kingfishers, and many others. The prairie-owl, living in the burrows
of the prairie-dog and of the armadillo, is a well-known example of
peculiar habit, and in the first case of curious partnership. Ground
nests, generally of the simplest character, with rough and scanty
accumulation of nest material, are made by swans, ducks, geese,
fowls, gulls, waterhens, corncrakes, etc. Mud nests, constructed from
damp earth, are well illustrated by the house-swallows, blue-creeper,
flamingo, etc. The common singing thrush is well known to make a
firm nest of clay and cow-dung mixed with moss. Carpenter-nests,
formed with more or less preparation in the holes of trees, are
used by woodpeckers and a few other arboreal birds. Platform nests,
simply consisting of flat seats, are formed by the ring and turtle
doves, by eagles, storks, and cranes. In some parts of the Continent
the flat nests formed by the storks on the tops of buildings are
familiar enough objects. Basket nests are such loosely interwoven
constructions of grass, stems, twigs, etc., as are made by the
crows, missel-thrushes, and most singing birds. The green weaving
birds (Ploceus pensilis) hang their loosely woven nests, with
downward directed opening, on the Madagascar trees. The South African
republican birds (Philitærus socius) form hundreds of hanging nests
on the branches, under the shelter of a common thatch. Woven nests
are the more delicately constructed and really woven constructions
of wool, hair, bark, grass, etc., which are made by such birds as
the goldfinch, the Baltimore bird, and very many others. Sewed
nests, composed of leaves sewed together by the beak as needle, are
well illustrated by various species of Icterus, and by the Indian
tailor-bird (Orthotomus bennetii). Felt-work nests are woven from
the wool of plants or animals, sometimes with other material in
addition; the humming-birds and the bullfinch form beautiful nests
of this fashion. Cement nests are bound together by a viscid and
very adhesive secretion, which is mixed with saliva, and used to
glue the materials of the nest together. The nests of the American
swallow, the edible birds’ nests of the Salangani, sought after as
luxuries by Chinese and others, are of this cemented type. Dome or
moss nests are roofed in above and have an entrance on the side. The
common wren, the water-wagtail, and the tits build on this principle.
The beautiful bottle-shaped nest of the titmouse is one of the
best examples. The parasite habit is well known among cuckoos and
cowbirds. The nest of another bird is utilized to the future loss
of the rightful inmates, and with obvious economy of labor on the
part of the intruders. Thus sparrows usurp the nests of swallows,
and starlings those of woodpeckers. Pheasant and partridge eggs are
sometimes found in the same nest, and the same has been observed
in many cases--_e. g._, gull and eider-duck. When artificial nests
are forthcoming, birds are glad to be relieved of the labor of
construction, and different birds thus sometimes share a common box.
The resorts of birds, when convenient nooks are available, are often
extremely curious.
It is a well-known fact that comparatively few birds (at any rate
outside of the tropics) remain in the same place all the year round.
They do not hibernate, but migrate on the approach of cold. Some we
know as winter visitors, returning north again in spring, most we
know only in summer, for in autumn they fly to the warmer south; a
third set we call “birds of passage”; for these we only know somewhat
incidentally as they pass through on their way elsewhere.
[Illustration: Birds of all Climes
1, Crested Grebe (_Podiceps_); 2, Toucan; 3, Hoopoe; 4, Parrot
(_Melopsittacus_); 5, Barn Owl; 6, Crested Penguin; 7, King Vulture;
8, Ruff (_Pavoucella_); 9, Horned Owl; 10, Wood Duck; 11, Kingfisher;
12, Capuchin Bird; 13, Grouse; 14, Spoonbill; 15, California Quail]
Thus the swallow, the cuckoo, the nightingale, etc., come to Britain
in summer and breed there, being winter residents further south; the
fieldfare, jacksnipe, bean-goose, redwing, and some others, reach
Great Britain in winter, being summer residents and breeders further
north; while the little sandpipers are familiar examples of the true
birds of passage which we know only for a short time as they rest on
our shores in the journey south in autumn, and north again in spring.
These three classes are obviously only different cases of one fact of
migration. Almost all birds are in some degree migratory. Those
which breed in the equatorial regions are the chief exceptions,
and even they pass from hill to valley and back again. Forms, too,
which seem to be constant residents of a non-tropical country are
in many cases known to exhibit a partial or a very local migration.
