Wearing resistance, wettability, morphology and mechanical properties of feathers of the American barn owl (Tyto furcata pratincola)

The American barn owl (Tyto furcata pratincola) evolved morphological specializations that facilitate silent flight. Their hunting success depends on these specializations so one would expect the barn owl to invest more in the maintenance of their overall plumage condition compared to other same sized birds like the domestic pigeon (Columba livia domestica). This would include for example to have significantly shortened molting cycles in order to replace worn and broken feathers. However, molting cycles of barn owls are significantly longer than those of domestic pigeons. One possible explanation would be that barn owl feathers are inherently more resistant against mechanical wear. During this study, the decrease of the water contact angle after artificially induced mechanical wear was used as a proxy for mechanical wear. Additionally, the feather specimens were investigated in regard to macroscopically visible signs of wear by rating them after each step of artificially induced mechanical wear using a feather condition score. The data suggests that barn owl flight feathers are more resistant against mechanical wear than pigeon flight feathers. This occurs most likely due to the higher stiffness of pigeon feathers which is most likely an adaptation to high speed flight. During the contact angle measurements that were done to evaluate the impact of artificially induced mechanical wear on the contact angle, the special wetting properties of barn owl flight feathers came into focus. Although feathers have extraordinary non-wetting properties in terms of high contact angles, water drops roll-off easily from pigeon feathers but stick to barn owl flight feathers. This can be described using the roll-off angle and the contact angle hysteresis. The velvet-like surface structure of flight and body feathers of the barn owl and the pigeon were removed by shaving. A comparison of the roll-off angle and contact angle hysteresis before and after shaving suggested that the velvet-like surface structure is responsible for the adhesion of water drops to barn owl flight feathers. This phenomenon did not distinctly occur on feathers that lack a velvet-like surface structure like pigeon feathers or barn owl body feathers. The material properties of feathers, especially of silently flying species like the American barn owl, are relatively poorly understood. Since most of the interaction between the wing and the air depends on the material properties and morphology of the barbs that form the airfoil, investigation of this is of special interest. The modulus of elasticity of the barbs of the 9th primary feather of the American barn owl and the domestic pigeon were analyzed. Tensile tests were used to measure force and strain of barbs. Together with the measurement of the cortex area of the barbs, the stress was calculated and used to determine the modulus of elasticity. Surprisingly, the modulus of elasticity was not constant along a feather barb but increased from the base of the barb towards the tip. It was generally higher in pigeon flight feather barbs in comparison to barn owl flight feather barbs. This is most likely an adaptation to high speed flight of pigeons and at the same time reinforces the hypothesis that the better wearing resistance of barn owl feathers is due to a lower stiffness, respectively a lower modulus of elasticity which leads to softer and more flexible feathers which reduces the likelihood of breakage. Since a feather barb is neither an isotropic nor a homogeneous structure, it became important to analyze the morphological structure, as well. Five parameters of the barb cross-section (total area, cortex area, pith area, outer and inner perimeter of the cortex) were characterized by fluorescence microscopy utilizing the autofluorescence of feather keratin. All morphological parameters were maximal at the base and decreased linearly towards the barb tip in both species. Differences of the five parameters between the species were mainly found in barbs of the outer vane. The ventral region of the barbs was distinctly elongated and bent in barn owl barbs, while no such elongation was visible in pigeon barbs. I hypothesize that this comma-shaped elongation is capable of covering the gaps that occur between two neighboring barbs. This would have an impact on the airflow over the ventral side of the wing. If this specialization is capable of avoiding sound-generating vortices that emerge between gaps of neighboring barbs, it would be potentially an additional adaptation to enable silent flight in barn owls