Most honest signalling theory predicts that signals must be costly to prevent the signaller from conveying false information to the receiver. However, there have been few direct measures of the costs associated with signalling. We measured oxygen consumption during begging in 5- and 10-day-old tree swallow (Tachycineta bicolor) nestlings to determine if there was an energetic cost associated with this signal, and if so, whether it increased with increasing begging intensity. Our results indicated that both 5- and 10-day-old nestlings used significantly more oxygen when begging than when resting. Although both the active metabolic rate and the resting metabolic rate were significantly higher in the older nestlings than in the younger nestlings, the scope (ratio of active metabolic rate to resting metabolic rate) of begging was the same in both age groups. Our results indicated that in general begging required 28% more energy than resting in tree swallow nestlings. Energy expenditure was not related to begging intensity in 5-day-old nestlings; however, these values were significantly related in the older nestlings. Although the energetic cost of begging seems relatively low, it may be important when the total energy budget of the nestlings is considered.