This is true, for instance, of the common wren and the red grouse
in the north of Scotland. All birds breed in the colder regions
of their migration. Changes in food supply and the temperature
are the most important conditions impelling them to shift their
habitats. The general trend of migration is always, as indicated,
toward the equator in autumn, from the equator in spring; but the
investigations of the British Migration Committee have clearly shown
that the courses often come to be circular. The flight is the more
universal in a country the more marked the contrast between summer
and winter. The annual migration from breeding areas too cold for
winter residence and food supply to warmer subsistence areas can not
be understood apart from the history of climates. When the European
climate was more equable, it was virtually indifferent to the birds
where they went. As it grew colder the birds had to fly further and
further south every few winters. Migration has become an inherited
habit, for they set about it before the impelling conditions are
directly present. According to Wallace, natural selection has
played an important part in confirming this habit. Many facts about
migration are still utterly obscure. The power birds have of flying
straight and of returning to the same locality is very marvelous. It
must be remembered that a continuous tradition is sustained; those
who have made the journey before guide the others. Doubtless they
have memory for great landmarks. They fly across the shallower parts
of the Mediterranean, where a chain of islands in this submerged
tract long remained to guide them. The smaller birds usually keep
nearer the ground; but it must not be forgotten that the flight
is usually mostly accomplished by night. Birds generally meet in
concourses, and migrate in flocks. Only a few fly alone. Sometimes
the old males remain, while the others “flit.” The return northward
is more rapid, without young ones or weaklings. The males often
return first.
As birds have a full active life, with considerable variety of
function, in usually complex environment, since, as we have already
noticed, their sense-organs and nervous systems are highly developed,
considerable exhibition of intelligence is to be expected. They seem
to have great vividness of sense impressions, to judge from their
power of recalling old haunts and old friends. Birds often return
to the same place season after season, and they have been known to
recognize an owner after the lapse of years. Their quickness of ear
and power of retention are evidenced by the power some possess of
learning to repeat sounds, both words and tunes. Some have exhibited
marked fondness for music, and the æsthetic tastes of the bower-bird
excite deserved admiration. Much more is known in regard to their
marvelous hereditary, general, and largely automatic reasonable
habits or “instincts” than in regard to their power of individually
adapting their conduct to novel circumstances. Their beautiful and
adroit contrivances of nest-building are very familiar instances of
the former, but many instances of the latter have been recorded.
As to feelings, it is hardly necessary to refer to their unexampled
exhibition of sexual emotion in song and dance, parade, and display,
or to the marvelous parental love and sacrifice expressed in their
nest-building labors, in their prolonged incubation, and in their
care for and courage in defending their brood. Subtler emotions of
jealousy, both in connection with and altogether apart from sex, of
affection for owners or associations, of sympathy for wounded or
enfeebled fellows, are also not rarely exhibited. That a bird singing
continuously for hours does not represent a rare height of emotion
is not to be believed. It may be fairly said that the joyous song of
the lark “at heaven’s gate” is an eloquent expression of emotion only
surpassed perhaps by human music.
MIMICRY
--DAVID ROBERTSON
In ordinary language a person who can imitate the accent, manner
of talking, and acting of another is said to be a good mimic. In
biology, however, the term mimicry is used in a metaphorical sense,
being applied to the resemblance which one species of animal or
plant frequently shows to another. This resemblance is usually
of a protective character. It is evident that if the resemblance
which a defenceless species of animal often has to a species well
furnished with natural offensive and defensive weapons were a mere
freak of nature, no satisfactory and philosophical explanation of the
phenomenon could be given.
Scientific investigators have to lay aside their wonder, and
laboriously set about finding a solution to the most intricate and
puzzling phenomena both in natural and physical science.
Mimicry was first used by Mr. W. H. Bates to denote the advantageous
and generally protective resemblance assumed by one species of animal
or plant to another.
It will be seen further on that the resemblance is not confined to
one species of animal to another species of animal, and one species
of plant to a plant of quite a distinct species, but that it also
exists between animals and plants.
Mr. A. R. Wallace, who, by his most patient and skilful researches
in the domain of animal life, has clearly defined and limited the
term mimicry as applied in biology, says: “A certain species of plant
or animal possesses some special means of defence from its enemies,
such as a sting, a powerful and disagreeable odor, a nauseous taste,
or a hard integument or covering. Some other species, inhabiting the
same district or part of it, and not itself provided with the same
means of defence, closely resembles the first species in all external
points of form and color, though often very different in structure
and unrelated in the biological order.”
