National Geographic : 1951 Jun
814 Mother's Nesting Impulse Is So Strong that It Overcomes the Fear of Man Ruby-throated hummingbird makes her nest of pussy-willow down, fastens it with cobwebs, and trims it with lichens. She curves the rim inward so the two bean-size eggs will not spill out in high winds. Other birds she drives away. Her carefree mate never comes home; he retires to his one-man club, or territory. all his time singing and defending the terri tory rather than sitting on eggs. In birds like the indigo bunting, the gold finch, and most warblers, whose males are brightly colored and the females plain, the males do not sit on the eggs at all, though they do help feed the young. The male rose breasted grosbeak is one brightly colored male that does sit on eggs, but the ruby throated hummingbird not only passes up egg sitting but disdains all household duties as well. Every bird species lays eggs of definite size, shape, color, and markings. There is, like wise, a rather definite clutch number for each species, from which they seldom depart. Auks and murres lay but a single egg; humming birds, 2; robins, 3 to 5; chickadees, 5 to 8; grouse, 8 to 15; and so forth. The number is probably an adaptation to the dangers to which the eggs and young are subjected. Each egg has a definite incubation period, or time required for hatching. This ranges from the 10 days of a cowbird to the 78 days of a royal albatross, with the average around 12 or 14 days for small birds like robins and sparrows. Some species, like most of the shore birds, the grackles, and warblers, regularly have only one brood in a season; others, like the doves, sparrows, and thrushes, may have two or three. Most birds, if the nest is broken up, will attempt to nest again. Chiselers of the Bird World There are a few birds, like the Old World cuckoos, the African honey guides, certain weaverbirds, and the New World cowbirds, that have lost entirely their parental instincts and have become social parasites. Laying their eggs in other birds' nests, they let the foster parent hatch the eggs and rear the young. Some of the birds, like the wrens, catbirds, and robins, respond by throwing out the cow birds' eggs. Others, like the yellow warblers, frequently bury the cowbirds' eggs in the bottom of the nest. But the majority of small birds just accept and hatch the eggs and rear the young cowbirds (pages 795, 804). Often this costs them their own young, which are smaller, grow more slowly, and are eventually smothered or crowded out of the nest.