262 Unversity of California Publications in Zoology (Vor. 138 
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 haye 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 Phaéthon 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 
