THE BIBBS. 203 



at their base by smaller feathers called coverts, the three 

 growing on the upper arm or humerus being, called ter- 

 tiaries. 



The large tail- or quill-feathers are called rectrices, be- 

 cause they act together as a rudder in flight. There are 

 usually twelve of them which are attached to the last tail- 

 vertebra, and they can be moved up and down, singly or to- 

 gether, or spread out like a fan. Their bases are covered 

 by the tail-coverts. 



Over the tail-bone (coccyx), which ends in a single bone, 

 called the " ploughshare," which very early in life consists 

 of several tail-vertebrae, are usually sebaceous glands, which 

 secrete an oil, used by the bird in oiling and dressing or 

 "preening" its feathers. In some birds, as the cock and 

 turkey, the head and neck are ornamented with naked folds 

 of the skin called "combs" and "wattles." 



At the close of the breeding season birds moult their 

 feathers; but some birds moult twice and thrice. The 

 quill-feathers are usually shed in pairs, but in the ducks 

 they are shed at once, so that these birds do not at this time 

 go on the wing, while the males put off the highly-colored 

 plumage of the days of their courtship, and assume for sev- 

 eral weeks a dull attire. In the ptarmigan both sexes not 

 only moult after the breeding season is over into a gray 

 suit, and then don a white winter suit, but also wear a third 

 dress in the spring. In the northern hemisphere the males 

 of many birds put on in spring bright, gay colors. Other 

 parts are also shed ; for example, the thin, horny crests on 

 the beak of the white pelican after the breeding season, are 

 shed like the horns from the head of deer. Even the whole 

 covering of the beak and other horny parts, like those about 

 the eyes of the puffins and dwarf -auks, may also be regularly 

 shed, while the claws of ptarmigans are regularly cast ofE. 



The power of remaining a long time in mid-air is in- 

 creased by the large air-cells, which are pockets 'filled with 

 air from the lungs. There are nine of these air-sacs — three 

 near the clavicle, four in the thorax, and two in the abdo- 



