Supplementary Figure 2: Attachment of the harness to a zebra finch

The bird is anesthetized with Isoflurane (3% induction, 1.5% support) delivered through a silicon tube going to the beak. The harness (Fig. 1a) was fabricated in advance from a piece of Velcro strip of 15x22 mm and a BüroLine rubber band of 40x1.3 mm (www.bueroline.com, Cat. #155012). The rubber band is sewn to the Velcro strip at two places, the knots of the thread are fixed by cyanoacrylate glue. (a) The harness is placed on the bird; the loops of the rubber band on the chest are attached to each other with a piece of thin laboratory/surgical rubber glove. (b) The rubber endings are glued together with a drop of cyanoacrylate, a small piece of paper is placed underneath to prevent gluing the feathers. (c) Bottom view of the attached harness. (d) Top view of the attached harness. The tightness of the harness is adjusted via the length of the rubber ring on the chest. It was found that tension of the harness has little impact on the quality of sound recordings. The tension should be such as to prevent any unnecessary pressure on the animal and to prevent sliding of the backpack on the bird’s back; a loose harness can disturb the animal as much as an overly tight harness. To attach the logger onto the Velcro we recommend placing forceps between the animal’s back and the harness to push against the forceps and not against the back. To remove the logger we recommend holding the Velcro strip of the harness in one hand and the backpack in the other hand.