begin to exhibit old-growth characteristics at about 175 to 250 years of age. Moss, on which marbled murrelets nest, forms on the limbs of Douglas-fir that are more than 150 years old. The four tree nests found before 1990 shared the following characteristics: (1) located in a large tree (>47 inches [120 cm] d.b.h.) with an open crown structure, (2) on a moss-covered limb that is camouflaged, partially shaded, and approximately horizontal with a diameter (including associated moss) of at least 14 inches (36 cm), and (3) located within the middle or lower part of a live crown. However, Marshall stated that because of their low aerial buoyancy marbled murrelets often nest high in the treetops or on steep slopes. Habitat must be sufficiently open to allow for easy flight. All marbled murrelet nests found in Washington, Oregon, and California were located in old-growth trees that ranged from 38 inches (88 cm) d.b.h. to 210 inches (533 cm) d.b.h. with a mean of 80 inches (203 cm) d.b.h. Nests were located high above the ground and had good overhead protection but allowed easy access to the exterior forest. It was initially believed that marbled murrelets might use the same nest in successive years but there has been little evidence of this. Stand size is also important in nest sites. Marbled murrelets more commonly occupy stands greater than 500 acres (202 ha) than stands less than 100 acres (40 ha). However, they may nest in remnant old-growth trees or groves that are surrounded by younger trees. In California, marbled murrelets are usually absent from stands less than 60 acres (24 ha) in size. In Washington, marbled murrelets are found more often when old-growth and mature forests make up over 30% of the landscape. Fewer marbled murrelets are found when clearcut and meadow areas make up more than 25% of the landscape. Concentrations of marbled murrelets offshore are almost always adjacent to old-growth or mature forests onshore, although marbled murrelets may not use the interior of dense stands. Where large trees are absent in the northern parts of marbled murrelet range, marbled murrelets nest in depressions on the ground, in rock cavities on the ground, or on rock outcrops. Marbled murrelets are both ground nesters and tree nesters where forests and treeless areas meet. In northern regions where coniferous forests nest sites are unavailable, marbled murrelets occupy alpine or tundra near the