Abstract
A breeding population of black-browed albatrosses has been known to exist at the Ildefonso Archipelago, Chile, for >90 years but the population has never been censused using scientifically defendable methods. To estimate population size, and examine the accuracy and practicality of various census methods, the population was censused in the 2002/2003 breeding season using (a) ground-truthed aerial photography, (b) yacht-based photography, (c) ground counts, (d) quadrat sampling and (e) point-distance sampling. Compared to ground-truthed aerial photography (judged the most accurate) yacht-based photography underestimated population size by 55%, ground counts by 13%, quadrat sampling by 11% and point-distance sampling by 9%. Ground-truthed air photography revealed that in the 2002/2003 breeding season 47,000 pairs of black-browed albatrosses bred at the Ildefonso Archipelago. A repeat aerial census in 2006 suggested the size of the breeding population had not changed in the 4 years between the two censuses. After the Falkland Islands/Islas Malvinas, South Georgia and Diego Ramirez, the Ildefonso Archipelago holds the fourth largest population of black-browed albatrosses in the world.
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Acknowledgments
We are grateful to the Armada de Chile for permission to work at Ildefonso, to Roger Wallis, skipper of the yacht ‘Spirit of Sydney’ for his seamanship in landing us on Ildefonso, to Sergio Cortes (in 2002) and Luis Utmann (2006) for their skill in piloting the twin otter, to Jay Watson for assistance on the ground and Rachel Alderman for assistance with the counts on the air photos in 2002. Carlos A. Moreno was funded by Proyecto Fondo de Investigacion Pesquera 2006–2030. Comments by Javier Arata, Barry Baker, Sally Poncet and Barbara Wienecke improved a draft.
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Appendices
Appendix 1
Appendix 2
In 2006 a total of 31,770 black-browed albatrosses were counted on the air photographs of Ilsa Grande. This figure corresponds to the 30,680 albatrosses counted on Isla Grande in 2002 (see Table 2). Both are raw (uncorrected) counts. The estimate in 2006 is 3.6% higher than in 2002 but was conducted three weeks earlier in the breeding season. There is also considerable within-hour variation in the ratio of nesting-to-total birds for any given days (see Fig. 4), which confounds the assessment of differences between years. For these reasons the result for 2006 is not considered to indicate an increase in the size of the breeding population at Ildefonso in the 4 years between the censuses.
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Robertson, G., Moreno, C.A., Lawton, K. et al. Comparison of census methods for black-browed albatrosses breeding at the Ildefonso Archipelago, Chile. Polar Biol 31, 153–162 (2008). https://doi.org/10.1007/s00300-007-0342-7
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DOI: https://doi.org/10.1007/s00300-007-0342-7