Nesting success and nesting height in the critically endangered Medium Tree Finch (Camarhynchus pauper)

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).