Distribution and Abundance of Darwin's Finches and Other Land
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Distribution and abundance of Darwin’s finches and other land birds on Santa Cruz Island, Gala´ pagos: evidence for declining populations M ichael D vorak,Birgit F essl,Erwin N emeth,Sonia K leindorfer and S abine T ebbich Abstract Population monitoring is a vital tool for conserva- of the 30 resident species of land birds (Parent et al., 2008). tion management and for testing hypotheses about popula- Although several island populations of Darwin’s finches tion trends in changing environments. Darwin’s finches on and mockingbirds have been lost since the arrival of Santa Cruz Island in the Gala´pagos archipelago have experi- humans (Steadman, 1986), no land bird species in the enced habitat alteration because of human activity, introduced Gala´pagos has gone extinct in historic times (Dvorak et al., predators, parasites and disease. We used point counts to 2004; Grant & Grant, 2008b; O’Connor et al., 2010c). conduct systematic quantitative surveys of Darwin’s finches However, three species, mangrove finch Camarhynchus and other land birds between 1997 and 2010.Thetemporal heliobates, medium tree-finch Camarhynchus pauper and analysis revealed that six of the nine species investigated Floreana mockingbird Mimus trifasciatus, are categorized declined significantly and that this decline was most pro- as Critically Endangered on the IUCN Red List of Threat- nounced at higher elevations in humid native forest and ened Species (IUCN, 2010). agricultural areas; the highland areas have been most affected Human activity over the past 500 years has dramatically by introduced species or direct human impact. Five of the six altered most of the Gala´pagos islands. Santa Cruz Island declining species are insectivorous, which suggests that changes was the last island to be colonized by humans; its first in insect abundance or insect availability are a critical factor in settlers arrived in the 1920s. The following decades saw the declines. Further study is required to test this idea. Other the almost complete disappearance of the native humid factors including habitat alteration and introduced parasites or forest on Santa Cruz and its replacement with introduced pathogens may be contributing to the observed declines. vegetation. Since the 1990s the ecological impact of human presence has accelerated because of drastic ecological and Keywords Camarhynchus, Certhidea, Darwin’s finches, social changes (Watkins & Cruz, 2007; Gonza´lez et al., distance sampling, Gala´pagos, Geospiza, population estimate, 2008). Santa Cruz now receives the highest number of Santa Cruz Island tourists per year in the Gala´pagos and exhibits the second highest degree of degradation in several of its vegetation This paper contains supplementary material that can be zones (Watson et al., 2010). found online at http://journals.cambridge.org Here we use quantitative census data to describe the distribution and abundance of the land birds of Santa Cruz. By comparing data for 1997–1998 with data for 2008–2010 Introduction we analyse population changes and discuss the effects of drastic ecological changes on bird populations. To evaluate ourteen species of Darwin’s finches (Geospizinae) the impact of these changes for conservation of individual Foccur in the Gala´pagos archipelago, representing 47% species we estimated population sizes for 2008, the year with the most complete survey coverage. MICHAEL DVORAK* BirdLife Austria, Vienna, Austria BIRGIT FESSL* (Corresponding author) Charles Darwin Foundation, Puerto Ayora, Santa Cruz Island, Gala´pagos, Ecuador, and Durrell Wildlife Con- Study area servation Trust, Les Augre`s Manor, Trinity, JE35BP, Jersey, Channel Islands. E-mail [email protected] 2 The 986 km Santa Cruz Island, which has a maximum ERWIN NEMETH Konrad Lorenz Institute, Vienna, Austria, and Max Planck altitude of 864 m, is the second largest island in the Institute for Ornithology, Seewiesen, Germany Gala´pagos archipelago, with 88% of its surface protected as SONIA KLEINDORFER Flinders University, School of Biological Sciences, Bedford Park, Adelaide, Australia part of the Gala´pagos National Park (Servicio Parque Nacional Gala´pagos, 2006). As a result of precipitation and SABINE TEBBICH University of Vienna, Department of Cognitive Biology, Vienna, Austria temperature patterns, several vegetation zones occur along *Contributed equally to this article an altitudinal gradient. Besides the coastal strip the following 1971 26 2010 14 2011 zones are commonly distinguished (Wiggins & Porter, ; Received July . Revision requested January . 