tamed, they cannot be preserved long in captivity, even in their
own country, when fed only on syrup. Audubon states, that when thus
fed they only live a month or two and die apparently starved; while
if kept in a room whose open windows are covered with a fine net,
so as to allow small insects to enter, they have been kept for a
whole year without any ill-effects. Another writer, Mr. Webber,
captured and tamed a number of the Ruby-throat in the United States.
He found that when fed for three weeks on syrup they drooped, but
after being let free for a day or two they would return to the open
cage for more of the syrup. Some which had been thus tamed and set
free, returned the following year, and at once flew straight to the
remembered little cup of sweets. Mr. Gosse in Jamaica also kept
some in captivity, and found the necessity of giving them insect
food; and he remarks that they were very fond of a small ant that
swarmed on the syrup with which they were fed. It is strange that,
with all this previous experience and information, those who have
attempted to bring live humming-birds to this country have fed them
exclusively on syrup; and the weakness produced by this insufficient
food has no doubt been the chief cause of their death on, or very
soon after, arrival. A box of ants would not be difficult to bring
as food for them; but even finely-chopped meat or yolk of egg would
probably serve, in the absence of insects, to supply the necessary
proportion of animal food.
_Nests._--The nests of the humming-birds are, as might be expected,
beautiful objects, some being no larger inside than the half of a
walnut-shell. These small cup-shaped nests are often placed in the
fork of a branch, and the outside is sometimes beautifully decorated
with pieces of lichen, the body of the nest being formed of cottony
substances and the inside lined with the finest and most silky
fibres. Others suspend their nests to creepers hanging over water,
or even over the sea; and the Pichincha humming-bird once attached
its nest to a straw-rope hanging from the roof of a shed. Others
again build nests of a hammock-form attached to the face of rocks
by spiders’ web; while the little forest-haunting species fasten
their nests to the points or to the under-sides of palm-leaves or
other suitable foliage. They lay only one or two white eggs.
_Geographical Distribution and Variation._--Most persons know that
humming-birds are found only in America; but it is not so generally
known that they are almost exclusively tropical birds, and that the
few species that are found in the temperate (northern and southern)
parts of the continent are migrants, which retire in the winter to
the warmer lands near or within the tropics. In the extreme north of
America two species are regular summer visitants, one on the east
and the other on the west of the Rocky Mountains. On the east the
common N. American or Ruby-throated humming-bird extends through
the United States and Canada, and as far as 57° north latitude, or
considerably north of Lake Winnipeg; while the milder climate of
the west coast allows the Rufous Flame-bearer to extend its range
to beyond Sitka to the parallel of 61°. Here they spend the whole
summer, and breed, being found on the Columbia River in the latter
end of April, but retire to Mexico in the winter. Supposing that
those which go furthest north do not return further south than the
borders of the tropics, these little birds must make a journey of
full three thousand miles each spring and autumn. The antarctic
humming-bird visits the inhospitable shores of Tierra-del-Fuego,
where it has been seen visiting the flowers of fuchsias in a
snow-storm, while it spends the winter in the warmer parts of Chili
and Bolivia.
In the south of California and in the Central United States three
or four other species are found in summer; but it is only when
we enter the tropics that the number of different kinds becomes
considerable. In Mexico there are more than thirty species, while
in the southern parts of Central America there are more than double
that number. As we go on towards the equator they become still
more numerous, till they reach their maximum in the equatorial
Andes. They especially abound in the mountainous regions; while
the luxuriant forest plains of the Amazons, in which so many other
forms of life reach their maximum, are very poor in humming-birds.
