Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:
Broad-tailed hummingbirds often nest in quaking, slender deciduous trees with smooth, gray-green bark found in the Colorado Rockies of the Western United States. After flying some 2,000 kilometres north from where they have wintered in Mexico, the hummingbirds need six weeks to build a nest, incubate their eggs, and raise the chicks. A second nest is feasible only if the first fails early in the season. Quality, not quantity, is what counts in hummingbird reproduction. A nest on the lowest intact branch of an aspen will give a hummingbird a good view, a clear flight patch, and protection for her young. Male hummingbirds claim feeding territories in open meadows where, from late May through June, they mate with females coming to feed but take no part in nesting. Thus when the hen is away to feed, the nest is unguarded. While the smooth bark of the aspen trunk generally offers a poor grip for the claws of a hungry squirrel or weasel, aerial attacks, from a hawk, owl, or gray jay, are more likely. The choice of where to build a nest is based not only on the branch itself but also on what hangs over it. A crooked deformity in the nest branch, a second, unusually close branch overhead, or proximity to part of a trunk bowed by a past ice storm are features that provide shelter and make for an attractive nest site. Scarcely larger than a halved golf ball, the nest is painstaking constructed of spider webs and plant down, decorated and camouflaged outside with paper-like bits of aspen bark held together with more strands of spider silk. By early June it will hold two pea-sized eggs, which each weigh one-seventh of the mother's weight, and in sixteen to nineteen days, two chicks.
Question: What aspect of broad-tailed hummingbird behaviour does the passage mainly discuss?
