A fixed energetic ceiling to parental effort in the great tit?

1. To elucidate the links between avian brood size, parental effort and par ental investment, we measured daily energy expenditure (DEEfem), condition (residuals of mass on tarsus) and feeding rate in female great tits Parus m ajor L. rearing broods in which the number of young was either reduced, unm anipulated or enlarged. 2. Female condition was negatively correlated with manipulation when measur ed at the nestling age of 8 days (measured during the day), which suggests a shift in allocation from self-feeding to chick-feeding. However, there wa s no detectable manipulation effect on condition measured at the nestling a ge of 12 days (measured during the night). Either female condition was only affected by manipulation in the early nestling phase or the females adjust ed their diurnal mass trajectory in response to brood size manipulation. Mo re detailed data are required to verify this point. There were no indicatio ns of a fitness cost associated with the condition during the day, but cond ition at night was positively related to winter survival. Since manipulatio n only affected condition during the day, there was no link between manipul ation and winter survival. 3. The duration of the working day was not affected by manipulation and fem ale feeding rate tended to flatten off with manipulated brood size. Similar ly, brood reduction resulted in a lower DEEfem, whilst brood enlargement ha d no effect. This suggests that females worked at an energetic ceiling when rearing an unmanipulated brood. However, the level of this 'ceiling' in DE Efem was not fixed: it differed between years. This leads us to conclude th at the observed ceiling was imposed by extrinsic factors (e.g. available fo raging time) and not by an intrinsic factor such as maximum energy assimila tion rate. We hypothesize that time limitation was the cause for the observ ed ceiling in energy expenditure and that the annual variation in the level of this ceiling was due to annual variation in ambient temperature. 4. A cost of reproduction was previously demonstrated in this population: b rood enlargement caused a reduction in the incidence of second clutches. Ho wever, since DEEfem did not differ between control and enlarged broods, we judge it unlikely that daily energy expenditure is a general predictor for parental investment.