Intra-Island Variation in the 'Elepaio on the Island of Hawai'i

Abstract

The 'Elepaio (Chasiempis sandwichensis) exhibits great vari- ation in plumage color on the Island of Hawai'i. Analysis of specimens from localities throughout the island reveals that the variation is geographically based, and that three subspecies can be distinguished: C. s. bryani, a pale form found at high elevation on the southwestern slopes of Mauna Kea; C. s. ridgwayi, a dark form found in wet windward forests; and C. s. sandwich- ensis, an intermediate form found in forests of the Kona region. Several zones of primary intergradation and one of possibly secondary intergradation occur, but appear to be narrow. The forms C. s. sclateri of Kaua'i and C. s. gayi of O'ahu exhibit no similar intra-island variation. Variation among the Hawai'i subspecies probably evolved in response to local variation in rainfall, and follows the predictions of Gloger's Rule, but the selective forces involved are obscure. The 'Elepaio (Chasiempis sandwichensis) is a monarchine flycatcher that comprises a genus endemic to the Hawaiian Islands. 'Elepaios occur on Kaua'i, O'ahu, and Ha- wai'i, but are enigmatically absent from oth- er seemingly suitable islands. Great plum-