262 University of California Piiblications in Zoology [Vol. 13 



Although usually entirely covered by the contour feathers, 

 filoplumes are occasionally developed to an extraordinary degree. 

 In many passerine birds they may be seen with a hand-lens project- 

 ing beyond the tips of the contour feathers on the nape, while in 

 other closely allied species they may not be exposed at all. In 

 Planesticus migratorius and Sialia mexicana occidentalis, for example, 

 they are plainly visible in the unruffled plumage, while in Hylocichla 

 guttata they are not exposed at all. In many genera of Pycnonotidae 

 they are long and hairlike, giving a conspicuously hairy appearance 

 to the plumage in the region of the nape and upper back. 



The only place in which I have found filoplumes really conspicuous 

 is in the plumage of cormorants, more or less in all of the species 

 examined, especially on the neck and upper back, although to some 

 extent on the breast, belly, and rump as well. In these birds many 

 of the filoplumes are long and largely exposed, and have the vanes 

 developed to a very unusual extent. In males they are pure white and 

 show up conspicuously as white streaks against the deep greenish- 

 black color of the contour feathers, while in females they are buffy 

 brown and inconspicuous against the brown plumage. Although 

 their development is very variable, some of them have barbs borne 

 on the terminal four-fifths of the shaft, though there are only about 

 15 per centimeter on each side, i. e., they are 0.6 mm. or more apart. 

 They are set at a very sharp angle with the shaft, so that they make 

 narrow but fairly dense vanes. The barbs bear very numerous 

 barbules, about 35 per millimeter on each side. They are of a filament- 

 ous downy type, but only 0.02 mm. long, and not spread apart to form 

 broad vanules. Although the barbules of the plumules of these 

 species are filamentous, with practically no indication of nodes except 

 at the extreme tip, the nodes on the barbules of the filoplumes are 

 characterized by well-developed sharp prongs, thus resembling the 

 nestling feathers. Though far from what would be expected, filo- 

 plumes are by no means conspicuously developed in allied families 

 of Steganopodes ; in fact, I have searched in vain for them in dried 

 skins of Pelecanus erythrorhynchus and P. californicus, Plotus an- 

 hinga and Phaethon longicauda. Since filoplumes occur in the most 

 diverse group of birds, and yet attain such a variable degree of 

 development in birds within the same order, it is a natural presump- 

 tion that they are of some use in the economy of nature, and are not 



