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ESM-1
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Experimental design
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To test the feeding preference of the focal birds towards conspecifics that were healthy or visibly
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diseased, we built two identical experimental cages (figure ESM-1.1). Each cage was divided
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into three compartments by two wire-mesh dividers (mesh size: 1.3 × 2.6 cm). The focal
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individual was placed in the middle compartment (figure ESM-1.1, “B”; 61 × 46 × 40 cm), and
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the healthy and diseased conspecifics were placed in either of the smaller side compartments
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(figure ESM-1.1, “A” or “C”; 22 × 46 × 40 cm). This allowed all three individuals to have visual
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and auditory contact with each other, while only the focal bird could directly interact, but
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without actual physical contact, with either the healthy or diseased individual. The location of
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diseased conspecifics was alternated between trials. Each compartment always contained two
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wooden perches and a water bowl. Depending on the stage of the experiment, a maximum of
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four small food dishes (5 × 1.5 cm) with ad libitum pelleted food (Roudybush Maintenance Diet,
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Roudybush, CA) were placed on both sides of the dividers, thus providing the two side
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compartments with one food dish and the centre compartment with two. As a result of this setup,
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the focal and either conspecific (healthy or diseased) were approximately 2 - 3 cm apart when
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feeding simultaneously. Each cage was placed on a stand in an empty room (2 × 2.5 × 2 m),
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which otherwise contained only a video camera on a tripod. To avoid infection of healthy
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individuals by MG deposited in the cage by the experimentally infected conspecifics,
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experimental cages, food dishes and water bowls were disinfected between trials. None of the
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focal or healthy conspecific house finches developed any physical signs of MG infection during
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or after the experiment. All housing protocols and experimental procedures were approved by
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Virginia Tech’s Institutional Animal Care and Use Committee prior to the commencement of
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research.
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Figure ESM-1.1. Experimental set-up whereby a focal healthy house finch (placed in
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compartment B) has a choice of feeding next to a healthy or diseased conspecific (placed in
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either compartment A or C). House finches on either side of the mesh cage barrier (illustrated by
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black dashed lines) can interact and aggressively displace each other without physical contact.
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The placement of food dishes (black rectangles), water bowls (grey circles) and perches (grey
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dotted lines) are shown.
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