Fig. 1. Schematic drawing of a nest-box modified into a metabolic chamber. To ensure the top of the nest-box was airtight, a sheet of rubber was inserted between nest-box and lid (a) and a cork was placed in the entrance hole (b). Reference air was measured close to the inflow of the nest-box (arrows underneath nest-box; c), while sample air was drawn from the nest-box via a tube near the entrance hole (d). The thickness of the nest was determined by the thickness of the nest cup (e) and the height of the nest rim (f).