Summary
Black-browed albatrosses breed at four known locations in Chile: Diego de Almagro (15,900 pairs, Lawton et al. 2003), Evangelistas (4,700; Arata et al. 2003), Ildefonso (47,000; this presentation) and Diego Ramirez (55,300; Robertson et al. unpublished). After the Falkland Islands/Islas Malvinas (382,000 pairs; Huin 2001), Chile (123,000 pairs) holds the second largest populations of black-browed albatrosses in the world. The population at the Ildefonso islets has been censused only three times in recorded history - in 1914 (Beck, in Murphy 1936), 1984 (Clarke et al. 1992) and 2001 (Aguayo-Lobo et al. 2001). However, each census was made at sub-optimal stages in the breeding cycle using inexact techniques (either by eye or video from an offshore position), which casts doubt over accuracy. Ildefonso is reasonably typical of many locations in the Southern Hemisphere where albatrosses breed: it is remote, difficult to land on and sparsely vegetated, all of which have implications for choice of census method and accuracy. To determine the status of black-browed albatrosses at Ildefonso, and to determine the most accurate census method, we estimated population size on 26 October 2002 using five techniques: aerial photography with ground truthing (the ‘control’), yacht-based photography, ground counts, quadrat sampling and point distance sampling. Air photography yielded an estimated 46,995 nests; since all albatrosses were visible from the air this number is believed to be the size of the breeding population in October 2002. All other census methods underestimated population size: yacht-based photographic counts by 55%, ground counts by 13%, point distance sampling by 11% and quadrat sampling by 2%. Choice of census method involves trade-offs between cost of ‘vehicle’ (aircraft, yacht, vessel to land ground parties), labour, disturbance to breeding albatrosses and required accuracy. Disregarding yacht-based photography (due to the large error) at Ildefonso ground truthed air photography was by far the most accurate and least intrusive method tested. Air photography also has the advantage of producing archival images, which is important to the accurate assessment of status and trends in the long term.