The conditions necessary to produce this power in its fullest
extent are,—large and strong pectoral muscles; great extent
of surface, as well as peculiarity of form, in the wing:
and feathers of firm texture, strong in the shaft, with the
filaments of the plume arranged and connected to resist
pressure from below. The extent of surface, the form and
other peculiarities of the wings, have been already noticed,
and the anatomical part only requires to be briefly described.
A certain degree of weight is necessary to flight*, and this
is imparted by large pectoral muscles; the power of these
muscles may he estimated by the depth of the keel, and the
breadth of the sides of the breast-bone or sternum; as
affording extent of surface for the attachment of those large
muscles by the action of which the wings are moved.
As an illustration of this form, the figure, inserted as a
vignette, on the opposite page, is a representation about one-
fouith less than the natural size of the breast-bone of a young
male Peregrine Falcon, which exhibits the depth of the keel,
the breadth of the sides, as well as the strength of the coracoid
bones; and the power of flight peculiar to all the species of
Falcons is still further illustrated by the form and substance
of the forked bone or furcula, commonly called the merrythought
or wish-bone, which is circular, broad and strong,
affording a permanent support to the shoulders. This furcula
represents the clavicles of mammals.
Though the subject of the figure may be taken to shew
the general form of the sternal apparatus in the Falconidce,
various members of the Family present some differences which
it may not be inexpedient to notice briefly. Thus the size of
the foramina or holes pierced in the posterior portion of the
sides of the breast-bone varies not inconsiderably in certain
* Those who wish to study the mechanics of flight cannot do better than consult
Prechtl’s 1 Untersuchungen über den Flug der Vogel’ (Wien: 1846) and
Prof. Marey’s ‘Mémoire sur le vol desinsectes et des oiseaux’ as published in the
‘ Aúnales des Sciences Naturelles’ (Zoologie, 5th ser. vol. xii.). In English the
essays of Dr. Pettigrew (Trans. Linn. Soe. vol. xxvi.) and Captain F. W. Hutton
(Phil. Mag. August, 1869) may also be mentioned in connection with the same
subject; but some of the conclusions of both the writers last named have been
impugned.
species. In the Eagles of the genera Aquila and Haliceetus
these foramina usually disappear entirely, each side of the
breast-bone consisting of an uninterrupted convex plate,
and the posterior portion is somewhat tapering instead of
being broader than the middle. In the Osprey (Pandion)
the hinder margin is still further altered in outline, and it is
possible that this peculiarity may he in some way connected
with the bird’s habit, as described, of taking its prey under
water: for, as will subsequently be mentioned, species whose
nature it is to seek their food by diving, and are thereby
subjected to a greater or less amount of pressure in proportion
to the depth of water, not unfrequently undergo a
considerable modification of this part of the sternum as compared
with those nearest to them in general structure, but
not so expert in their subaquatic feats.
By an extended examination of the different species of
Buzzards and Harriers, it will be found that the characters
described as necessary to produce rapid flight decline gradually.
The sternum decreases in size, the keel loses part
of its depth and the coracoid bones and furcula become more
slight.
A representation of the sternum of a Vulture has already
been given and that of an Owl will be immediately inserted,
to afford a comparative view of the size and structure of the
same parts in these Families of the Birds-of-Prey.