Breeding Ecology of the Hawaii Common Amakihi

Abstract

The ecology and breeding biology of the Common Amakihi (Hemignathus virens) was studied on Mauna Kea, Hawaii, from 1970 through 1981. Common Amakihi retained mates for more than one season and returned each year to the same area to breed. There was an excess of unmated males in the population. First-year birds were rarely successful in their initial breeding attempt. The species had a protracted breeding season, with nests found in every month of the year except August through October. However, the most intensive breeding occurred from March to May, that period of greatest mamane (Sophora chrysophylla) flowering. Ritualized courtship in the Common Amakihi included high altitude and low altitude chases. Territories encompassed the nest site and all food resources, and was a fixed location that often had the same boundaries in succeeding years. The size and location of territories were related to available food resources. Nests were statant, open cupped, with mamane trees the preferred nesting substrate. Clutch size averaged 2.5 eggs and varied between years such that, in a year of low food availability, birds with smaller clutch sizes fledged more young than did those with larger clutches. Incubation period was 14 days and clutches hatched asynchronously. Difference in nestling sizes resulted in brood reduction through elimination of the smallest individual in years of decreased food supply. The nestling period averaged 17 days and young usually fledged synchronously. Both parents fed the young but only the female incubated and brooded. Based on 150 nests with complete records, hatching success was 53.5%, fledging success was 64.9%, and overall reproductive success was 34.7%. The major mortality factor during the breeding cycle was nest desertion by the adults. There were six major factors which influenced productivity on Mauna Kea. The two most important were length of the breeding season and clutch size. Other factors were territoriality, hatching sequence, nestling growth rates, and nestling mortality.