320 Animal Life 
Purely surface-swimmers, like Gulls (Fig. 8), sit high, especially in the stern, and there 
are many gradations between these two extremes. The Grebes, however, although 
thorough divers, sit high ordinarily, but will sink almost level with the surface at any 
alarm. 
In diving, some birds at all events, such as Grebes and the Diving Ducks, use both 
feet together, as may be easily seen in the case of the latter in St. James’s Park when 
Pochards are diving near the bridge. Under ordinary circumstances grebes and diving 
ducks seem not to open their wings under water, but when chased the latter do so, 
keeping the wings half open. Guillemots fly under water in this way, not using their 
feet at all, and I have seen the elegant Pheasant-Tailed Jagana (Hydrophasianus chirurgus), 
essentially a light-floating surface-bird, do the same thing. 
Penguins are specially constructed for sub-aquatic flight; their wings, which only 
move at the shoulder-joint, beimg perfect paddles, and their feet not being used for 
propulsion under the surface (Hig. 9). They 
differ from other diving-birds m being able to 
swallow prey under water. The only aquatic 
Passerines, the Dippers or Water-Ouzels, also fly 
under water, using the wings half closed; their 
feet are lke those of ordimary Thrushes, and I 
am not aware that they are brought into play 
In swimming. 
It is, of course, the flight of birds which 1s 
their most interesting form of movement, and the 
one which is the most difficult to understand in 
some of its phases. Birds may fly in two ways— 
by beating the air with the wings, or by glding 
with these motionless, although in the latter case 
there must be some previous impulse to give 
momentum. In beating, the wing is brought 
downwards and forwards—much more forwards 
than one is apt to suppose, although instan- 
taneous photographs have made more familar 
poses which Japanese artists had apparently been 
able to appreciate from studying birds with the 
naked eye (Fig. 10). The down-stroke of the wing 
occupies more time than its upward recovery, and 
is that which constitutes the propelling force. 
During its continuance, the hinder edge of the 
wing is forced up and the tips of the primaries 
bent by the resistance of the air, so great is the force employed in the stroke. 
At the close of the down-stroke the wing moves at first backwards, and then, being 
sharply flexed, rises upwards, while the plane of the wing is altered so that its lower 
surface looks forwards instead of backwards as in the down-stroke. The end of the 
humerus, or upper wing-bone, in performing these movements describes a downwardly 
inclined ellipse, the down-stroke occupying the front half of this. Both wings always 
move together, unlike the legs in walking. 
The exertion in this kind of flight is greatest on rising; a Pigeon’s wings touch 
each other at the up-strokes and nearly so at the down-strokes, when the bird starts, and 
for this reason very large birds, such as the Condor and Adjutant, often have great 
difficulty in rising, though they fly superbly when well under way. It is indeed 
probable that, even were the struthious birds provided with wings and pectoral muscles 
