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 Camarhynchus pauper. We used acoustic surveys during 2004, 2008 and 2013 to compare the estimated population size of C. pauper and other land bird species in a remnant patch of Scalesia forest. First, we compared song in C. pauper and C. parvulus and the recently discovered Camarhynchus hybrid group to justify our use of acoustic surveys to detect population trends given contemporary hybridisation between C. pauper and C. parvulus. Song differed significantly between C. pauper versus C. parvulus and hybrid birds, but not between C. parvulus versus hybrid birds. Second, we compared population size estimates. Camarhynchus pauper declined 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 ∼ 419 C. pauper males in the Scalesia forest (estimate for Scalesia 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 Scalesia habitat between 2004 and 2013: Dendroica petechia (+256%), Crotophaga ani (+254%) Geospiza fuliginosa (+23%), and Myiarchus magnirostris (+11%) increased, while the ground finch G. fortis (-76%) decreased. Understanding why C. pauper is declining while other land bird species are increasing in the same habitat requires continued inquiry and monitoring efforts.