STRUCTURE OF BIRDS. 141 
Except in the Penguins, the feathers of a bird are never evenly 
distributed over the body, but are arranged in long rows or tracts 
(pteryle) separated by more or less wide spaces (apteria). In those 
birds which have no down the spaces may be seen at once by raising the 
contour-feathers, The form and arrangement of these tracts and spaces 
are definite, and characteristic of whole families or orders of birds, and 
are important for purposes of classification. 
The contour-feathers of the head and body overlap one another, 
and their arrangement resembles that of the scales in a reptile or fish. 
The large feathers, which fringe the hinder border of the wing and 
those of the tail, have a peculiar arrangement. The former, called 
the flight-feathers (remiges) (fig. 2 1&2), overlap one another laterally, 
so that their free edges face outwards, towards the front of the extended 
wing. They are divided into two series, primaries and secondaries. 
The primary quills are closely attached to the bones of the hand and 
vary in number from nine to twelve; while the secondary quills 
extend from the wrist inwards to the elbow-joint, and vary from six 
(Humming-birds and Swifts) to thirty-seven (Albatros). 
‘The large quills of the tail (rectrices) (fig. 1 13) serve for steering 
purposes. They rise, like the ribs of a fan, from a common base formed 
by the last bone of the vertebral column. 
Covering of Beak and Feet [Figs. 4, 8, & 9].—The beak is always, 
and the feet are usuaily, devoid of feathers, and encased in a horny 
covering. The beak is formed by the prolongation of the jaws, which 
in modern birds never bear teeth, and its sheath (rhamphotheca) is 
either formed of a single piece or made up of numerous separate 
elements, when it is said to be compound. In some birds, such as 
the Puffins, parts of the beak are periodically shed. The horny 
covering of the feet (podotheca) is generally made up of numerous 
small pieces which take the form of overlapping plates or scales 
(fig. 4 5,6). 
Oil-glands.—With few exceptions, birds have a singular apparatus 
for secreting oil situated on the root of the tail. With the beak they 
press out a drop of oil from this gland, to lubricate and polish their 
plumage. 
Moult.—The renewal of plumage is a process familiar to all under 
the term “moult” (ecdysis). It occurs at least once a year, and 
generally twice, in the spring and autumn, when the old worn-out 
feathers are shed and replaced by new ones. ‘The spring-moult only 
affects the smaller feathers, but in autumn the change is complete, and 
generally results in considerable differences of colour constituting the 
“seasonal plumages” of so many birds, such .as the Grebes, Divers, 
and Weaver- Finches. 
