Comparative use of three methods for estimating the population size of a transferred island endemic: the endangered Seychelles White-eye Zosterops modestus

Abstract

Post-release monitoring is an important aspect of species transfers, providing a basis for conservation and management actions to achieve long-term survival of the species. Between 2001 and 2003, the Seychelles White-eye Zosterops modestus became established on Frégate Island following the transfer of 37 adult colour-ringed birds from Conception Island. Capture and colour ringing of birds was undertaken at regular intervals to keep the majority of the population banded. As the population grew, sampling methodologies became necessary to estimate its size accurately. Here, we compare the results obtained between November 2005 and January 2006 using three different methods: (1) capture-mark-relocate (NOREMARK); (2) point-transect with ‘distance sampling’, and (3) direct systematic surveys. Fifty point counts of 10 min each were conducted and replicated three times. The estimate by capture-mark-relocate was 77 individuals (72–83; P < 0.05) with better precision over distance sampling: 78 individuals (44–136; P < 0.05). The result from direct systematic surveys (81) indicated that estimates obtained with the two indirect methods were reliable. Based on present and previous method comparisons, we recommend using the capture-mark-relocate method for its higher precision and the NOREMARK program to determine sizes of island populations with a significant proportion of marked individuals.