16S rRNA seq of Junco hyemalis: preen oil
Swab samples were stored at -80 C until analysis. We extracted genomic DNA from the swabs using the QIAgen Powerlyzer PowerSoil DNA Isolation kit, with minor modifications to the manufacturers protocol (Whittaker et al., 2019) . Due to the low microbial biomass and high concentration of background host DNA in these swab samples, we amplified and sequenced the bacterial DNA using a nested PCR approach (Fan et al., 2009;Yu et al., 2015) . The first round of amplifications included 15 cycles, with each reaction containing 1 l each of the universal 16S rRNA gene primers 27f-CM (5-AGA GTT TGA TCM TGG CTC AG-3) and 1492R (5-ACG GCTACC TTG TTA CGA CTT-3), 12.5 l of 2X GoTaq Green Master Mix (Promega, Madison, WI, USA), and 3.0 l purified DNA. Following an initial 5 min incubation at 95C, the cycling parameters were 94C for 30 s, 50C for 30 s and 72C for 120 s. We sequenced the V4 region of the 16S rRNA gene on the Illumina MiSeq platform using a V2 500 cycle MiSeq Reagent Kit (Illumina MS102- 2003), and the dual indexing sequencing strategy developed by Kozich et al. (Kozich et al., 2013) .
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