Brazil, being more hilly and with more variety of vegetation, is
richer, but does not equal the Andean valleys, plateaux, and volcanic
peaks. Each separate district of the Andes has its peculiar species
and often its peculiar genera, and many of the great volcanic
mountains possess kinds which are confined to them. Thus, on the
great mountain of Pichincha there is a peculiar species found at
an elevation of about fourteen thousand feet only; while an allied
species on Chimborazo ranges from fourteen thousand feet to the
limits of perpetual snow at sixteen thousand feet elevation. It
frequents a beautiful yellow-flowered alpine shrub belonging to the
Asteraceæ. On the extinct volcano of Chiriqui in Veragua a minute
humming-bird, called the little Flame-bearer, has been only found
inside the crater. Its scaled gorget is of such a flaming crimson
that, as Mr. Gould remarks, it seems to have caught the last spark
from the volcano before it was extinguished.
Not only are humming-birds found over the whole extent of America,
from Sitka to Tierra-del-Fuego, and from the level of the sea to the
snow-line on the Andes, but they inhabit many of the islands at a
great distance from the mainland. The West Indian islands possess
fifteen distinct species belonging to eight different genera, and
these are so unlike any found on the continent that five of these
genera are peculiar to the Antilles. Even the Bahamas, so close to
Florida, possess two peculiar species. The small group of islands
called Tres Marias, about sixty miles from the west coast of Mexico,
has a peculiar species. More remarkable are the two humming-birds of
Juan Fernandez, situated in the Pacific Ocean, four hundred miles
west of Valparaiso in Chili, one of these being peculiar; while
another species inhabits the little island Mas-afuera, ninety miles
further west. The Galapagos, though very little further from the
mainland and much more extensive, have no humming-birds; neither
have the Falkland islands, and the reason seems to be that both
these groups are deficient in forest, and in fact have hardly any
trees or large shrubs, while there is a great paucity of flowers
and of insect life.
_Humming-birds of Juan Fernandez as illustrating Variation and
Natural Selection._--The three species which inhabit Juan Fernandez
and Mas-afuera present certain peculiarities of great interest. They
form a distinct genus, Eustephanus, one species of which inhabits
Chili as well as the island of Juan Fernandez. This, which may be
termed the Chilian species, is greenish in both sexes, whereas
in the two species peculiar to the islands the males are red or
reddish-brown, and the females green. The two red males differ
very slightly from each other, but the three green females differ
considerably; and the curious point is, that the female in the
smaller and more distant island somewhat resembles the same sex
in Chili, while the female of the Juan Fernandez species is very
distinct, although the males of the two islands are so much alike. As
this forms a comparatively simple case of the action of the laws of
variation and natural selection, it will be instructive to see if we
can picture to ourselves the process by which the changes have been
brought about. We must first go back to an unknown but rather remote
period, just before any humming-birds had reached these islands. At
that time a species of this peculiar genus, Eustephanus, must have
inhabited Chili; but we must not be sure that it was identically the
same as that which is now found there, because we know that species
are always undergoing change to a greater or less degree. After
perhaps many failures, one or more pairs of the Chilian bird got
blown across to Juan Fernandez, and finding the country favourable,
with plenty of forests and a fair abundance of flowers and insects,
they rapidly increased and permanently established themselves on the
island. They soon began to change colour, however, the male getting
a tinge of reddish-brown, which gradually deepened into the fine
colour now exhibited by the two insular species, while the female,
more slowly, changed to white on the under-surface and on the tail,
while the breast-spots became more brilliant. When the change of
colour was completed in the male, but only partially so in the
female, a further emigration westward took place to the small island
Mas-afuera, where they also established themselves. Here, however,
the change begun in the larger island appears to have been checked,
for the female remains to this day intermediate between the Juan
Fernandez and the Chilian forms. More recently, the parent form has
again migrated from Chili to Juan Fernandez, where it still lives
side by side with its greatly changed descendant.[15] Let us now
see how far these facts are in accordance with the general laws of
variation, and with those other laws which I have endeavoured to
show regulate the development of colour.[16]
[15] In the preceding account of the probable course of events in
peopling these islands with humming-birds, I follow Mr. Sclater’s
paper on the _Land Birds of Juan Fernandez_,--_Ibis_, 1871, p. 183.
In what follows, I give my own explanation of the probable causes
of the change.
