XVlil INTRODUCTION. 



A peculiar gland exists in most birds under the skin 

 on the upper part of the tail bone, secreting an oily matter 

 with which birds smear or preen their feathers to prevent 

 them from getting wet, and it is therefore most developed 

 in water birds. 



Most birds are monogamous, a few, chiefly among the 

 Gallinacese, being polygamous, and the males of these are 

 very pugnacious, especially at the breeding season. The 

 continuation of the species is effected by eggs, which are 

 either laid in nests, or simply on the ground. Some nests 

 are beautifully and most artificially made, as in the Weaver 

 birds, the Tailor birds, Hone5^suckers, Orioles, &c.; and are 

 sometimes lined with feathers, or other soft materials, but 

 less so, in hot countries, than in cold or temperate climes. 

 Others are simply made of a few sticks and roots ; in a 

 few families with mud agglutinated by saliva ; and some, 

 (the Collocalia}) construct their nests almost entirely of 

 inspissated saliva. Many birds nidificate in holes of trees, 

 bome in holes in banks, or in clefts of rocks, and others on 

 the ground. Most birds make their nests solitary. A good 

 many, however, such as some Swallows and Swifts, Bee- 

 eaters, Weaver birds, and others, build in company. 



The eggs vary in number ; few in birds of prey, numer- 

 ous in Gallinacea3 and many in water-birds ; moderate in 

 most Insessores, though numerous in a very few genera. 

 The colour is often uniform in certain families and tribes ; 

 white in Owls, Swifts, most Fissirostral birds, and in most 

 of the Scan sores ; much mottled and variegated in most 

 Grallatores and Natatores ; blue in the Herons, and in 

 many Thrushes, Accentor, true Cuckoos, &c,, and vari- 

 ously spotted, and streaked in others. Most birds hatch 

 their own eggs. The true Cuckoos deposit their eggs in 

 the nests of other birds, and the Ostrich leaves her eggs 



