When different introduced species across trophic levels (ectoparasite, predator) invade island systems, they may pose significant threats to nesting birds. In this study, we measure nesting height and causes of offspring mortality in the critically endangered Medium Tree Finch (Camarhynchus pauper), an island endemic restricted to Floreana Island on the Galápagos archipelago. Considering all nests at which a male built a nest, sang and attempted to attract a female (N = 222 nests), only 10.4% of nests produced fledglings (5% of nests had total fledging success, 5.4% of nests had partial fledging success). Of the 123 nests chosen by a female, 18.7% produced fledglings and of 337 eggs laid, 13.4% produced fledglings. Pairing success was higher for older males, but male age did not predict nesting success. All nests with chicks were infested with Avian Vampire Fly larvae (Philornis downsi). We attributed the cause of death to Vampire Fly if chicks were found dead in the nest with fly larvae or pupae (45%) present. We inferred avian (either Asio flammeus galapagoensis or Crotophaga ani) predation (24%) if the nest was empty but dishevelled; and Black Rat (Rattus rattus) predation (20%) if the nest was empty but undamaged. According to these criteria, the highest nests were depredated by avian predators, the lowest nests by rats, and intermediate nests failed because of Vampire Fly larvae. In conclusion, there is no safe nesting height on Floreana Island under current conditions of threats from two trophic levels (introduced parasitic dipteran, introduced mammalian/avian predators).

We have been monitoring nesting outcome at nests of the critically endangered Medium Tree Finch (Camarhynchus pauper) on Floreana Island, Galapagos since 2004. Nests were monitored following our standardised protocol that we developed in 2000 on Santa Cruz Island and maintained throughout the study on Floreana Island. Nests were routinely inspected, with binoculars and ladder during 2004 to 2006, and since 2008 with a borescope, every three days during incubation and every two days during the nesting phase to confirm activity. 

Sheet 2 of the excel file describes each variable.