Altitudinal Variations in Nests of the Hawaiian Honeycreeper Hemignathus virens virens

Abstract

We examined more than 90 nests of the Common Amakihi (Hem- ignathus 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 signifi- cantly 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. The Common Amakihi (Hemignathus virens, family Drepanididae) is an abundant and widespread forest bird of the Hawaiian islands. The subspecies from the Island of Hawaii (H. v. virens) occurs at elevations between 650 and 3,000 m. Some populations nest in areas that are extremely wet and warm (rain forests), whereas others nest in relatively dry cold areas (upland savannahs); all commonly do so dur- ing the rainy season (Berger 1969, Eddinger 1970, van Riper 1978). On the Island of Ha- waii, their breeding season extends from late October through August. We speculated that the small size of the ama- kihi (13-l 4 g), coupled with the extreme vari- ations in moisture and temperature to which populations are exposed while nesting, might have led to altitudinal differences in nest struc- ture. For example, breeding success in rain for- ests might require a nest that will drain readily and dry quickly, but one that need not be high- ly insulated because of the mild and fairly con- stant temperatures in such habitats. In fact, amakihi on the Island of Kauai sometimes brace their nests against tree trunks where they are soaked by water running down the bark during heavy rains. In contrast, amakihi nest- ing at high elevations on the Island of Hawaii, where it is decidedly drier and colder, may require a more highly insulated nest, but one that is still capable of drying efficiently. Here, a nest with a liner might be more adaptive. In order to address these possibilities, we con- ducted qualitative and quantitative studies of m&e than 90 amakihi nests collected at many different elevations on the Island of Hawaii.