Dynamic interaction between an alien parasite and its hosts

Invasive species are viewed as a significant component of global change and are considered major drivers of extinctions of birds on islands. However, natural systems that have been recently invaded by a parasite are interesting from a scientific point of view because they allow the monitoring of host and parasite populations in the first stages of their interaction. The interaction between the recently invaded parasitic fly, Philornis downsi, and its principal hosts on the Galapagos Islands, Darwin`s finches, provides an ideal model for tracking adaptation processes from the onset. The flies lay their eggs in bird nests and the developing larvae feed on blood from the nestlings. The parasite reduces fledgling success of Darwin`s finches, which leads to substantial population declines. However, the effect of the parasite on chick mortality varies greatly among Darwins finch species and the number of larvae per nest has increased in some species during the last two decades but declined in others, indicating a dynamic system. A long-term dataset, also provides evidence for a shift in the life cycle of the parasite: before 2010, we only found larvae in nests with chicks, but now we also record them in incubating nests where they feed on incubating females. We hypothesize that in Galapagos, competition among flies is high which in turn promotes earlier attacks on finch nests. To test this hypothesis, we will first investigate which factors influence the shift in the life cycle of the fly. Additionally, we will assess whether this shift can also be found in the ancestral population on mainland Ecuador. Secondly, we will investigate why chick mortality varies among parasitized Darwins finch species. We will compare the warbler finch which has a breeding success of 30 % successful nests with the closely related small tree finch, which has less than 10% success. With field experiments we will test whether the two species respond differently to parasitism with nest abandonment during the incubation phase and whether they differ in their ability to compensate for the detrimental effects of parasitism with increased parental care. A third aim is to test whether larvae grow faster in small tree finch nests than in warbler finch nests and whether the adult flies prefer small tree finch nests during the nestling phase. Philornis downsi is a major threat to the Galapagos avifauna. Results from our studies will be used by members of the international Philornis working group to develop strategies for controlling this highly damaging parasitic fly.

The avian vampire fly, an invasive parasite introduced to the Galapagos Islands in the mid-20th century, poses a serious threat to the survival of Darwin's finches and other small endemic bird species. The fly lays its eggs in bird nests, and its larvae feed on the blood of nestlings and incubating females, drastically reducing fledgling success. This has led to significant population declines. On the Galapagos Islands, competition among female flies is intense, with multiple females often laying eggs in the same nest, resulting in high parasite numbers and early chick death. Our data indicate that the intense competition among female flies promotes earlier attacks on finch nests, as the fly larvae need to reach the final larval stage or pupation before the chicks die. As a result, the parasite's virulence increases, critically endangering several populations of Darwin's finches and the endemic Little Vermilion Flycatcher. Our project made a significant contribution to the protection of Galapagos birds by developing methods to eliminate larvae from their nests. For instance, we provided nest material treated with insecticide, which was readily incorporated into nests by a high percentage of Darwin's finches and Vermilion Flycatchers. This method led to high larval mortality and a marked increase in breeding success. In the case of the Little Vermilion Flycatcher, we combined this approach with habitat management, removing invasive plants that hinder prey capture. Together, these strategies helped prevent the extinction of a genetically unique population on Santa Cruz Island. We also developed a new technique that involves spraying the outside of nests with insecticide. Surprisingly, instead of flying, the flies walk into the nest. Thereby, they come into contact with the treated material and dye. This technique effectively prevents infestation. However, the impact of the parasite on chick mortality varies significantly among Darwin's finch species. Over the past two decades, the number of larvae per nest has increased in some species while declining in others. For example, warbler finches were once more affected by the parasite than the closely related small tree finches, but recently, small tree finches have become the preferred host. One possible reason for this shift is that the larger nests of the small tree finch offer better insulation, providing more favourable temperature conditions for larval growth. Additionally, small tree finches breed later and more synchronously in the year, as they rely more on specific food sources, such as spiders. However, this once adaptive strategy may now be an ecological trap, as parasitic pressure increases later in the season.