The Encyclopedia Americana (1920)/Flowers, Fertilization of, by Birds
FLOWERS, Fertilization of, by Birds.— That insects and especially bees, carry pollen from flower to flower as they visit them in succession in search of the nectar hidden deeply in their corollas, is familiarly known to most readers. The insects entering a flower brush against the anthers, rub off the pollen (usually a little sticky) and in entering the next flower leave some of it on the pistil by means of which it reaches the seed. Many flowers appear to have become adapted in their structure to this external means of cross-fertilization (q.v.).
It is not so well known, however, that birds perform a similar service to plants which, although far more limited than that of insects, is still of great importance in certain plant families, especially in the tropics. A large variety of minute insects penetrate blossoms in search of food and some virtually live there as long as the flowers last. This fact is well known to birds of many sorts and the long slender bills characteristic of some families are especially adapted to probing the big, tubular, nectar-holding and fly-infested corollas produced by tropical trees and herbs. Thus the banana plantations of the West Indies and Central America may be said to depend largely for successful crops on the prevalence of the banana quit (Certhiola flaveola), a small, plainly dressed creeper that searches the blossoms for these insects; in doing so the creeper's head becomes heavily dusted with yellow pollen, which is rubbed off against the pistil of other flowers as it pushes its head into them. These quits, or sugar-birds, of which tropical America has several species, have long curved bills and tongues frayed at the tip into a tiny brush. They search many sorts of flowers for insects, but neglect the nectar, or at any rate that is not the first attraction. The hummingbirds, however, which may be said as a class to live on what they find in flowers (accessible to them in the tropics the year round), feed upon both the insects and the sugary juices to be found there. For this their structure particularly adapts them. Their small size permits them to enter completely into the larger flowers, while their powerful wings enable them to poise in front of smaller ones and explore them with the long slender bill characteristic of the tribe. See Hummingbird.
About 500 species of hummingbirds have
been described, differing much in details of size
and color, especially in respect to the beak, for
the straight bill of a Docmastes may measure
five inches, while in Rhamphomicron it is only
a quarter of an inch long. In some the bill
curves slightly upward, in others downward and
in Eutoxeres it is bent like a sickle. All these
varieties of shape indicate special requirements,
that is, the choice of particular blossoms; and
Fritz Müller says that various species of
abutilon in southern Brazil are sterile unless
fertilized by the one kind of hummer that
frequents each one. The hummingbirds dart
into and out of a long list of tropical blossoms,
gathering and leaving pollen, and several other
cases of mutual dependence between them and
plants might be mentioned. Thomas Belt
(‘Naturalist in Nicaragua’ London 1873)
describes the flowers of certain vines of the
genus Marcgravia in which the corolla
consists in part of a ring of pockets, or “pitchers”
in various positions. In spring these fill with
a sweetish liquid that attracts insects in large
numbers, and these in turn lure hummingbirds,
which alone among birds can reach the freely
swaying flowers; but in order to rifle these
sweet pockets, the bird must push past the
generative parts (stamens and pistil) and thus
give and take pollen on its glittering head or
breast. The sabre-like flower of Erythrina
offers a still more curious case of the need of
hummingbirds, help in effecting healthful
cross-fertilization.
The hummingbirds of America are represented in the warm parts of the Old World by the sun-birds and honey-suckers, which also are small and brilliantly plumaged, have rather long, and usually curved, fine-pointed bills, and protrusile tongues with split or else brush-like tips, with which it is easy to entangle a small bug or to extract nectar from its deep cup, for the tongue of these birds may be rolled into a sucking-tube. These resemblances to the American hummingbirds, with which they have no real relationship, merely show the acquirement by different kinds of birds of similar means to the same end — the rifling of flowers of honey and attendant mites of insects. The honey-suckers are Australian and are especially addicted to the eucalyptus trees and shrubs that profit by feeding the beautiful little creatures. The eucalyptus groves of Australia are also frequented by the peculiar parrots called lories or brush-tongued parrakeets. They are distinguished by the dense coating of papillæ on the tongue, with which they lick up honey and insects together from the big nectarous blossoms; and more than one writer has mentioned that their foreheads are smeared with yellow pollen as they fly from tree to tree.
It is a very significant fact that Australia and its neighboring islands are deficient in insects, especially bees and butterflies. It was necessary to import and acclimate bumblebees before clover could be raised there for fodder. This deficiency in healthful cross-fertilization for plants is supplied by birds and bats. Interdependence of certain tropical trees and lowlier plants and their birds is illustrated elsewhere. The West Indian logwood is not found except where a certain sugar-bird lives; and the same companionship is noticeable in the East Indies, according to Layard. Forbes (‘Wanderings in the Eastern Archipelago’ 1885), notes a plant in Sumatra dependent on a certain spider-eater (bird). Darwin concluded "that the beaks of birds are specially adapted to the various flowers that they visit"; and Grant Allen believes, conversely, that “many of the most brilliant and beautiful bell-shaped tropical flowers have been specially developed to suit the tastes and habits of those comparatively large and powerful fertilizers.”