1916] Chandler: Structure of Feathers 259 



3. Oil-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 into several divisions, 

 which further split into barbs (see Chandler, 1914). There is evi- 

 dence, in Circus at least, that the feathers in the circlet are definite 

 in number. 



4. Nestling Feathers 



Although no thorough systematic study of the microscopic struc- 

 ture of nestling 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 (pi. 13, 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 

 teleop tiles, 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 carinate 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 (pi. 34, fig, 96), the barbules being filamentous with short, 

 sharp prongs at the nodes (see Studer, 1878). In ducks, e. g.. Anas 

 platyrhynchos, the nestling down differs widely from that of the 

 adults (pi. 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 pi. 35, fig. 104, with 

 pi. 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- 