In South America there are certain butterflies, the Heliconidæ,
which are remarkable for the variety and beauty of their colors;
but they are incapable of rapid and sustained flight, and would for
this reason fall an easy prey to insect-eating birds. Their wings,
however, are never found among those rejected by insectivorous
birds--in places where the remains of other butterflies frequently
cover the ground. The Heliconidæ possess a powerfully disagreeable
and pungent odor, which is so little volatile as to cling to the
fingers for several days after handling one of these insects. Mr.
Wallace inferred from this that they have a disagreeable taste, and
would not on that account be eaten by birds. This was subsequently
found by Mr. Belt to be the case.
Belonging to the family of the Pieridæ, which is quite distinct from
the family of the Heliconidæ, and the greater number of which are
white, there is a genus of small butterfly named Leptalis, which is
eaten by birds. Some species of the genus Leptalis are white, like
their allies among the Pieridæ, but the majority of the Leptales have
an exact resemblance to some species of the Heliconidæ as far as
regards the peculiar shape and color of their wings.
The structure of the two families is completely different; in
spite of this the resemblance is so strikingly close that both the
experienced entomologists Mr. Bates and Mr. Wallace often at the
time of capture mistook the one for the other, and only discovered
their mistake by a closer examination. This has been looked upon as
the most typical example of true mimicry, and is interesting from
the fact that it is the first instance to which the term mimicry was
applied.
It is necessary to distinguish carefully between true mimicry and
several similar though superficial modes of resemblance which occur
among organic beings. Several orchids resemble flies or spiders, but
this is merely a case of accidental resemblance.
Among animals of a higher order than insects mimicry very seldom
occurs.
Among mammals, all of which belong to the vertebrates, mimicry is
seldom found, and it is supposed that only one genuine case has been
observed.
Cladobates, an insect-eating genus found in the Malayan region,
includes many species closely resembling squirrels both in size and
color, as well in regard to the bushiness and position of the tail.
It is supposed by Mr. Wallace that Cladobates, owing to its
resemblance to the harmless fruit-eating squirrel, may be enabled to
approach insects and birds upon which it lives.
Cuckoos bear a considerable resemblance to hawks; the cuckoo tribe
being weak and defenceless will in this way be enabled to elude the
voracious hawks.
There is a genus of dull-colored birds in Australia and the Moluccas
named Tropidorhynchus. These birds are large, active, and strong,
with powerful claws and sharp beaks. They congregate in flocks, and
are remarkably aggressive, driving away crows and even hawks.
In these same countries a genus of the group orioles lives, named
Mimeta. These are much weaker than their allies the golden orioles,
and besides are devoid of their brilliant colors, being usually
olive-green or brown. It is a very common thing to find species of
the Mimeta resembling Tropidorhynchi living on the same island.
The Tropidorhynchus bouruensis and Mimeta bouruensis are both found
in the island of Bouru, the latter of which mimics the former as
described by Mr. Wallace:
“The upper and under surfaces of the two birds are exactly of
the same tints of dark and light brown. The Tropidorhynchus has
a large, bare, black patch round the eyes; this is copied in the
Mimeta by a patch of black feathers. The top of the head of the
Tropidorhynchus has a scaly appearance from the narrow scaly-formed
feathers, which are imitated by the broader feathers of the Mimeta,
having a dusky line down each. The Tropidorhynchus has a pale ruff,
formed of curious recurved feathers on the nape (which has given
the whole genus the name of friar birds); this is represented in
the Mimeta by a pale band in the same position. Lastly, the bill of
the Tropidorhynchus is raised into a protuberant keel at the base,
and that of the Mimeta has the same character, although it is not
a common one in the genus.” The result is that when superficially
examined the birds seem to be identical, though possessed of
important structural differences, and placed wide apart in any
natural arrangement.
Mr. Wallace mentions some curious cases of mimicry among reptiles,
where a venomous tropical genus of snakes, Elaps, belonging to
America, is closely mimicked by several genera of harmless snakes.
It is in a special degree among insects that cases of mimicry are
most frequently found.
Genuine cases of mimicry are not so easily shown to exist among
plants. The resemblance between white dead nettle (Lamium album)
and the stinging nettle, as well as between other labiates and the
stinging nettle, may be considered to be a case of real mimicry as
defined above.
The true stinging nettles are avoided by animals, owing to their
possession of stinging hairs, which contain an acid fluid capable of
causing pain and producing blisters.
