In invasive parasites, generalism is considered advantageous during the initial phase of introduction. Thereafter, fitness costs to parasites, such as host-specific mortality, can drive parasites towards specialism to avoid costly hosts. It is important to determine changes in host specificity of invasive populations to understand host-parasite dynamics and their effects on vulnerable host populations. We examined changes in mortality in the introduced avian vampire fly (Philornis downsi) (Diptera: Muscidae), a generalist myasis-causing ectoparasite, between 2004 and 2020 on Floreana Island (Galápagos). Mortality was measured as the proportion of immature larvae found upon host nest termination. Over the time period, the avian vampire fly was most abundant and had low mortality in nests of the critically endangered medium tree finch (Camarhynchus pauper) and had the highest mortality in nests of hybrid tree finches (Camarhynchus spp.). Low larval mortality was also found in small tree (Camarhynchus parvulus) and small ground finch (Geospiza fuliginosa) nests. Selection could favour avian vampire flies that select medium tree finch nests and/or avoid hybrid nests. Overall, the finding of differences in avian vampire fly survival across host species is parsimonious with the idea that the introduced fly may be evolving towards host specialisation.

Avian vampire fly (Philornis downsi) intensity (total number of parasites collected per nest) was collected from nests of Darwin's finches monitored in the highlands of Floreana Island, Galapagos across 10 years: 2004, 2005, 2006, 2008, 2010, 2012, 2013, 2014, 2016 and 2020. Floreana rainfall data (sum of annual rainfall; mm) were collected via satellite sourced from CPC Global Unified Precipitation Data provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, downloaded from the Galápagos Vital Signs website by the Galápagos Conservancy. Nests of three host species and one hybrid, small ground finch (Geospiza fuliginosa), small tree finch (Camarhynchus parvulus), medium tree finch (C. pauper) and the hybrid tree finch (C. parvulus x C. pauper including introgressed individuals) were monitored during incubation and nesting. Host species was determined morphologically in the field and hybrids were confirmed via microsatellite analysis. Brood size was collected using borescope. Upon nest termination (host death, as determined via borescope, or fledging) nests were collected and dismantled to collect the total number of P. downsi (larvae, pupae, puparia and adult flies). Dead nestlings were soaked in ethanol for 24 hours to collect any larvae residing in the nares or ear canals and these larvae were added to the total intensity. For a subset of these nests, all P. downsi larvae were identified to age class: first instar, second instar and third instar. First and second instar are unable to survive to adulthood upon termination of the nest and therefore were used to caculate to proportion of unsuccessful P. downsi per nest.