Avian population trends in <italic>Scalesia</italic> forest on Floreana Island (2004-2013): Acoustical surveys cannot detect hybrids of Darwin’s tree finches <italic>Camarhynchus</italic> spp.

Summary: Floreana 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 Finch <italic>Camarhynchus pauper</italic>. We used acoustic surveys during 2004, 2008 and 2013 to compare the estimated population size of <italic>C. pauper</italic> and other land bird species in a remnant patch of <italic>Scalesia</italic> forest. First, we compared song in <italic>C. pauper</italic> and <italic>C. parvulus</italic> and the recently discovered <italic>Camarhynchus</italic> hybrid group to justify our use of acoustic surveys to detect population trends given contemporary hybridisation between <italic>C. pauper</italic> and <italic>C. parvulus</italic>. Song differed significantly between <italic>C. pauper</italic> versus <italic>C. parvulus</italic> and hybrid birds, but not between <italic>C. parvulus</italic> versus hybrid birds. Second, we compared population size estimates. <italic>Camarhynchus pauper</italic> declined by 52% between 2004 and 2013 (with a 10% increase since 2008); <italic>C. parvulus</italic>/hybrid increased by 45% between 2004 and 2013 (with 28% decrease since 2008). In 2013, there were ∼ 419 <italic>C. pauper</italic> males in the <italic>Scalesia</italic> forest (estimate for <italic>Scalesia</italic> habitat 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 highland <italic>Scalesia</italic> habitat between 2004 and 2013: <italic>Dendroica petechia</italic> (+256%), <italic>Crotophaga ani</italic> (+254%) <italic>Geospiza fuliginosa</italic> (+23%), and <italic>Myiarchus magnirostris</italic> (+11%) increased, while the ground finch <italic>G. fortis</italic> (-76%) decreased. Understanding why <italic>C. pauper</italic> is declining while other land bird species are increasing in the same habitat requires continued inquiry and monitoring efforts. [ABSTRACT FROM AUTHOR]