Feather material
For contour feathers, abdominal feathers were selected as the most
likely to interface with water. Our primary source was the same as used
in an earlier study of water birds (Rijke 1970). Here, water birds are
defined as birds that have habitats with open water and land birds as
those that have not. A list of the species in this study is compiled in
Table 1, using English names and taxonomic sequence suggested by
Handbook of the Birds of the World (HBW) (Del Hoyo et al. 1997 – 2013).
The values for r and d of these feathers, measured at the
mid-part of the vane, were collected at the time of the 1970 study using
a transmission light microscope equipped with a calibrated scale ocular.
The data have been reproduced for convenience in Table 1. We see no
reason to suspect the accuracy and precision of these data to be
anything less than of those collected with electronic imaging
techniques.
The values for barb length l of the closed pennaceous portion of
the contour feathers were measured at the mid-part of the vane to the
nearest half millimeter using a traveling microscope. At least three
feather specimens of each species were examined. For the calculation of
body feather density and the extent of contour feather overlap, we
measured the length of the rachis Lf to the
nearest millimeter. The extent of overlapping can be approximated by the
product of Lf and the square root of the number
of feathers per surface area.
To estimate the latter, we made use of the data on number of feathers
and body weights as reported by several authors (Wetmore 1936; Hutt and
Ball 1933; Dwight 1900; McGregor 1903; Knappen 1932; Lowe 1933; Kuhn and
Hesse 1957). For the weights of the birds we used as our source HBW (Del
Hoyo et al. 1997 – 2013), the weight ranges for both male and female
birds being averaged for our purpose. By fitting a second-order
polynomial to these data (ignoring those on very small birds and
penguins), an estimate of the number of contour feathers as a function
of the mass of the bird was obtained. For the relationship between body
surface area and body mass, expressions proposed by Perez, Moye and
Pritsos (2014) and Mitchell (1936) were used to estimate surface area as
a function of body weight. Combining the results of these two sets of
calculations, contour feather densities expressed in number of feathers
per surface area were found to be approximately 100,000 to 150,000 per
m2 for water birds weighing less than 1.2 kg for all
species studied. This number increases with body weight to
200,000/m2 at about 7 kg. The extent of feather
overlap, according to these calculations, yields about 10 to 15 feathers
in a stack for families in the lower weight range with twice that number
for heavier birds. Land birds show an average of nine feathers in a
stack.