Volume 52, No. 1

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Breeding population size of the Pink-footed Shearwater Ardenna creatopus on Isla Mocha, Chile.



1Oikonos-Ecosystem Knowledge, Avenida Cerro El Plomo 5931, Los Condes, Santiago, Chile *(
2Center for Ecosystem Sentinels, Department of Biology, University of Washington, Washington, 98195, USA
3U.S. Geological Survey, Western Ecological Research Center, Santa Cruz, California, 95060, USA
4University of Puget Sound, Tacoma, Washington, 98416, USA
5Corporación Nacional Forestal, Isla Mocha, Provincia de Arauco, Chile


CARLE, R.D, VARELA, T., COLODRO, V., CLARK-WOLF, T., FELIS, J., HODUM, P., ASTETE CASTILLO, F.J. & LÓPEZ, V. 2024. Breeding population size of the Pink-footed Shearwater Ardenna creatopus on Isla Mocha, Chile. . Marine Ornithology 52: 85 - 96

Received 14 July 2023, accepted 20 December 2023

Date Published: 2024/04/15
Date Online: 2024/04/09
Key words: population model, burrowing seabird, Juan Fernández Islands, stratified-random sampling, fisheries bycatch


Species population estimates are a fundamental component of conservation planning, but there are deficiencies in reliable data for many seabirds. The Pink-footed Shearwater Ardenna creatopus is a seabird that breeds on three islands worldwide, with the largest population on Isla Mocha, Chile. We aimed to update the breeding population estimate of Pink-footed Shearwaters on Isla Mocha, comparing results from design- and model-based estimation methods. We counted shearwater burrows in 220 randomly generated five-meter-radius plots across pre-defined strata on Isla Mocha. We estimated total number of burrows using area-based extrapolation (design-based method), and separately using a model predicting burrow density based on habitat (model-based method). We multiplied burrow abundance estimates by burrow occupancy for final population estimates. The stratum-area-weighted burrow density estimate for the 15.8 km2 study area was 0.0106 burrows·m-2 (standard error [SE] = 0.0030). The average island-wide proportion of occupied burrows was 0.758 (standard deviation [SD] = 0.121). The design-based method estimated 168 209 burrows (95% confidence interval [CI] = 74 715-261 704, coefficient of variation [CV] = 0.28), and 127 503 breeding pairs (95% CI = 87 610-167 395). The model-based method estimated 233 436 burrows (95% CI = 151 237-332 179, CV = 0.19) and 181 859 breeding pairs (95% CI = 95 773-267 945, CV = 0.24). These population estimates are greater than previous estimates for Isla Mocha, whose means ranged from 19 440-42 095 breeding pairs. Because our study design differed from those used to generate previous estimates, our estimate should be considered a stand-alone result rather than an increase in the breeding population. Because of the low fit of the model-based result, the design-based result may be a more reliable estimate to use for species management efforts. Based on our estimate, approximately 90% of the Pink-footed Shearwater world population breeds on Isla Mocha, and with its restriction to only three breeding localities world-wide, the species remains vulnerable. The full manuscript in Spanish can be found in Appendix 1, available on the website.


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