302 BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 
even cling to rocks or other vertical surfaces; and their nidification 
presents nothing that may be deemed peculiar or even specially 
characteristic. In their flight and manner of procuring their food, 
however, they differ strikingly from all other birds, in these respects 
closely resembling certain insects, especially the crepuscular hawk- 
moths (Sphingide). Their food, consisting mainly of small insects 
but in part also of the nectar of flowers, is mostly gleaned from 
blossoms, before which they poise, with wings so rapidly vibrating 
as to be invisible except as a dim haze or halo partly surrounding 
the body and producing the humming sound from which these birds 
derive their vernacular name, the bill thrust inside the flower and 
the slender, semitubular tongue extended into the depths of the 
blossom. Some species, instead of feeding from flowers, glean their 
insect food from the bark of forest trees, following along the branches 
in suspended flight in the same manner that the others pass from 
flower to flower. In their feeding from flower to flower, Humming 
Birds, like bees, butterflies, and moths, perform the same office in 
the economy of nature as insects by transferrmg pollen from one 
bloom to another, and thus assisting in the fertilization of plants. 
In flying from one point to another, the flight of Humming Birds, 
while essentially direct, is usually more or less undulating, and so 
extremely rapid that the eye can scarcely follow. Often this flight 
is accompanied (at least in the case of males of some species) by a 
more or less remarkable screeching or grating sound, produced 
mechanically by some peculiarity of wing-structure. 
Diminutiveness of size and metallic brilliancy of coloring are the 
chief external characteristics of Humming Birds, though exceptions 
to both occur; and in these respects they, as a group, have no 
rivals. Unfortunately stuffed specimens convey but a faint idea of 
their splendid coloring, for the perfection of their changeable reful- 
gence can be fully realized only in the living bird, whose every 
change of position flashes to view a different hue—emerald green 
replacing ruby red, sapphire blue succeeding fiery orange, or either 
becoming opaque velvety black—according to the angle at which 
the sun’s rays touch the feathers, an effect which can only partially 
be imitated with the stuffed specimen by artificially changing its 
position with reference to the light. Many species have a spot of 
the most luminous or brilliantly metallic color (usually green) that 
it is possible to imagine on the forehead at the base of the bill, this 
spot being surrounded by the most intense velvety black—evidently 
to enhance the brilliancy of the ornament by contrast, just as a 
jeweler would, for the same purpose, display a diamond or other 
gem against a background of black velvet. Often there is a spot of 
brilliant color and one of a contrasting hue just below it, the result 
