`Apapane and Hawai`i `Amakihi occur on other islands and are somewhat more numerous at about 93,000 and 44,000 individuals, respectively, but these represent a large proportion of each species' total population. Haleakala supports about 18,000 `I`iwi, another species of global conservation concern. The entire population of about 34,000 Maui Alauahio is also restricted to east Maui. The endangered Maui Parrotbill and Akohekohe or Crested Honeycreeper number about 500 and 3,700 individuals, respectively, and occur only in high elevation forests on northern and eastern Haleakala.
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The Halekala IBA supports one of the greatest concentrations of endemic Hawaiian forest birds, including the entire population of several species that are endangered or of global conservation concern. The area is 46,602 hectares (115,106 acres) in size and includes Haleakala National Park, Hanawi State Natural Area Reserve, Ko`olau, Hana, Kipahulu, Kahikinui, Kula, and Makawao State Forest Reserves, Polipoli Spring State Recreation Area, State Department of Hawaiian Home Lands, The Nature Conservancy's Waikamoi Preserve, and some private lands. Invasive alien plants dominate much of the lowlands on all sides. The drier southern and western slopes have been extensively altered by human development, timber harvesting, grazing, and agriculture. Dense rainforests on the northern and eastern slopes have hindered human development, and these areas contain some of the most intact native ecosystems in Hawai`i. The windward slopes in particular have been eroded by numerous streams. Three large canyons cut the flanks of Haleakala, Ko`olau Gap on the north and Kaupo Gap and Kipahulu Valley on the south. The crater and summit of Haleakala consist of bare lava and ash punctuated by many cinder cones, with only sparse vegetation. Vegetation becomes shorter at higher elevations, giving way to subalpine shrubland and grassland above about 2,000 meters (6,600 feet).
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The northern and eastern slopes are covered in dense rainforest, the southern and western slopes support dry shrubland and mesic forest. Annual rainfall ranges from about one meter (39.3 inches) in the southwest to over seven meters (275 inches) at around 1,000 meters elevation on the northeastern face. Prevailing northeasterly trade winds cause high rainfall on the northern and eastern slopes, while the southern and western sides of the mountain are drier. The Haleakala Important Bird Area encompasses most of the higher elevations of the volcano, from near sea level on the northern coast to the summit at 3,055 meters (10,023 feet). Haleakala means "house of the sun" and is also the name of the active shield volcano that forms the eastern part of the Hawaiian Island of Maui.