(ii 
we) 
1916] Chandler: Structure of Feathers 2 
3. Oul-gland Feathers 
In the majority of birds the oil-gland, occurring on the rump, 
is furnished with a circlet of feathers at or near its apex, while in 
others the sides of the glandular swelling are furnished with small 
feathers and the apical circlet is missing. The presence or absence 
of this circlet has been used extensively as a taxonomic character. 
When present the feathers constituting the circlet are of modified 
plumule type. In Circus hudsonius, for instance, the shaft is entirely 
missing, the calamus splitting immediately imto several divisions, 
which further split into barbs (see Chandler, 1914). There is evi- 
dence, in Circus at least, that the feathers in the cirelet are definite 
in number. 
4. Nestling Feathers 
Although no thorough systematie study of the microscopic struc- 
ture of restling feathers has been attempted, a brief survey of a 
few diverse types brings out some interesting facts. The highest 
development of nestling feathers is displayed in ostriches, in which 
there is a well-formed calamus. The distal portion of a number of 
the barbs is more or less expanded and flattened (pl. 18, fig. 2d), 
giving the plumage of the chick a very coarse, bristly appearance. 
The barbules are numerous, and similar in structure to those of the 
teleoptiles, only very much shorter. In rheas, as in ostriches, barbules 
are borne only on the basal portion of the barbs, the tips being 
hairlike. 
In earinate birds the structure of the down barbules of nestling 
feathers is nearly always less specialized than that of the down of 
adults. In penguins the minute structure of the neossoptiles or 
nestling feathers is exactly similar to that of the down of the tel- 
eoptiles (pl. 34, fig. 96), the barbules being filamentous with short, 
sharp prongs at the nodes (see Studer, 1878). Im ducks, e. g., Anas 
platyrhynchos, the nestling down differs widely from that of the 
adults (pl. 35, fig. 104), being exactly similar in form to the down 
of penguins. The barbules are short and filamentous, with a series of 
prongs at the nodes, those nearer the tip of the barbules being more 
prominent than those near the base (compare pl. 35, fig. 104, with 
pl. 34, fig. 96). The yellow color so characteristic of ducklings is due 
entirely to structural interference of light. In rails, although the 
adult down is widely different from that of either ducks or penguins, 
the nestling down is very similar to that of both these birds, the char- 
