Mismatched reproduction is energetically costly for chick feeding female great tits

1. Climate change has caused a phenological mismatch between the timing of reproduction and the local food peak in many bird species. Late breeding birds therefore experience reduced food availability during chick rearing and are thus predicted to have an increased energy expenditure. Observational studies, however, show mixed results, perhaps because they compare energy expenditure across rather than within individuals at different levels of food availability. 2. In a cross foster experiment, we measured daily energy expenditure (DEE) twice within individuals during chick feeding (when chicks were 6 and 14 days old) for 28 free-living female great tits (Parus major). To avoid confounding effects of chick age, these females reared on both occasions a standardized foster brood of eight 10-day-old chicks during the 24-h measuring period. For all birds, food availability declined between the two measurements. 3. We show that DEE during chick feeding increased within females when food availability decreased. Variation in DEE within females is partly explained by brood visit rates, food availability and temperature. 4. DEE during chick feeding could be affected by the investment in previous stages of the reproductive attempt. However, energy expenditure during chick feeding was not correlated to energy expenditure during egg laying, measured in these same females. 5. Understanding of energetic costs during all phases of the reproductive cycle is important to forecast the consequences of climate warming on timing of reproduction.