At temperate latitudes, altricial birds and their nestlings need to handle night temperatures well below thermoneutrality during the breeding season. Thus, energy costs of thermoregulation might constrain nestling growth, and low nocturnal temperatures might require resources that parents could otherwise have invested into nestlings during the day. To manipulate parental work rate, we performed brood size manipulations in breeding marsh tits (Poecile palustris). Nest box temperatures were always well above ambient temperature and increased with increasing brood size. In line with predictions, a large majority of females (but no males) made use of this benign environment for roosting. Furthermore, females tending enlarged broods, thereby having to work harder during the day, reduced their body temperature at night. This might have reduced nocturnal energy expenditure. Our finding that a higher proportion of enlarged, as compared to control, females continued to use the nest box as roosting sites even after a simulated predation event despite increased vulnerability to predation, further highlighting the need for energy conservation in this group. High nest box attendance and reduced body temperature in brood-reduced females may indicate that these females prioritised self-maintenance by initiating other costly physiological adjustments, e.g. moult, when relieved from parental work. We suggest that the energy demand for defending homeothermy is an element of the general trade-off between current and future reproduction, i.e. between daytime investment in food provisioning and the potential short- and long-term costs of a reduction in body temperature and increased predation risk.

mainNest = Identity of nest box Idfemale = Identity of female Year = 1 denotes 2010 and 2 denotes 2011 Date = The date of female body temperature measurement, 1 = 1 May Femtemp = Female body temperature Ambientnight = Ambient temperature during the night of female body temperature measurement Boxnight = The temperature within the nest box Day = The age of the nestlings when the female body temperature was measured Exp = Experimental category; 1 = enlarged, 2 = control, 3 = reduced broods Brood = Number of nestlings in the brood Femalemass = mass of the female at nestling age 14 days Femage = female age; 2 = a female in her second calendar year etc. Nestl mass = The mean mass of the nestlings in a brood Femaletars = tars of the female at nestling age 14 days Fem cond = Female condition estimated as the residual mass from the relationship between mass and tarsus length of the femalesinoutFemid = Female identity Year = 1 denotes 2010 and 2 denotes 2011 Exp = Experimental category; 1 = enlarged, 2 = control, 3 = reduced broods Inout = Denotes if the female is in or out of the box during the second night visit. 1 = in, 2 = out Age = female age; 2 = a female in her second calendar year etc. NestT = The temperature within the nest box