STRUCTURE OF BIRDS. 141 



Except in tl-e Penguins, the feathers of a bird are never evenly 

 distributed over the body, but are arranged in long rows or tracts 

 (pterylai) 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 i &i), overlap one another lateralry, 

 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 usually, 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. 



