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.