We examined more than 90 nests of the Common Amakihi (Hemignathus virens virens) from the Island of Hawaii to determine if their placement, composition, or insulation varied with meteorological conditions at the time of nesting. Common Amakihi nest chiefly during the wet season. The nests were always within the canopy of the nest trees and consequently were probably shielded from rain. The nests from a warm rain forest on Kohala Mountain were significantly higher in the canopy and nearer each tree' s center than the nests from Mauna Kea, a cold upland Savannah. On Mauna Kea, nests were nearer the edge of the canopy at higher elevations, i.e., in a location where they would benefit from radiant solar energy. Nests from Kohala Mountain lacked liners and were more porous than those from Mauna Kea. These features permitted the nests to dry rapidly. Nests from Mauna Kea, in contrast, were always lined, which retarded drying and increased their insulating capacity-features appropriate for the drier, colder conditions in areas where they were built. All of the nests were excellent windscreens. The thermal conductance of nests from Mauna Kea diminished with altitude, i.e., nests at higher elevations had more insulation than those at lower elevations. This trend was associated with differences in the nests' walls, which were denser (but not thicker) at higher elevations. The nest' s thermal conductance can be used to estimate the energetic expense of incubation. For Common Amakihi, the energy required to keep a clutch at incubation temperature may be as much as 0.115 W or 47% of the birds' metabolic rate at rest.

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The ventral apterium of free-living female White-crowned Sparrows (Zonotrichia leucophrys gambelii) becomes an incubation patch during the breeding season. At this time, it loses its feathers, increases in wet and defatted dry weight, and undergoes marked histological alterations. At times of year other than the breeding season, the apterium consists of a low squamous epidermis and a thin, poorly vascularized dermis of dense connective tissue. The dermis is separated from subcutaneous tissue by an internal elastic lamina. During the breeding season, the epidermis is a proliferative, stratified squamous epithelium with well-defined basal, intermediate, transitional, and cornified layers; and the dermis consists of a superficial layer of collagen and a deep layer of highly vascular areolar connective tissue, noticeably edematous and mildly inflamed. Blood vessels are frequently in large groups in the center of the dermis. Edema and hypervascularity are most pronounced during incubation, but the epidermis is best developed during egg-laying. The apterium reverts to its basal state after the incubation period. Captive females, which do not breed, do not develop incubation patches. Estrogen is apparently responsible for feather loss and collagen synthesis. It and other unidentified hormones (probably prolactin and/or androgens produce the hypervascularity, edema, and epidermal growth.