Avian population trends inScalesiaforest on Floreana Island (2004-2013): Acoustical surveys cannot detect hybrids of Darwin’s tree finchesCamarhynchusspp.

Abstract

SummaryFloreana Island has the highest proportion of local land bird extinctions on the Galápagos Archipelago, and is home to the range-restricted and critically endangered Medium Tree FinchCamarhynchus pauper. We used acoustic surveys during 2004, 2008 and 2013 to compare the estimated population size ofC. pauperand other land bird species in a remnant patch ofScalesiaforest. First, we compared song inC. pauperandC. parvulusand the recently discoveredCamarhynchushybrid group to justify our use of acoustic surveys to detect population trends given contemporary hybridisation betweenC. pauperandC. parvulus. Song differed significantly betweenC. pauperversusC. parvulusand hybrid birds, but not betweenC. parvulusversus hybrid birds. Second, we compared population size estimates.Camarhynchus pauperdeclined by 52% between 2004 and 2013 (with a 10% increase since 2008);C. parvulus/hybrid increased by 45% between 2004 and 2013 (with 28% decrease since 2008). In 2013, there were ∼ 419C. paupermales in theScalesiaforest (estimate forScalesiahabitat only) and ∼ 2,537 males on Floreana Island (estimate for the entire available highland habitat). Not all species showed a pattern of decline in the highlandScalesiahabitat between 2004 and 2013:Dendroica petechia(+256%),Crotophaga ani(+254%)Geospiza fuliginosa(+23%), andMyiarchus magnirostris(+11%) increased, while the ground finchG. fortis(-76%) decreased. Understanding whyC. pauperis declining while other land bird species are increasing in the same habitat requires continued inquiry and monitoring efforts.