal barbules of the barb next more distal. They are arranged in an overlapping fashion like shingles on a roof, having their dorsal aspect exposed to air or water. This continuous-looking, hook-and-flange arrangement provides the distal one-third with the rigidity so critical for its mechanical properties. It also confers water repellency and resistance to water penetration to the body plumage.
The water repellency of this part of the contour feather can be rated by the value of the wettability parameter (r + d )/r,where 2r denotes the diameter of the approximately circular or elliptical cross-section of the barb and 2d the separation of the barbs measured in the plane of the long axes of the barbs (Cassie and Baxter 1945; Moilliet 1963; Rijke 1970). Water repellency, expressed in terms of the contact angle with which a drop of water rests on a feather surface, is proportional to (r + d )/r , but resistance to water penetration, expressed as the pressure required to force water through the barbs and barbules, is inversely proportional to this parameter as well as to r . Values for the parameter range from about 2.5 for penguins (Spheniscidae ) to 7 or more for typical land birds, implying that the contour feathers of penguins have poor water repellency, but excellent resistance to water penetration. For the contour feathers of land birds, it is the other way around.
The contribution of barbules to water repellency and resistance to water penetration is not based on the same mechanism as applies to barbs. Instead, barbules provide an interlocking mechanism by preventing the barbs from separating under mechanical forces, for instance, when water penetrates between barbs. They do so by increasing their own separation with their hooks sliding in the flanges of the adjoining barbules. As a result, the pressure required to force water through the feather is determined only by the diameter and spacing of the barbs without recourse to the barbules. Noteworthy is that the wettability parameter for barbules is more or less constant for all bird families at about 4.5 and does not vary with the feeding habits of water bird families as it does for barbs (Rijke et al. 1989).
In this paper, we consider the effects of mechanical forces, specifically the impact of diving, plunging, and alighting, on contour feathers and the structural properties that are identifiable as adaptations to these forces by comparisons of different, ab inito determined, foraging niches. In studies where the trait patterns between biological groups such as foraging niches are compared, it is important to consider the phylogeny of the species in the groups. The standard of establishing evolutionary trends across phylogeny is based on generalized least squares estimation of coefficients for linear models (Blomberg et al. 2003; Adams and Collyer 2018). However, this analysis and its variations are prone to statistical inaccuracy due to high type I errors and use of phylogenetic simulation. Therefore, this simulation infers incorrect parameter estimates if phylogeny is not conditioned in the analysis. Our hypothesis is that the contour feathers of water birds exhibit, in addition to water repellency and resistance to water penetration, morphological and mechanical features that are advantageous for specific aqueous habitats and behavioral patterns.