Effects of observers using different methods upon the total population estimates of two resident island birds

-During a 5-week study of the Nihoa Millerbird and Nihoa Finch, we censused birds using these techniques: two line transect methods, a variable-distance circular plot method, and spot-mapping of territories (millerbirds only). Densities derived from these methods varied greatly. Due to differences in behavior, it appeared that the two species reacted differently to the observer. Millerbirds appeared to be attracted to a moving observer, perhaps to forage on insects; finches appeared to be attracted to a stationary observer in order to feed on seabird eggs temporarily abandoned during the count. Although these behaviors may be unusual, they dramatically demonstrate that no single census method will suffice for all species. The method that assures the least observer effect will provide the most accurate population estimate. The ornithological literature contains few serious attempts to determine total species populations; the few cases are of very rare or insular populations, usually both. Because of the lack of immigration and emigration geographically inherent in island environments, the census of resident island bird populations can provide important insights into many questions of avian biology. We discuss here two such species’ populations and demonstrate the potential of different census methods that result in quite different population estimates. Nihoa Island, a volcanic high island remnant, is the easternmost of the Northwestern Hawaiian Islands, which largely make up the Hawaiian Islands National Wildlife Refuge. The island is 63.2 ha in area, rising to 273 m, with an average southward facing slope of 45”. The east, west, and north coasts are sheer cliffs, and the south coast consists of low (lo-20 m) cliffs skirted by rock benches. There is one nearly inaccessible beach. The vegetation is very low, rarely exceeding 0.75 m high, and is largely made up of three shrub species (Sida fallax, Solanum nelsoni, and Chenopodium oahuense). Nihoa is one of the only two sizable high islands (Necker is the other) of the Northwestern Hawaiian Islands. As such, it supports a biota that is unique in several ways, as compared to the refuge’s atolls. Nihoa has endemic plants (Herbst in Clapp et al. 1977), several arthropod taxa (Beardsley 1966) and a very dense population of seabirds (including large numbers of shearwaters, petrels, and terns). Nihoa also has two endangered, endemic passerines, the Nihoa Millerbird (Sylviidae: Acrocephalus familiaris kingi) and the Nihoa Finch (Drepanididae: Psittirostrn ultima). The Laysan Millerbird (A. f. ’ Department of General Science. University of Hawaii, 2450 Campus Road, Honolulu, Hawaii 96822. t USDA Forest Service, Institute of Pacific Islands Forestry, II51 Punchbowl Street, Honolulu, Hawaii 96813. familiaris), now extinct, was the only other native Hawaiian sylviid. The Nihoa Finch and its close relative the Laysan Finch (P. cantans), are two of the only four extant finch-billed drepanidids, all of which are endangered (USFWS 1980). Survival of these two finches on two tiny islands will be dependent on management programs that successfully prevent introduction of exotic biota and other forms of human disturbance . The difficulty of successfully landing on Nihoa (approximately 50% of all attempts succeed), and the rigorous field conditions, explain why so little is known of the biology of the millerbird and the finch. Aside from a 12-day field expedition in 1969 by John Sincock of the U.S. Fish and Wildlife Service and Ernest Koska, then of the Hawaii Division of Fish and Game, only about 70 days have been spent in the study of the biota of Nihoa (Clapp et al. 1977). Periodic, usually annual, visits by USFWS biologists have rarely entailed more than a brief census of the two land birds. As a result, population estimates for the millerbird and finch have fluctuated greatly, in part no doubt a result of sampling intensity, as well as a reflection of population trends. Estimates of millerbirds have varied from as low as 41 to as high as 592. Estimates of finches have ranged from 1318 to 6686 (Sincock in Clapp et al. 1977). Recent (1977-1979) millerbird estimates by Sincock (pers. commun.) range from 127 +11% (95% confidence limits) to 490 ? 60%. Sincock’s finch estimate for 1979 was 3612 ? 40%. Obtaining total population estimates of these two resident island birds is desirable for at least two reasons: (1) knowledge of the total population size would enhance our understanding of population dynamics and limiting factors for these species, and (2) development and implementation of management plans would be aided by accurate information on total population numbers.