The number of species of humming-birds now known to exist considerably
exceeds 400; and, though none departs very widely from what a
morphologist would deem the typical structure of the family, the
amount of modification, within certain limits, presented by the
various forms is surprising and even bewildering to the uninitiated.
But the features that are ordinarily chosen by systematic
ornithologists in drawing up their schemes of classification are found
by the "trochilidists," or special students of the _Trochilidae_,
insufficient for the purpose of arranging these birds in groups, and
characters on which genera can be founded have to be sought in the
style and coloration of plumage, as well as in the form and
proportions of those parts which are most generally deemed sufficient
to furnish them. Looking to the large number of species to be taken
into account, convenience has demanded what science would withhold,
and the genera established by the ornithologists of a preceding
generation nave been broken up by their successors into multitudinous
sections--the more adventurous making from 150 to 180 of such groups,
the modest being content with 120 or thereabouts, but the last
dignifying each of them by the title of genus. It is of course obvious
that these small divisions cannot be here considered in detail, nor
would much advantage accrue by giving statistics from the works of
recent trochilidists, such as Gould,[10] Mulsant[11] and Elliot.[12]
It would be as unprofitable here to trace the successive steps by
which the original genus _Trochilus_ of Linnaeus, or the two genera
_Polytmus_ and _Mellisuga_ of Brisson, have been split into others, or
have been added to, by modern writers, for not one of these professes
to have arrived at any final, but only a provisional, arrangement; it
seems, however, expedient to notice the fact that some of the authors
of the 18th century[13] supposed themselves to have seen the way to
dividing what we now know as the family _Trochilidae_ into two groups,
the distinction between which was that in the one the bill was arched
and in the other straight, since that difference has been insisted on
in many works. This was especially the view taken by Brisson and
Buffon, who termed the birds having the arched bill "colibris," and
those having it straight "oiseaux-mouches." The distinction wholly
breaks down, not merely because there are _Trochilidae_ which possess
almost every gradation of decurvation of the bill, but some which have
the bill upturned after the manner of that strange bird the
avocet,[14] while it may be remarked that several of the species
placed by those authorities among the "colibris" are not humming-birds
at all.
In describing the extraordinary brilliant plumage which most of the
_Trochilidae_ exhibit, ornithologists have been compelled to adopt the
vocabulary of the jeweller in order to give an idea of the
indescribable radiance that so often breaks forth from some part or
other of the investments of these feathered gems. In all, save a few
other birds, the most imaginative writer sees gleams which he may
adequately designate metallic, from their resemblance to burnished
gold, bronze, copper or steel, but such similitudes wholly fail when
he has to do with the _Trochilidae_, and there is hardly a precious
stone--ruby, amethyst, sapphire, emerald or topaz--the name of which
may not fitly, and without any exaggeration, be employed in regard to
humming-birds. In some cases this radiance beams from the brow, in
some it glows from the throat, in others it shines from the
tail-coverts, in others it sparkles from the tip only of elongated
feathers that crest the head or surround the neck as with a frill,
while again in others it may appear as a luminous streak across the
cheek or auriculars. The feathers that cover the upper parts of the
body very frequently have a metallic lustre of golden-green, which in
other birds would be thought sufficiently beautiful, but in the
_Trochilidae_ its sheen is overpowered by the almost dazzling
splendour that radiates from the spots where Nature's lapidary has set
her jewels. The flight feathers are almost invariably dusky--the
rapidity of their movement would, perhaps, render any display of
colour ineffective: while, on the contrary, the feathers of the tail,
which, as the bird hovers over its food-bearing flowers, is almost
always expanded, and is therefore comparatively motionless, often
exhibit a rich translucency, as of stained glass, but iridescent in a
manner that no stained glass ever is--cinnamon merging into crimson,
crimson changing to purple, purple to violet, and so to indigo and
bottle-green. But this part of the humming-bird is subject to quite as
much modification in form as in colour, though always consisting of
ten _rectrices_. It may be nearly square, or at least but slightly
rounded, or wedge-shaped with the middle quills prolonged beyond the
rest; or, again, it may be deeply forked, sometimes by the overgrowth
of one or more of the intermediate pairs, but most generally by the
development of the outer pair. In the last case the lateral feathers
may be either broadly webbed to their tip or acuminate, or again, in
some forms, may lessen to the filiform shaft, and suddenly enlarge
into a terminal spatulation as in the forms known as "racquet tails."
The wings do not offer so much variation; still there are a few groups
in which diversities occur that require notice. The primaries are
invariably ten in number, the outermost being the longest, except in
the single instance of _Aithurus_, where it is shorter than the next.
The group known as "sabre-wings," comprising the genera
_Campylopterus_, _Eupetomena_ and _Sphenoproctus_, present a most
curious sexual peculiarity, for while the female has nothing
remarkable in the form of the wing, in the male the shaft of two or
three of the outer primaries is dilated proximally, and bowed near the
middle in a manner almost unique among birds. The feet again,
diminutive as they are, are very diversified in form. In most the
tarsus is bare, but in some groups, as _Eriocnemis_, it is clothed
with tufts of the most delicate down, sometimes black, sometimes buff,
but more often of a snowy whiteness. In some the toes are weak, nearly
equal in length, and furnished with small rounded nails; in others
they are largely developed, and armed with long and sharp claws.
Apart from the well-known brilliancy of plumage, of which enough has
been here said, many humming-birds display a large amount of
ornamentation in the addition to their attire of crests of various
shape and size, elongated ear-tufts, projecting neck-frills, and
pendant beards--forked or forming a single point. But it would be
impossible here to dwell on a tenth of these beautiful modifications,
each of which as it comes to our knowledge excites fresh surprise and
exemplifies the ancient adage--_maxime miranda in minimis Natura_. It
must be remarked, however, that there are certain forms which possess
little or no brilliant colouring at all, but, as most tropical birds
go, are very soberly clad. These are known to trochilidists as
"hermits," and by Gould have been separated as a subfamily under the
name of _Phaethornithinae_, though Elliot says he cannot find any
characters to distinguish it from the _Trochilidae_ proper. But sight
is not the only sense that is affected by humming-birds. The large
species known as _Pterophanes temmincki_ has a strong musky odour,
very similar to that given off by the petrels, though, so far as
appears to be known, that is the only one of them that possesses this
property.[15]
[Illustration: From _The Cambridge National History_, vol. ix.,
"Birds," by permission of Macmillan & Co. Ltd.