**4. Discussion**

The present study has shown that adaptations in feather microstructure and body surface area in contact with water that bring about a reduction in viscous and frictional drag while swimming increase according to the bird's intimacy with open water. Swimming and diving birds, such as penguins and grebes, benefit the most from reduced viscous drag, more so than plungers such as gannets. Aerials such as terns even less so, but much more than herbivore surface feeders such as ducks.

The body feathers of ducks, in turn, appear to be better adapted to their watery habitat than those of aquatic ground feeders such as herons or kingfishers. The single herbivore aquatic ground feeder in this study, the Spotted Dikkop, is a bird of open scrubby habitat with comparatively little interaction with open water. Its drag coefficient is more in line with those of land birds in which adaptations to locomotion in water are not expected to have evolved.

Land birds do not only show drag coefficients higher than those of water birds, they also show no significant difference among the two foraging niches examined in this study. This is in line with expectation as their lack of interaction with open water and their locomotion in air only suggest that forces that foster reduced drag in water have been absent in their evolutionary history.

Of the three methods of statistical analyses, the phy-ANOVA test shows us that allowing for phylogenetic relatedness negates any differences among feather microstructure that may exist. Only for land birds would non-significance be expected. There is no doubt that group aggregation among the 48 water bird species is quite strong which detracts from the reliability of our positive and negative findings. Adding more species to the study or identifying more foraging niches could, statistically speaking, affect the results either way depending on numbers of species and their phylogenetic relatedness. Alternatively, it could be argued that relatedness is not necessarily a force that would make the evolution of an isolated trait impossible. Several examples support this notion. For instance, the Flightless cormorant (*Phalacrocorax harrisi*) is undoubtedly closely related to all other cormorants, yet a small change in the diameter and spacing of its barbs has rendered the bird better adapted to its bottom feeding habits than other cormorants are. The contour feathers of Brown pelicans (*Pelecanus occidentalis)* that, unlike their congeners, dive from the air to procure their prey, are more water repellent than those of other pelicans that catch their fish while swimming. Similar considerations apply to the differences in the contour feathers of dippers (*Cinclidae*) [6].

As argued above, a conventional statistical test while avoiding the condition of equal sample size and variance among populations, may the more suitable. Following this line of thought, the non-parametric variety of analysis would show that among group 1 consisting of all 48 aquatic birds, no significance is apparent, but when compared to land birds, it is. Subdividing into swimmers and waders shows comparison of the first group with land birds to be significant whereas that of waders with land birds is not. However, comparison between swimmers and waders is significant again indicating that, in terms of feather microstructure, waders stand between swimmers and land birds, but closer to land birds. This interpretation is entirely plausible, particularly if we assume that water birds have evolved from land birds.

In summary, the length-to-width ratio of the dorsal aspect of the distal onethird of abdominal feathers, the part that is in contact with water in aquatic birds, varies with the extent of interaction with open water as formulated by our hypothesis. This ratio and the total drag coefficient, composed of viscous pressure and frictional drag and calculated from Reynolds-averaged Navier–Stokes equations, are lowest for swimming and diving birds and increase for birds with less intimacy with open water. The highest values were found for land birds that have no open water in their habitat.

Due to the limited number of foraging niches and close phylogenetic relatedness among water bird families, statistically significant differences among water birds was not observed if allowance for phylogeny was made. However, using conventional statistical tests, in particular the non-parametric variety that does not assume conditions of equal sample size and variance, did show significant results when comparing water birds with land birds, swimming birds with land birds and

**47**

**Author details**

Roelof D. Coertze1

Charlottesville, USA

and Arie M. Rijke2

\*Address all correspondence to: amr@virginia.edu

University, Potchefstroom, South Africa

provided the original work is properly cited.

\*

1 Department of Microbiology, School of Biological Sciences, North-West

2 Department of Materials Science and Engineering, University of Virginia,

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds*

swimming birds with waders, but not waders with land birds. This finding suggests, in terms of feather microstructure, a closer evolutionary relationship between waders and land birds than between waders and swimmers. In line with expectation, land birds showed no significant differences in their contour feather geometry that

*DOI: http://dx.doi.org/10.5772/intechopen.96994*

could be related to interaction with open water.

*Viscous Drag Reduction and Contour Feather Geometry in Water and Land Birds DOI: http://dx.doi.org/10.5772/intechopen.96994*

swimming birds with waders, but not waders with land birds. This finding suggests, in terms of feather microstructure, a closer evolutionary relationship between waders and land birds than between waders and swimmers. In line with expectation, land birds showed no significant differences in their contour feather geometry that could be related to interaction with open water.