2 Accepted 1 March 2011. First published online 23 November 2011. Fig. 1): (1) Dry zone, southern slope (0–120 m, 531.7 km ), ª 2011 Fauna & Flora International, Oryx, 46(1), 78–86 doi:10.1017/S0030605311000597 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.139, on 28 Sep 2021 at 17:34:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605311000597 Darwin’s finches on Santa Cruz Island 79 FIG. 1 The extent and location of the vegetation zones on Santa Cruz Island, based on data from Huttel (1990), and the locations of the 233 survey points (Table 1) used in 2008. The inset indicates the location of Santa Cruz in the Gala´pagos archipelago. characterized by dry forest and scrub land dominated by Data collection deciduous trees, mainly palo santo Bursera graveolens, cacti Opuntia echios, Jasminocereus thouarsii and various spe- Point counts were used for all surveys. Data were collected 2 1997 cies of scrubs. (2) Dry zone, northern slope (191.5 km ), during the early breeding season of land birds in 17 20 1998 15 17 covered by high palo santo forest. (3)Transitionzone(120– ( January– March), ( January– February), 2 2008 29 15 2010 13 10 300 m, 99.2 km ), a dense, mainly deciduous forest ( January– March) and ( January– 2010 dominated by the endemics pega-pega Pisonia floribunda, March) by MD (all years), BF (all but ) and EN 2010 5 07 00 guayabillo Psidium galapageiums and matazarno Piscidia ( ). Counts lasted for minutes between . and 2 11 00 carthagenensis.(4)Scalesiazone(300–650 m, 1.8 km ), an . Darwin’s finches and other land birds on the evergreen forest dominated by treelike Scalesia peduncu- Gala´pagos show high singing activity during the breeding lata; trunks and branches are densely covered with epi- season and mostly inhabit areas of dense vegetation, and phytes (mosses, liverworts, ferns and others); most of this therefore direct observations are unreliable. As our aim was zone and smaller parts of the transition and fern zones have to calculate relative and absolute densities in a comparable been converted to agriculture. (5)Agriculturalzone way we noted only singing birds (presumed to be territory 2 (114.2 km ), mainly farms and grazing land, with intro- holding males) to avoid counts of non-singing females or duced trees and shrubs (e.g. Psidium guajava, Cedrela juveniles. For Gala´pagos mockingbird Mimus parvulus, odorata, Rubus spp.), almost completely confined to the Gala´pagos flycatcher Myiarchus magnirostris and smooth- wetter southern side of the island. (6)Fernzone(above650 billed ani Crotophaga ani all observations (except over- 2 m, 17.7 km ), where vegetation consists mostly of ferns, flying individuals) were tallied. However, the first two grasses and the shrub Miconia robinsoniana; many of the species have a tendency to follow an observer and we did introduced plant species from adjacent farmland have not therefore include these species in the between year 3 invaded the fern zone (Watson et al., 2010). (7)Cinchona comparison (Table ). The dark-billed cuckoo Coccyzus 2 zone (12 km ), areas of the fern zone that are now over- melacoryphus was also excluded from this analysis be- grown with forests of Cinchona pubescens (Ja¨ger et al., cause its singing activity seems to be less correlated with 2009). breeding activity and territories than other species and is dependent on other, unknown, factors. Distances of the birds to the observers were estimated to the nearest 5 m Methods between 0 and 20 m, and to the nearest 10 m beyond 20 m. Nomenclature of Darwin’s finches follows Petren In all years we started with calibration sessions for distance et al. (1999). estimation. ª 2011 Fauna & Flora International, Oryx, 46(1), 78–86 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.139, on 28 Sep 2021 at 17:34:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605311000597 80 M. Dvorak et al. A snapshot approach was used. At the beginning we ments (Marsden et al., 1997; Marsden, 1999; Cramp et al., 2 counted all singing birds as rapidly as possible; then, during 2009). Densities (singing males per km ) were calculated for the remaining minutes, we counted any additional birds in each individual species in each vegetation zone; 95% confi- the context of the position and behaviour of the individuals dence intervals were calculated using bootstrapping meth- already registered. Using this approach we decreased the risk ods. All observations were entered into Distance in grouped of counting the same individuals twice (e.g. while moving format; intervals of either 10 or 20 m were used in the around) and increased the probability of recognizing new analyses. Estimated distances were inspected for outliers that birds that were silent during the initial phase. We avoided could make model fit problematic, and any found were counting birds that were obviously not present at the point excluded. Combinations of all key models and adjustments initially, such as individuals moving into the area during the provided by Distance were tested. The detection curve based 5-minute counting period. Points were generally counted on the model that best fitted the data was chosen automat- once per year as it was assumed that the census period covered ically by Distance using Akaike’s Information Criterion.