Though incubation falls ordinarily to the lot of the female, there are many species in which both sexes alternate in this duty. This is true in the ostrich, various auklets, herons, grebes, petrels, etc. The male rose-breasted grosbeak (Zamelodia ludovici ana) broods devotedly, though in this case his action would seem disadvantageous, as his plumage is strikingly variegated with black and white, and the rose-red spot on his breast often shows above the rim of the nest, while his mate is inconspicuous in a brown, streaked dress. In the phalaropes, the bustard-quails (Tur nix) , the emu and the rhea the male alone incubates, the female taking no part in this duty. It is believed that this also obtains in other shore-birds.
The megapodes or mound-builders (Megapodidae), found from the Nicobars and Philippine islands to Australia, carry on no incubation whatever. For example, the scrub-fowl (Megapodius duperryi), of Northern Territory and Queensland in Australia, scrapes together a mound of sand and earth, to which a small amount of vegetation is added, during the wet season. The mound may be used for a number of consecutive seasons; and mounds 14 ft. in height and 35 ft. in diameter have been discovered. The eggs of one pair of birds are buried in the top of such a mound, and, through heat generated by decomposition of the included vegetation, are maintained at an even temperature of 95° F. The Mallee fowl (Leipoa ocellata) of Australia constructs a much smaller nest by scraping out a hole in the ground 2 f t. wide and I ft. deep, with the excavated earth piled up around the opening, so that the whole resembles the crater of a volcano in miniature. The cavity is filled with leaves, twigs and vegetable debris, scraped up by use of wings and feet for yards around. This is left un covered for four or five months until it is soaked by rains, when decomposition changes it into a hot-bed. A central chamber is excavated in the top at the proper time, and the debris taken out, mixed with sand and returned. In a few days another excavation is made, and an egg placed within it, so that it stands with the small end down. On warm days when the sun shines the birds make a cavity in the top of the mound to catch the rays of warmth. In damp, rainy weather the conical peak of the mound is covered with sticks and rubbish that assist in turning water ' that might penetrate to the eggs. The temperature about the eggs is maintained from 90° to 97° F until they are hatched. During dry seasons, when there is no moisture to promote the necessary decomposition of vegetable material to produce heat, the birds are said not to breed. On the other hand recent researches show that
in some species at least there is no generation of heat by decompo sition within the nests, and the comparatively low temperature necessary for development is maintained by oxidations within the egg itself. The nesting activities of these curious birds are strongly reminiscent of the breeding-habits of the reptilian group, from which the class of birds as a whole is descended.
The period of incubation varies greatly in different groups, in general, being longer in large birds. The period may be shortened slightly by slight increase in incubation temperature or prolonged somewhat by irregular attention on the part of the parent. The incubation period of the emu is said to range from 56 to 63 days, that of the ostrich is reported as 42 days, the domestic mallard requires four weeks, and the domestic fowl three weeks. Most small perching birds require from 12 to 14 days, with magpies and jays running from 16 to 18 days. The shortest incubation period known is that of the American cowbird (Molothrus ater), whose eggs hatch after ion days' incubation.
In the gallinaceous birds, e.g., the domestic fowl, and many other species, the nest is used merely to house and incubate the eggs and the young, which are born covered with down, and follow the mother in search of food as soon as their plumage is dry. In some mound birds the young are hatched with developed wing-quills, so that they are able to fly within half-an-hour of hatching. In most tree-nesting species the young undergo a shorter period of development in the egg, and consequently must remain in the nest under close parental care for a period after hatching. The nest is thus a home during early juvenile develop ment. Food is sought by the parents and brought to the young, and what guard may be possible is maintained against destructive enemies. In hole-nesting species like the hoopoes and wood peckers there is indifference to nest sanitation, as immunity against dangerous bacteria that multiply actively when heat and moisture are available has apparently been developed. Such nests become extremely foul as the period of occupancy progresses. Hawks, herons and numerous others, even when very young, instinctively void their excrement beyond the nest, so that such unsanitary condition is obviated. In most of the smaller perching birds the excrement of the young is of such a consistency that it maintains a globular form and is removed from the nest by the parent. In a few finches, as the American goldfinch and crossbills, the ex creta are voided by the young about the margin of the nest and are not removed.