We measured energy expenditure in free-living great tits (Pares major) during the active (day) and the inactive period (night) with the aim of determining whether great tits compensate for energy costs made during periods of high activity in periods of low activity. If such compensation occurs, inferences from measurements of energy expenditure over a 24 h period, with regard to the energy costs of the different behavioural elements, may obscure relations between parental effort and energy expenditure. Also, energy budgets, based on estimates of time budgets combined with a fixed cost for each behavioural category observed, may be unreliable if animals are able to compensate for energy costs made during periods of high activity in periods of low activity.
Laboratory studies have revealed a reduction in resting metabolic rate (RMR) when birds are forced to work harder during the day, but it has not yet been investigated whether such nocturnal savings are also made under natural conditions.
We manipulated brood size in a free-living population of great tits to create a difference in the demands of the nest, measured effort (feeding visits) and daily energy expenditure (DEE). In order to test whether compensation occurred we measured both DEE over 24 h, and resting metabolic rates (RMR) of female great tits at night. DEE and feeding rate differed between the experimental groups, being higher in females rearing enlarged broods, but we did not find evidence of nocturnal saving.