Physical Principles of Water Penetration and Repellency of Porous Surfaces and their Relevance to Feathers

The present study highlights water penetration and repellency of feathers and these factors’ relation to bird behavior and habitat in light of information that has become available since 1985. We present the physical principles of water penetration and repellency of porous surfaces and their relevance to feathers. The study shows that the requirements for water repellency of feathers and those for resistance to water penetration are partly in conflict with each other. Modern studies seeking to understand the relationship between surface morphology, wettability, and droplet sliding in plants and insects as well as electron microscopy of biological structures such as butterfly wings have confirmed and expounded the original concepts that underlie the water repellency of rough and porous surfaces. Both aspects can be presented and evaluated by the same parameter, expressed in terms of the width 2r and spacing 2d of the barbs, (r + d)/r. This parameter is small for water birds, particularly for those that dive frequently and are concerned with water penetration. It is larger for terrestrial birds, which are concerned primarily with water repellency. In each family of water birds, with some exceptions, a balance between water repellency and resistance to penetration exists as an adaptation to their particular habitat and behavioral pattern. For example, we discuss the habit of cormorants and darters of spreading their wings after a period in the water in relation to the value of the parameter. We postulate that information on habitat and behavior and, indirectly, the family identity of water birds, including those of fossil taxa, can be inferred from this parameter. It is likely that many more examples of contour-feather data have been gathered. However, the above examples suffice to suggest that each feather substructure represents an evolutionary adaptation to a specific set of behavioral patterns and habitat conditions.