[16] See _Macmillan’s Magazine_, Sept. 1867, “On the Colours of
Animals and Plants,” and Chapters V. and VI. of the present volume.
The amount of variation which is likely to occur in a species will
be greatly influenced by two factors--the occurrence of a change
in the physical conditions, and the average abundance or scarcity
of the individuals composing the species. When from these or other
causes variation occurs, it may become fixed as a variety or a
race, or may go on increasing to a certain extent, either from a
tendency to vary along certain special lines induced by local or
physiological causes, or by the continued survival and propagation
of all such varieties as are beneficial to the race. After a
certain time a balance will be arrived at, either by the limits of
useful variation in this one direction having been reached, or by
the species becoming harmoniously adapted to all the surrounding
conditions; and without some change in these conditions the specific
form may then remain unaltered for a very long time; whence arises
the common impression of the fixity of species. Now in a country
like Chili, forming part of a great continent very well stocked with
all forms of organic life, the majority of the species would be in
a state of stable equilibrium; the most favourable variations would
have been long ago selected; and the numbers of individuals in each
species would be tolerably constant, being limited by the numerous
other forms whose food and habits were similar, or which in any way
impinged upon its sphere of existence. We may, therefore, assume
that the Chilian humming-bird which migrated to Juan Fernandez
was a stable form, hardly if at all different from the existing
species which is termed Eustephanus galeritus. On the island it met
with very changed but highly favourable conditions,--an abundant
shrubby vegetation and a tolerably rich flora; less extremes of
climate than on the mainland; and, most important of all, absolute
freedom from the competition of rival species. The flowers and
their insect inhabitants were all its own; there were no snakes
or mammalia to plunder its nests; nothing to prevent the full
enjoyment of existence. The consequence would be, rapid increase
and a large permanent population, which still maintains itself;
for Mr. Moseley, of the _Challenger_ expedition, has informed the
writer that humming-birds are extraordinarily abundant in Juan
Fernandez, every bush or tree having one or two darting about it.
Here, then, we have one of the special conditions which have always
been held to favour variation--a great increase in the number of
individuals; but, as there was no struggle with allied creatures,
there was no need for any modification in form or structure, and
we accordingly find that the only important variations which have
become permanent are those of size and of colour. The increased
size would naturally arise from greater abundance of food with a
more equable climate throughout the year, the healthier, stronger,
and larger individuals being preserved. The change of colour
would depend on molecular changes in the plumage accompanying the
increase of size; and the superior energy and vitality in the male,
aided by the favourable change in conditions and rapid increase
of population, would lead to an increased intensity of colour,
the special tint being determined either by local conditions or
by inherited tendencies in the race. It is to be noted that the
change from green to red is in the direction of the less refrangible
rays of the spectrum, and is in accordance with the law of change
which has been shown to accompany expansion in inorganic,--growth
and development in organic forms.[17] The change of colour in the
female, not being urged on by such intense vital activity as in
the case of the male, would be much slower, and, owing probably to
inherited tendencies, in a different direction. The under-surface
of the Chilian bird is ashy with bronzy-green spots on the breast,
while the tail is entirely bronze-green. In the Juan Fernandez
species the under-surface has become pure white, the breast-spots
larger and of a purer golden-green, while the whole inner web of
the tail-feathers has become pure white, producing a most elegant
effect when the tail is expanded.
[17] See “Colours of Animals,” _Macmillan’s Magazine_, Sept. 1877,
pp. 394-398, and Chapter V. in the present volume.
We may now follow the two sexes to the remoter island, at a period
when the male had acquired his permanent style of colouring, but
was not quite so large as he subsequently became; while the change
of the female bird had not been half completed. In this small and
comparatively barren island (a mere rock, as it is described by
some authors) there would be no such constant abundance of food,
and therefore no possibility of a large permanent population; while
the climate would not differ materially from that of the larger
island. Variation would therefore be checked, or might be stopped
altogether; and we find the facts exactly correspond to this view.
