Abundance, distribution, and population trend of the Hawaiian Hawk; 1998-2007.

Abstract

The current population size and distribution of the endemic Hawaiian Hawk (or`Io in Hawaiian, Buteo solitarius) on Hawai`i Island was determined as part of a reevaluation of the species’ endangered status. Locations originally sampled in 1998 were resurveyed in 2007 with variable circular plot (VCP) count methods to produce habitat-specific estimates of density and assess trends in abundance. In addition, we developed a method for correcting density estimates for the unobserved movement of hawks attracted to call playbacks. We estimated that the 5,755 km2 breeding range harbored 3,239 hawks (95% CI = 2,610 to 3,868) in 1998 and 3,085 hawks (95% CI = 2,496 to 3,680) in 2007. No significant difference in densities was found among years at either regional or island-wide scales. However, pooled 1998 and 2007 densities did show significant differences among habitats and regions. Our 1998 and 2007 population estimates are as much as twice that of previous estimates, and we discuss reasons for this including differences among studies in the accuracy of distance estimation, accounting for unobserved hawk movement, and the extent of area used for the extrapolation of mean densities. The Hawaiian Hawk appears to persist as a viable population well-distributed throughout forest and adjacent habitats on Hawai`i Island. However, with a population comprised of about 3,000 individuals confined to a single island, the species is vulnerable to the effects of human-caused change to its habitat base.