State-space modeling of population sizes and trends in Nihoa Finch and Millerbird

Condor
By: , and 

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Abstract

Both of the 2 passerines endemic to Nihoa Island, Hawai‘i, USA—the Nihoa Millerbird (Acrocephalus familiaris kingi) and Nihoa Finch (Telespiza ultima)—are listed as endangered by federal and state agencies. Their abundances have been estimated by irregularly implemented fixed-width strip-transect sampling from 1967 to 2012, from which area-based extrapolation of the raw counts produced highly variable abundance estimates for both species. To evaluate an alternative survey method and improve abundance estimates, we conducted variable-distance point-transect sampling between 2010 and 2014. We compared our results to those obtained from strip-transect samples. In addition, we applied state-space models to derive improved estimates of population size and trends from the legacy time series of strip-transect counts. Both species were fairly evenly distributed across Nihoa and occurred in all or nearly all available habitat. Population trends for Nihoa Millerbird were inconclusive because of high within-year variance. Trends for Nihoa Finch were positive, particularly since the early 1990s. Distance-based analysis of point-transect counts produced mean estimates of abundance similar to those from strip-transects but was generally more precise. However, both survey methods produced biologically unrealistic variability between years. State-space modeling of the long-term time series of abundances obtained from strip-transect counts effectively reduced uncertainty in both within- and between-year estimates of population size, and allowed short-term changes in abundance trajectories to be smoothed into a long-term trend.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title State-space modeling of population sizes and trends in Nihoa Finch and Millerbird
Series title Condor
DOI 10.1650/CONDOR-15-214.1
Volume 118
Issue 3
Year Published 2016
Language English
Publisher Cooper Ornithological Club
Publisher location Santa Clara, CA
Contributing office(s) Pacific Island Ecosystems Research Center
First page 542
Last page 557
Country United States
State Hawai'i
Other Geospatial Nihoa Island
Online Only (Y/N) N
Additional Online Files (Y/N) N
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