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

P. Marcos Gorresen; Kevin W. Brinck; Richard J. Camp; Chris Farmer; Sheldon M. Plentovich; Paul C. Banko

doi:10.1650/condor-15-214.1

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

P. Marcos Gorresen·Kevin W. Brinck·Richard J. Camp·Chris Farmer·Sheldon M. Plentovich·Paul C. Banko

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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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Condor Article

Source-reported bounding extent: 23.05546886248638° to 23.064807339477476° latitude; -161.92892789840695° to -161.913800239563° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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BibTeX RIS

P. Marcos Gorresen, Kevin W. Brinck, Richard J. Camp, Chris Farmer, Sheldon M. Plentovich, Paul C. Banko. 2016. State-space modeling of population sizes and trends in Nihoa Finch and Millerbird. https://doi.org/10.1650/condor-15-214.1

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