The male, which had already acquired his colour, remains almost
undistinguishable from his immediate ancestral form; but he is a
little smaller, indicating either that the full size of that form
had not been acquired at the period of migration, or that a slight
diminution of size has since occurred, owing to a deficiency of
food. The female shows also a slight diminution of size, but in
other respects is almost exactly intermediate between the Chilian
and Juan Fernandez females. The colour beneath is light ashy,
the breast-spots are intermediate in size and colour, and the
tail-feathers have a large ill-defined white spot on the end of
the inner web which has only to be extended along the whole web
to produce the exact character which has been acquired in Juan
Fernandez. It seems probable, therefore, that the female bird has
remained nearly or quite stationary since its migration, while
its Juan Fernandez relative has gone on steadily changing in the
direction already begun; and the more distant species geographically
thus appears to be more nearly related to its Chilian ancestor.
Coming down to a more recent period, we find that the comparatively
small and dull-coloured Chilian bird has again migrated to Juan
Fernandez; but it at once came into competition with its red
descendant, which had firm possession of the soil, and had probably
undergone slight constitutional changes exactly fitting it to its
insular abode. The new-comer, accordingly, only just manages to
maintain its footing; for we are told by Mr. Reed, of Santiago,
that it is by no means common; whereas, as we have seen, the red
species is excessively abundant. We may further suspect that the
Chilian birds now pass over pretty frequently to Juan Fernandez,
and thus keep up the stock; for it must be remembered that whereas,
at a first migration, both a male and a female are necessary for
colonization, yet, after a colony is formed, any stray bird which
may come over adds to the numbers, and checks permanent variation
by cross-breeding.
We find, then, that all the chief peculiarities of the three allied
species of humming-birds which inhabit the Juan Fernandez group of
islands, may be fairly traced to the action of those general laws
which Mr. Darwin and others have shown to determine the variations
of animals and the perpetuation of those variations. It is also
instructive to note, that where the variations of colour and size
have been greatest they are accompanied by several lesser variations
in other characters. In the Juan Fernandez bird the bill has become
a little shorter, the tail-feathers somewhat broader, and the
fiery cap on the head somewhat smaller; all these peculiarities
being less developed or absent in the birds inhabiting Mas-afuera.
These coincident changes may be due, either to what Mr. Darwin
has termed correlation of growth, or to the partial reappearance
of ancestral characters under more favourable conditions, or to
the direct action of changes of climate and of food; but they show
us how varied and unaccountable are the changes in specific forms
that may be effected in a comparatively short time, and by means
of very slight changes of locality.
If now we consider the enormously varied conditions presented
by the whole continent of America--the hot, moist, and uniform
forest-plains of the Amazon; the open llanos of the Orinoco; the
dry uplands of Brazil; the sheltered valleys and forest slopes of
the Eastern Andes; the verdant plateaus, the barren paramos, the
countless volcanic cones with their peculiar Alpine vegetation; the
contrasts of the East and West coasts; the isolation of the West
Indian islands, and to a less extent of Central America and Mexico
which we know have been several times separated from South America;
and when we further consider that all these characteristically
distinct areas have been subject to cosmical and local changes, to
elevations and depressions, to diminution and increase of size, to
greater extremes and greater uniformity of temperature, to increase
or decrease of rainfall; and that with these changes there have
been coincident changes of vegetation and of animal life, all
affecting in countless ways the growth and development, the forms
and colours, of these wonderful little birds--if we consider all
these varied and complex influences, we shall be less surprised at
their strange forms, their infinite variety, their wondrous beauty.
For how many ages the causes above enumerated may have acted upon
them we cannot say; but their extreme isolation from all other
birds, no less than the abundance and variety of their generic and
specific forms, clearly point to a very high antiquity.
_The Relations and Affinities of Humming-birds._--The question of
the position of this family in the class of birds and its affinities
or resemblances to other groups, is so interesting, and affords
such good opportunities for explaining some of the best-established
principles of classification in natural history in a popular way,
that we propose to discuss it at some length, but without entering
into technical details.
