Discussion
Birds, in particular water birds, are protected from water penetrating
to the skin by the diameter and spacing of the barbs as determined by
the parameter (r + d )/r and the absolute value ofr (Baxter and Cassie 1945; Rijke and Jesser 2011). The smaller
these values are, the greater the resistance to water penetration. Only
darters are known to benefit from water reaching the skin in order to
reduce buoyancy while stalking prey on the bottoms of shallow lakes and
streams. Their contour feathers show unusually large values for the
parameter and lack barbules all together. However, all other species,
with the possible exception of the Flightless Cormorant (P.
harrisi ), have a contour feather structure that optimizes water
repellency and resistance to suit the specific requirements of their
habitat and behavior.
Swimming birds, by their weights, exert a pressure on their surface area
in contact with water that remains well below that required to force
water through the barbs. This is particularly true for the most aquatic
of families, but, as shown in Table 3, decreasingly so for the families
less intimate with open water. Swimmers are subject to a more or less
static equilibrium between the pressure exerted by the weight of the
bird and the capability of the outer contour feathers to resist
penetration. Once the pressure exceeds this resistance, the underlying
layers of feathers will eventually be penetrated as well and will
provide no further protection against wetting (Rijke, Jesser and Mahoney
1989).
Diving birds, on the other hand, are subject to much greater, albeit
temporary, pressures. On immersion, their bodies will be quickly
surrounded by water. Initially, some air will be expelled, but the
remaining air within the plumage as well as in air sacs and airways,
will be trapped and compressed by hydrostatic forces. As they dive
deeper, the pressure difference across the water-feather interface will
no longer increase, but balance out as the compliant feather coat
further compresses the trapped air at greater depths, thereby decreasing
the volume and the buoyancy of the bird.
For birds alighting on water, the pressure on impact will not be
balanced by compressed trapped air, but will instead produce a pressure
gradient with the atmospheric air in the plumage. It is not known if
this gradient is large enough to force water through the barbs of a
single contour feather or a stack of multiple feathers. The available
experimental data, few as they are, seem to suggest that each additional
feather layer adds another 50 percent increase to water resistance
(Rijke, Jesser and Mahoney 1989). Experiments of this kind, in which
water is forced through feathers, may well closely resemble the
conditions of birds landing on water. However, water on impact could
also reach the skin by the flexing and bending of stacked feathers. How
much each of these two dynamic mechanisms contributes to water
penetration, if at all, is unknown. It is likely, however, that stacked
layers mostly serve to reduce the bending and flexing of vanes in diving
and alighting birds - and thus aid in preventing water from reaching the
skin - but not in swimming birds.
As the data in Table 1 show, the contour feathers of penguins have barbs
that are much shorter and thicker, and therefore more resistant to
bending than those of less aquatic species: thirty times more so than
those of divers, grebes and cormorants, fifty times more so than those
of finfoots, jacanas and storm petrels, and six hundred times more so
than those of waders. Compared with those of land birds, these contour
feathers are more resistant to bending by as much as three orders of
magnitude. We posit that these differences in magnitude as well as the
wide range of resistance to bending represent evolutionary adaptations
to the forces of impact associated with specific feeding habits and
habitats.
The families in each foraging niche share a similar behavior with
respect to their feeding habits and interaction with water. This is
evident for families in the Aquatic Dive foraging niche, but less so for
the taxonomically more distant families in other niches. Penguins,
divers, grebes and cormorants all pursue their prey in much the same
way, but this holds less true for families in the other foraging niches.
In parallel with this observation, we find that the values forl /r are small for species in the Aquatic Dive niche, but
larger in the others. In other words, the most aquatic species have
stiff and very similar vanes in their contour feathers that resist
bending, providing increased protection against water reaching the skin,
whereas species with less interaction with open water have, apart from
more diverse feeding habits, more flexible and dissimilar vanes with no
such protection.
For swimming birds, we have seen that water may ultimately reach the
skin if the weight of the bird exceeds the pressure required to force
water through the barbs of the outer contour feathers, but in plunging
and diving birds or birds landing on water surfaces, water penetration
may also be caused by bending of the vanes. Closely stacked contour
feathers should impede bending, but to which extent is difficult to
measure experimentally. One would expect the denser the feather coat and
the more the feathers overlap the more restriction to bending is
attained. However, our calculations have shown there are approximately
100,000 to 150,000 feathers per m2 for water birds
weighing less than 1.2 kg regardless of group. Furthermore, the extent
of overlapping amounts to about 10 to 15 feathers in a stack for birds
in all groups with approximately double that number for heavy birds.
Apparently, feather overlapping is the same for all water birds and, as
a result, the restriction stacking provides to bending is also the same.
Only for birds weighing more than 1.2 kg do we find an increase in
feather density and overlap with weight: up to 250,000 per
m2 and stacks of 18 for the pink-backed pelican
(P. rufescens ). This observation is in line with expectation as
impact forces are directly proportional to mass.
The above findings may be explained by any of two or both possibilities:
1) the feather density and number of feathers in a stack are
sufficiently large to prevent feather bending regardless of behavioral
pattern and 2) the barb stiffness and resistance to water penetration of
the contour feathers of each species are large enough to prevent water
reaching the skin on their own account and do not benefit from a further
increase in feather density or stacking.
The results of phylogenetic ANOVA have demonstrated that regardless of
the phylogenetic relationships between bird species in this study, there
is a significant difference in feather microstructure between the water
bird groups. That no such significant difference was found for the land
bird feeding niches supports the hypothesis of this study that the
contour feathers of water birds exhibit features that are advantageous
for specific aqueous habitats and behavioral patterns such as diving,
plunging and alighting.
In summary, we have observed that the length and diameter of the barbs
of contour feathers vary considerably among water birds with their
stiffness parameters covering an eight-fold range however evolutionarily
adapted to a specific niche. By referring to the mechanical properties
of materials in general, we were able to show that short and thick barbs
are stiff and resist bending, whereas long and thin barbs are flexible
which facilitates bending. The value for l/r and, in particular
the deflection parameter (l/r )3. (r +
d )/r , is small for penguins, the most aquatic of bird families,
but increases by orders of magnitude for birds with less interaction
with open water. The families in each of these groups are taxonomically
different, but have in common their method of feeding. This is
particularly true for the species in the Aquatic Dive niche, but less so
for other niche representatives, which populate a wider range of
habitats and have more diverse feeding habits. This effect was not
observed among terrestrial birds, although other terrestrial traits may
remain conserved due to the birds’ respective niches.