Sample collection
We banded nestlings at 6-8 days old with a unique color band combination and a numbered metal band (National Band and Tag, Kentucky, USA). We collected fecal samples from nestlings opportunistically at this time. To collect fecal samples, we removed nestlings from the nest and held them over a sterile weigh boat until they defecated. We then moved the fecal sample from the tray to a sterile tube, placed it on ice in the field for up to 6 hours, and then stored it in a -20 °C freezer until the bacterial DNA was extracted. We transported the samples to the University of Connecticut and stored them in a -80 °C freezer for downstream 16S sequencing. Although studies show that the bacterial community in avian feces does not always represent the entire digesta of the host (e.g., in the cecum; Wilkinson et al., 2017), fecal samples are generally representative of the bacterial community in the large intestines (Videvall et al., 2018; Wilkinson et al., 2017) and are used when hosts cannot be euthanized. When nests were empty, they were collected and examined for P. downsi ; the full data set was reported in Knutie et al., (2023).