It would be clearly of advantage to another plant to resemble one
possessing such defensive armor as the stinging nettle.
There is another labiate, Ajuga ophrydis of South Africa, mentioned
by Mr. Mansel Weale. This labiate closely resembles an orchid, and
for this reason insects may be induced to visit the flower and thus
fertilize it.
Mr. Worthington Smith, the eminent fungologist, has found three rare
British fungi, each accompanying common species, which they closely
resembled; and one of the common species has a bitter nauseous taste.
In this case we have an example of genuine mimicry.
Dr. Hans Meyer has given in his valuable work, _Across East African
Glaciers_, some very striking instances of mimicry. He says:
“The similarity for the purpose of protection of the majority of the
great mammals, _i. e._, the likeness of the color of their coats, and
partly also of their external appearance, to the features and colors
of the regions which they inhabit, must strike every traveler with
astonishment.
“At a small distance the hartbeest (antelope), when stationary,
is really not distinguishable from red ant-heaps which everywhere
abound; the long-legged and long-necked giraffe can not be
distinguished from the dead trunk of a mimosa, the zebra from a
gray-brown clump of grass and thorn-scrub, the rhinoceros from a
fallen trunk of a tree. It is only when they move that they can be
distinguished. Nature has also extended this protective mimicry
(Schutzspiel) to the small insects; and perhaps for this reason they
often escape the eye specially in search of them; for butterflies and
grasshoppers look like dry twigs, the cicadæ like leaf-stalks, the
spiders like thorns, the phasmodæ like bare twigs, beetles like small
lumps of earth and small stones, moths like mosses and lichens.
“This protective mimicry is manifested not only in regard to the
colors and forms of the animals, but also as regards their movements,
or their manner of standing still, and in their preference for
certain localities appropriate to their disguise. There is protection
everywhere; protection against climatic extremes and against animal
foes; such varied and abundant protection as could only be developed
by natural selection in a primeval continent like Africa.”
In spite of the voluminous literature of “animal mimicry” since
Bates first published his classical memoir on the subject, the exact
nature of the process whereby insects and other creatures “mimic”
(though that is not the appropriate word) the appearance of other
species is still far from being understood. All we know is that this
power, this resemblance of a beetle or a butterfly to the ground upon
which it sits, the sticks among which it creeps, or the leaves among
which it flutters, helps to save it from destruction, while it is a
decided advantage to it to “mimic” another insect which is sedulously
avoided by birds. The observations in this byway of zoology are as
curious as any yet made. It is found, for instance, that an American
spider (Cyrlarachne) takes the semblance of a little land shell very
abundant in the localities which it frequents; and that another
species (Thomisus alcatorius), remarkable for the length of its
forelegs, so fastens itself on the stems of grasses as to be nearly
indistinguishable from the spikelets.
Some observations, for which we are indebted to M. Heckel of
Marseilles, throw a good deal of light on the origin of mimicry, at
least so far as the assumption of protective coloration is concerned.
There is a spider (Thomisus onustus) very common in the south of
France which conceals itself in the flower of a species of wild
convolvulus for the purpose of trapping two kinds of fly on which
it feeds. This convolvulus is found in three principal varieties:
white, pink with deeper spots of the same hue, and light pink forms
with a slight greenishness on the external wall of the flower. Each
of these three varieties is visited by the spider. But the varieties
of spider conform in hue to the varieties of the flower, and each
confines itself to the one which is most protective to it. If,
however, the animal is confined to a Dahlia versicolor, it conforms
to the hue of its new abode--that is, the pink one turns to red, and,
in like manner, if transferred to the yellow snapdragon, it takes
the color of this flower. They change in shade as the shade of their
host changes, and when pink, white, green, and yellow varieties are
confined together in a box they all become nearly white.
The question of protective coloration in fishes has of late received
some light which compels a revision of our former theories on the
subject. It has usually been held that the color of fishes is of
the mimicry order--that is, it has been acquired for the purpose of
deceiving their enemies. Trout will very commonly take the hue of
the river bottom over which they swim, and, as every one knows, it
is difficult to detect a flounder or other flat fish at rest, though
when it turns over the white under surface of its body instantly
reveals the creature’s presence. The hue of the upper surface is due
to the action of light. For when a sole was kept in a raised glass
case, with light directed upward from below, pigment formed on the
white side, and began to be absorbed on the one hitherto exposed to
the same agent.
