Abundance and Demography of the Hawaiian Hawk: Is Delisting Warranted?

Abstract

To provide the U.S. Fish and Wildlife Service (USFWS) with updated information on the Hawaiian hawk ('io, Buteo solitarius) for reconsideration of its current endangered status, we determined population size, distribution, habitat availability, survival, fecundity, and finite rate of increase (λ) of 'io during 1998 and 1999 on the island of Hawaii. We estimated the total population at 1,457 ± 176.3 SE 'io. Birds were broadly distributed around the island, but highest densities were found in mature native forest with a grass understory (0.57 ± 0.12 'io/km 2 ). We estimated that 58.7% of the island (6,144 km 2 ) was usable habitat for 'io. Of the usable habitat, 31.8% (1,954 km 2 ) currently is protected by state and federal forests, parks, and refuges. Based on an average density of 0.24 ± 0.08 'io/km 2 in those areas, we calculated that protected areas currently support 469 'io (95% CI: 244-901). In all habitats combined, first-year and adult survival was 0.50 ± 0.10 and 0.94 ± 0.04, respectively, and fecundity was 0.23 ± 0.04 female young/breeding female. Overall, λ was 1.03 ± 0.04. Elasticity analyses showed adult survival to be the most important parameter regulating finite growth of this population. The population appears viable based on λ is approximately 1 and high adult survival. Additionally, substantial areas of habitat are protected, 'io appear resistant to avian diseases found on the island, no evidence exists of negative impacts from contaminants, and the birds show the ability to use human-altered landscapes and exotic prey. Because of the short study duration, relatively low population size, variance around our estimates (abundance, fecundity, survival, λ), and environmental stochasticity, we believe that delisting to a nonthreatened status is unwarranted. However, we believe that downlisting to threatened status would be appropriate for this species, or embracing a listing as near threatened under the International Union for the Conservation of Nature (IUCN) Red List categories. Regardless of a change in political status, population size and especially adult survivorship should be routinely monitored.