and down as they do so. This display can continue for several minutes before ending abruptly; afterwards, the birds may sit quietly together for some time before separating again. They raise a single brood per year, nesting singly or in loose colonies of , with nests spaced at least apart. Unlike most other gulls, Bonaparte's gulls typically nest in trees — almost exclusively conifers, including black spruce, Atlantic white cedar, tamarack and jack pine. Both sexes help to build the nest, which is a platform of small sticks, sometimes with lichen, moss or marsh vegetation added, measuring about in diameter. Most nests are placed above the ground, and within of open water, though they have been found low as , as high as , and as far as from open water. They are only rarely placed on the ground. Adults are aggressive in defending their nests, chasing away even large potential predators such as hawks, common ravens and humans. There is some evidence that human activity in the area of their nests may cause decreased productivity — they are known to travel as far as a kilometer (about a half mile) to mob people, for example — but, contrarily, some choose to nest near human habitation. The female lays a clutch of , with most nests containing . The eggs are subelliptical, slightly glossy and smooth, measuring and weighing . They range in colour from pale to medium green, olive or buff, and may be variably marked with spots, blotches, or scrawls of brown, grey, violet, or black; these markings may be fine and evenly distributed over the entire surface of the egg, or thick and concentrated towards the egg's larger end. The eggs are incubated by both parents for . Studies have shown that breeding populations can be significantly impacted by bad weather. In one multi-year study in Churchill, Manitoba, for example, more than half of the eggs laid in observed nests were blown out of those nests during storms; only 42% of eggs survived to hatching. Hatchling Bonaparte's gulls are semi-precocial, emerging from their eggs covered with natal down feathers, and with eyes already opened. The down is yellowish-buff, greyer on the flanks and belly, and somewhat pinkish on the breast, with dark brown mottling on the upperparts and head. The bill is blue-black with a pinkish base, and the legs and feet are pinkish-buff. Young birds typically