Single bird

Flow-through respirometry was used to determine resting metabolic rate (RMR). Food was available until the onset of each experiment, and consequently the energetic contribution of digestion is included in estimates of RMR. In each experiment, a broiler was selected at random and placed into a chamber (volume 61 L) from which air was drawn at 50 L min⁻¹. The experimental period consisted of the bird quietly resting while V˙CO₂ was simultaneously recorded. An observer noted when standing and sitting postures occurred. Ambient CO₂ levels were recorded before and after measurement of the bird to allow for correction of any change in room-air composition. Laboratory conditions (temperature, humidity and light) were maintained to control factors potentially affecting choice of resting posture³³. The most stable 60-second V˙CO₂ plateau was considered representative of RMR in each posture. The birds showed no temporal preference for resting posture, and consequently sitting and standing occurred in no particular order. Data from 79 experiments (using 41 individuals; some birds were resampled at later developmental stages) were analysed. The relationship between increasing body mass and RMR in sitting (RMR_sit) and standing (RMR_stand) postures was determined using linear regression.