Experimental manipulation of parasites
We searched field sites daily for evidence of nest building by small ground finches. Once eggs were laid, we checked nests every other day until the nestlings hatched. Prior to egg hatching, we experimentally manipulated fly abundance via adding approximately 10 mL of 1% permethrin solution (Permacap; hereon, fumigated) or water (hereon, sham-fumigated) to the nest. Within two days of nestling hatching, we treated nests again to ensure the removal of all parasites. Briefly, we removed the contents of the nests (including the nestlings, unhatched eggs, and the nest liner), and treated them with either a permethrin solution or water by spraying approximately 5 mL into the nest where the larvae live. After treatment, we returned the dry nest liner, and placed nestlings back into the nest. We used permethrin because it is highly effective at removing vampire flies from the nests (Kleindorfer & Dudaniec, 2016). However, studies have found that permethrin can have sublethal effects on nestling birds (Bulgarella et al., 2020); therefore, we ensured that nestlings did not come into contact with the insecticide. Additionally, adults returned to their nests with no cases of abandonment from the treatment (Koop et al., 2013; Knutie et al., 2016).