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.