There is in the Eastern hemisphere, especially in tropical Africa
and Asia, a family of small birds called Sun-birds, which are
adorned with brilliant metallic colours, and which, in shape and
general appearance, much resemble humming-birds. They frequent
flowers in the same way, feeding on honey and insects; and all
the older naturalists placed the two families side by side as
undoubtedly allied. In the year 1850, in a general catalogue of
birds, Prince Lucien Bonaparte, a learned ornithologist, placed
the humming-birds next to the swifts, and far removed from the
Nectarinidæ or sun-birds; and this view of their position has gained
ground with increasing knowledge, so that now all the more advanced
ornithologists have adopted it. Before proceeding to point out the
reasons for this change of view, it will be well to discuss a few
of the general principles which guide naturalists in the solution
of such problems.
_How to Determine Doubtful Affinities._--It is now generally
admitted that, for the purpose of determining obscure and doubtful
affinities, we must examine by preference those parts of an
animal which have little or no direct influence on its habits
and general economy. The value of an organ, or of any detail of
structure, for purposes of classification, is generally in inverse
proportion to its adaptability to special uses. And the reason of
this is apparent, when we consider that similarities of food and
habits are often accompanied by similarities of external form or
of special organs, in totally distinct animals. Porpoises, for
example, are modified externally so as to resemble fishes; yet
they are really mammalia. Some marsupials are carnivorous, and are
so like true carnivora that it is only by minute peculiarities of
structure that the skeleton of the one can be distinguished from
that of the other. Many of the hornbills and toucans have the same
general form, and resemble each other in habits, in food, and in
their enormous bills; yet peculiarities in the structure of the
feet, in the form of the breast-bone, in the cranium, and in the
texture and arrangement of the plumage, show that they have no
real affinity, the former approaching the kingfishers, the latter
the cuckoos. Such structural peculiarities as these have no direct
relation to habits; and they are therefore little liable to change,
when from any cause a portion of the group may have been driven to
adopt a new mode of life. Thus all the Old World apes, however much
they may differ in size or habits, and whether we class them as
baboons, monkeys, or gorillas, have the same number of teeth; while
the American monkeys all have an additional premolar tooth. This
difference can have no relation to the habits of the two groups,
because each group exhibits differences of habits greater than often
occur between American and Asiatic species; and it thus becomes a
valuable character indicating the radical distinctness of the two
groups, a distinctness confirmed by other anatomical characters.
On the other hand, peculiarities of organization which seem
specially adapted to certain modes of life, are often diminished
or altogether lost in a few species of the group, showing their
essential unimportance to the type, as well as their small value
for classification. Thus, the woodpeckers are most strikingly
characterised by a very long and highly extensible tongue, with the
muscles attached to the tongue-bone prolonged backward over the head
so as to enable the tongue to be suddenly darted out; and also by
the rigid and pointed tail which is a great help in climbing up the
vertical trunks of trees. But in one group (the Picumni), the tail
becomes quite soft, while the tongue remains fully developed; and
in another (Meiglyptes) the characteristic tail remains, while the
prolonged hyoid muscles have almost entirely disappeared, and the
tongue has consequently lost its peculiar extensile power; yet in
both these cases the form of the breast-bone and the character of
the feet, the skeleton, and the plumage, show that the birds are
really woodpeckers; while even the habits and the food are very
little altered. In like manner the bill may undergo great changes;
as from the short crow-like bill of the true birds-of-paradise
to the long slender bills of Epimachinæ, which latter were on
that account long classed apart in the tribe of Tenuirostres, or
slender-billed birds, but whose entire structure shows them to be
closely allied to the paradise-birds. So, the long feathery tongue
of the toucans differs from that of every other bird; yet it is not
held to overbalance the weight of anatomical peculiarities which
show that these birds are allied to the barbets and the cuckoos.