Avian population survey in the Floreana highlands: is Darwin’s Medium Tree Finch declining in remnant patches ofScalesiaforest?

Abstract

SummaryIsland species typically exist in pathogen and predator sparse environments before human settlement, and are particularly vulnerable to the impacts of invasive species. In this study, we used the variable circular-plot method to estimate the density of birds in the highlands of Floreana Island, Galápagos Archipelago, where introduced parasites, predators, and habitat degradation are a known threat to endemic species. We recorded the number of birds seen and heard at 15 locations near Cerro Pajas Volcano in 2004 and 2008, an area that harbours the largest expanse of highland Scalesia forest on Floreana Island. We estimated the change in population density for nine bird species, including five species of Darwin’s finches. We specifically address changes in population density for the locally endemic Medium Tree Finch Camarhynchus pauper, which only occurs on Floreana Island and has a small population size. Comparing 2004 and 2008, our study found lower population density in the Medium Tree Finch, but stable population density in Small Tree Finch C. parvulus and Large Tree Finch C. psittacula. Based on data from three additional highland sites surveyed in 2008, we estimate that the maximum size of the Medium Tree Finch population is 1,620 individuals. In addition to the survey data, we observed breeding males in 2006 and 2008. We found: (1) low nesting success (six out of 63 nests produced fledglings) and high Philornis downsi parasite intensity, and (2) a biased age structure of the breeding population. No breeding males were one year old in 2006, and no males were five years old in either study year, indicating low reproductive success as well as limited lifespan. This research has contributed to the recent re-evaluation by IUCN, which has changed the Red List status of the Medium Tree Finch from ‘Vulnerable’ to ‘Critically Endangered’.

Conservation status of landbirds on Floreana: the smallest inhabited Galápagos Island

“…Our population estimate of between 3900 and 4700 territories of Medium Tree‐Finches in 2015 and 2016 (Table ) was higher than previous estimates based on studies conducted in 2008 (1620, O'Connor et al. , 2292, Peters and Kleindorfer ) and 2013 (2537 males, Peters and Kleindorfer ). Our density estimates (1.8–2.9 territories/ha) were more than double those calculated in either 2008 (0.6–1.0 birds/ha) (O'Connor et al.…”

“…In addition, O'Connor et al. () described a decline in the population of the endemic Medium Tree‐Finches ( Camarhynchus pauper ) on Floreana between 2004 and 2008. Given these reports of disappearing and declining populations of landbirds on Floreana, our objective was to conduct a comprehensive survey of landbirds encompassing the entire island.…”

Botfly Parasitism Effects on Nestling Growth and Mortality of Red-Crested Cardinals

“…Some authors have proposed birds should avoid selecting nesting sites where they are more exposed to parasites, and that selection for good sites would be the first line of defense against parasitism (Loye andCarroll 1991, 1998;O'Connor et al 2010b). We did not observe behavior by Red-crested Cardinals that suggested increased use of isolated patches of forest and earlier onset of breeding to avoid the higher parasite prevalence typical of riverine forest patches.…”

Divergent foraging behavior in a hybrid zone: Darwin’s tree finches (Camarhynchus spp.) on Floreana Island

“…Different fieldwork activities were done during the months January to March, which coincides with the onset of peak breeding activity in Darwin's finches. Birds were mist-netted and observed in the Scalesia forest at the base of Cerro Pajas volcano (1°17′S, 90°27′W, elevation 250-350 m) (O'Connor et al, 2010a All guidebooks refer to three Darwin's tree finch species on Floreana Island: small tree finch, medium tree finch, and large tree finch (C. parvulus, C. pauper and C. psittacula, respectively). But a recent study by Kleindorfer et al (2014a) found only two genetic groups and one hybrid cluster of tree finches on Floreana Island.…”