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Nest = 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 female