Green Warbler-finch, Certhidea olivacea, is a recently-split species endemic to the Galápagos Islands, Ecuador, and is found only on the islands of Santa Cruz, Isabela, Fernandina, Santiago, Baltra, Pinzón and Rábida (del Hoyo and Collar 2016) (Extent of Occurrence measured via Minimum Convex Polygon at 15,600km2). It occurs in a range of habitat types, but the abundance of individuals recorded on Santa Cruz in 2008-2010 appear to be highest in Scalesia and fern dominated habitats, and lowest in dry and agricultural areas (Dvorak et al. 2012). This study also estimated the number of singing males at 55,500 (37,300-80,600) on Santa Cruz alone (Dvorak et al. 2012), and so the global population will not approach the threshold for Vulnerable.
Dvorak et al. (2012) also compared measures of abundance (in terms of number of individuals sighted per point during point count observations) between 2008-2010 and 1997-1998. Combining the per point observations of this species for each time period and looking at an overall average for both 1997-1998 and 2008-2010 suggests that between the two survey periods (which roughly equates to 3 generations lengths), C. olivacea potentially declined by c.46% on Santa Cruz. The authors of this study put forward several reasons that may explain this trend. Anthropogenic habitat alteration may be a key threat with the areas most impacted by humans being the upper transition and Scalesia zones, which were traditionally the stronghold for the species (Dvorak et al. 2012). Scalesia has in particular been heavily impacted. It may have potentially covered most of the island (Stewart 1915), but now only occurs in small scattered patches, which have been invaded by non-native plants (Rentería and Buddenhagen 2006, Jäger et al. 2007, Dvorak et al. 2012). Plants are not the only invasives that may be affecting this species, with rats, mosquitoes and the parasitic fly Philornis downsi all potentially impacting on the avifauna of the Galápagos (Fessl and Tebbich 2002, Whiteman et al. 2005, Fessl et al. 2010, Dvorak et al. 2012).
Population trend information is not available across the species’s range and so we cannot directly estimate the rate of decline in this species. However, we request further information regarding whether the threats that have led to the decline on Santa Cruz may also be causing similar declines on other islands. If this was the case, then the species may warrant listing as Vulnerable under criteria A2bce+3bce+4bce. The species may also warrant listing as Vulnerable under criterion B1ab(iii,v) as the species is restricted to an Extent of Occurrence of <20,000km2, where there is a continuing decline in the area/quality of habitat and number of mature individuals, and likely occurs at 6-10 locations* given the threat from invasive species.
We welcome any comments or further information regarding this potential uplisting.
*Note that the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).
del Hoyo, J.; Collar, N. J. 2016. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2: Passerines. Lynx Edicions and BirdLife International, Barcelona, Spain and Cambridge, UK.
Dvorak, M.; Fessl, B.; Nemeth, E.; Kleindorfer, S.; Tebbich, S. 2012. Distribution and abundance of Darwin’s finches and other land birds on Santa Cruz Island, Galápagos: evidence for declining populations. Oryx 46(1): 78-86.
Fessl, B.; Tebbich, S. 2002. Philornis downsi – a recently deicovered parasite on the Galápagos archipelago – a threat for Darwin’s finches? Ibis 144: 445-451.
Fessl, B.; Young, H. G.; Young, R. P.; Rodríguez-Matamoros, J.; Dvorak, M.; Tebbich, S.; Fa, J. E. 2010. How to save the rarest Darwin’s finch from extinction: the mangrove finch on Isabela. Phil. Trans. R. Soc. Lond. B 365: 1019-1030.
IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.
IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.
Jäger, H.; Tye, A.; Kowarik, I. 2007. Tree invasion in naturally treeless environments: impacts of quinine (Cinchona pubescens) trees on native vegetation in Galápagos. Biol. Conserv. 140: 297-307.
Rentería, J. L.; Buddenhagen, C. E. 2006. Invasive plants in the Scalesia pedunculata forest at Los Gemelos, Santa Cruz, Galápagos. Noticias de Galápagos – Galápagos Research 64: 31-35.
Stewart, A. 1915. Some observations concerning the botanical conditions on the Galápagos Islands. Transaction of the Wisconsin Academy of Science and Arts and Letters 18: 272-340.
Whiteman, N. K.; Goodman, S. J.; Sinclair, B. J.; Walsh, T.; Cunningham, A. A.; Kramer, L. D.; Parker, P. G. 2005. Establishment of the avian disease vector Culex quinquefasciatus Say, 1823 (Diptera: Culicidae) on the Galápagos Islands, Ecuador. Ibis 147: 844-847.