DWELLINGS
--FRÉDÉRIC HOUSSAY
Animals construct dwellings either to protect themselves from the
cold, heat, rain, and other chances of the weather, or to retire
to at moments when the search for food does not compel them to be
outside and exposed to the attacks of enemies. Some inhabit these
refuges permanently; others only remain there during the winter;
others, again, who live during the rest of the year in the open air,
set up dwellings to bring forth their young, or to lay their eggs and
rear the offspring.
We shall find every stage, from that of beings provided for by
nature, and endowed with a special organ which secretes for them a
shelter, up to those who are constrained by necessity to seek in
their own intelligence an expedient to repair the forgetfulness of
nature.
Nearly all the Mollusca are enveloped by a very hard calcareous case,
secreted by their mantle: this shell, which is a movable house, they
bear about with them and retire into at the slightest warning.
Caterpillars which are about to be transformed into chrysalides weave
a cocoon, a very close dwelling in which they can go through their
metamorphosis far from exterior troubles. It is an organic form of
dwelling, or produced by an organ. It is not necessary to multiply
examples of this kind; they are extremely numerous. In the same
category must be ranged the cells issuing from the wax-glands which
supply bees with materials for their combs in which they inclose the
eggs of the queen with a provision of honey.
I do not wish to insist on creations of this kind which are
independent of the animal’s will and reflection. Near these facts
must be placed those in which animals, still using a natural
secretion, yet endeavor to obtain ingenious advantages from it
unknown by related species.
There is, for example, the Macropus viridiauratus, or paradise-fish,
which blows air bubbles in the mucus produced from its mouth. This
mucus becomes fairly resistant, and all the bubbles imprisoned and
sticking side by side at last form a floor. It is beneath this
floating shelter that the fish suspends its eggs for its little ones
to undergo their early development.
Certain tubicolar annelids, whose skin furnishes abundant mucus
which does not become sufficiently hard to form an efficacious
protection, utilize it to weld together and unite around them
neighboring substances, grains of sand, fragments of shell, etc. They
thus construct a case which both resembles formations by special
organs and manufacture by the aid of foreign materials. The larvæ
of Phryganea, who lead an aquatic life, use this method to separate
themselves from the world and prepare tubes in which to dwell. All
the fragments carried down by the stream are good for their labors
on condition only that they are denser than the water. They take
possession of fragments of aquatic leaves, and little fragments
of wood which have been sufficiently long in the water to have
thoroughly imbibed it, and so become heavy enough to keep themselves
at the bottom, or at least to prevent them from floating to the
surface. It is the larva of Phryganea striata which has been best
studied; those of neighboring species evidently act much in the same
way, with differences only in detail. The little carpenter stops a
fragment rather longer than his own body, lies on it and brings it
in contact with other pieces along his own sides. He thus obtains
the skeleton of a cylinder. The largest holes are filled up with
detritus of all kinds. Then these materials are agglutinated by a
special secretion. The larva overlays the interior of its tube with
a covering of soft silk which renders the cylinder water-tight and
consolidates the earlier labors. The insect is thus in possession of
a safe retreat. Resembling some piece of rubbish, it completes its
metamorphosis in peace, undisturbed by the carnivora of the stream.
Between the beings whom nature has endowed with a shelter and those
who construct it by their own industry, we may intercept those who,
deprived of a natural asylum and not having the inclination or the
power to make one, utilize the dwellings of others, either when the
latter still inhabit them, or when they are empty on account of the
death or departure of the owner. In the interior of the branchial
chamber of many bivalvular mollusca, and especially the mussel, there
lives a little crustaceous commensal called the pea-crab (Pinnoteres
pisum). He goes, comes, hunts, and retires at the least alarm within
his host’s shell. The mussel, as the price of its hospitality, no
doubt profits by the prizes which fall to the little crab’s claws. It
is even said that the crab in recognition of the benefits bestowed
by his indolent friend keeps him acquainted with what is passing on
around, and as he is much more active and alert than his companion he
sees danger much further away, and gives notice of it, asking for the
door to be shut by lightly pinching the mussel’s gill.
For birds like the cuckoo and the Molothrus it is not possible to
plead extenuating circumstances. They occupy a place in an inhabited
house without paying any sort of rent. Every one knows the cuckoo’s
audacity. The female lays her eggs in different nests and troubles
herself no further about their fate. She seeks for her offspring
a shelter which she does not take the trouble to construct, and
moreover at the same time assures for them the care of a stranger in
place of her own.