Assessing population size and structure for Andean Condor Vultur gryphus in Bolivia using a photographic ‘capture‐recapture’ method

Diego Méndez,Huw Lloyd,Stuart Marsden

doi:10.1111/ibi.12681

Abstract

The Andean Condor Vultur gryphus is a globally threatened and declining species. Problems of surveying Andean Condor populations using traditional survey methods are particularly acute in Bolivia, largely because only few roosts are known there. However, similar to other vulture species, Andean Condors aggregate at animal carcasses, and are individually recognizable due to unique morphological characteristics (size and shape of male crests and pattern of wing coloration). This provided us with an opportunity to use a capture‐recapture (‘sighting‐resighting’) modelling framework to estimate the size and structure of an Andean Condor population in Bolivia using photographs of individuals taken at observer‐established feeding stations. Between July and December 2014, 28 feeding stations were established in five different zones throughout the eastern Andean region of Bolivia, where perched and flying Andean Condors were photographed. Between one and 57 (mean = 20.2 ± 14.6 sd) Andean Condors were recorded visiting each feeding station and we were able to identify 456 different individuals, comprising 134 adult males, 40 sub‐adult males, 79 juvenile males, 80 adult females, 30 sub‐adult females and 93 juvenile females. Open population capture‐recapture models produced population estimates ranging from 52 ± 14 (se) individuals to 678 ± 269 individuals across the five zones, giving a total of 1388 ± 413 sd individuals, which is roughly 20% of the estimated Andean Condor global population. Future trials of this method need to consider explicitly knowledge of Andean Condor movements and home‐ranges, habitat preferences when selecting suitable sites as feeding stations, juvenile movements and other behaviours. Sighting‐resighting methods have considerable potential to increase the accuracy of surveys of Andean Condors and other bird species with unique individual morphological characteristics.

Highlights

The Andean Condor Vultur gryphus is a globally threatened and declining species
As field categorization by sex and age in Andean Condors is reasonably straightforward (Ferguson-Lees & Christie 2001) and their unique plumage characteristics and unique crest and dewlap characteristics in adult males show no change throughout periods of at least 3 months and may even extend for years or even the whole life of some individuals (Snyder et al 1987, McGahan 2011), the species can be surveyed using a sighting-resighting approach, as has been done to survey other animal species with individually unique morphological characteristics (Karanth & Nichols 2002, Silver et al 2004)
Using carcasses placed at observer-established feeding stations, we were able to attract and identify Andean Condor individuals based on unique morphological characteristics of bare skin and feather plumage (Figs 2 and S1)

Summary

Introduction

The Andean Condor Vultur gryphus is a globally threatened and declining species. Problems of surveying Andean Condor populations using traditional survey methods are acute in Bolivia, largely because only few roosts are known there. Similar to other vulture species, Andean Condors aggregate at animal carcasses, and are individually recognizable due to unique morphological characteristics (size and shape of male crests and pattern of wing coloration) This provided us with an opportunity to use a capture-recapture (‘sighting-resighting’) modelling framework to estimate the size and structure of an Andean Condor population in Bolivia using photographs of individuals taken at observer-established feeding stations. We use photographs of individual Andean Condors attracted to strategically sited feeding stations to assess the population size and structure of the species across the eastern Andes of Bolivia, corresponding to the largest portion of the species’ range in the country (Balderrama et al 2009). We used a sighting-resighting model to test the suitability of such a set of survey feeding stations to generate population estimates and make recommendations for employing this method for future surveys of this and other bird populations

Methods

Results

Conclusion