Abstract
Invasive alien parasites and pathogens are a growing threat to biodiversity worldwide, which can contribute to the
extinction of endemic species. On the Gala´pagos Islands, the invasive parasitic fly Philornis downsi poses a major threat to
the endemic avifauna. Here, we investigated the influence of this parasite on the breeding success of two Darwin’s finch
species, the warbler finch (Certhidea olivacea) and the sympatric small tree finch (Camarhynchus parvulus), on Santa Cruz
Island in 2010 and 2012. While the population of the small tree finch appeared to be stable, the warbler finch has
experienced a dramatic decline in population size on Santa Cruz Island since 1997. We aimed to identify whether warbler
finches are particularly vulnerable during different stages of the breeding cycle. Contrary to our prediction, breeding success
was lower in the small tree finch than in the warbler finch. In both species P. downsi had a strong negative impact on
breeding success and our data suggest that heavy rain events also lowered the fledging success.