ORIGINAL ARTICLE Intra-individual variation in body temperature and pectoral muscle size in nestling Alpine swifts Apus melba in response to an episode of inclement weather Pierre Bize Æ Aure ´lie Klopfenstein Æ Caroline Jeanneret Æ Alexandre Roulin Received: 25 October 2006 / Revised: 4 January 2007 / Accepted: 17 March 2007 / Published online: 19 June 2007 Ó Dt. Ornithologen-Gesellschaft e.V. 2007 Abstract The Alpine swift (Apus melba) forages on in- sects caught exclusively on the wing, implying that dependent nestlings face acute food shortage in periods of cold and rainy weather. Therefore, there should be strong selection on nestling swifts to evolve physiological strate- gies to cope with periods of undernutrition. We have investigated intra-individual changes in nestling pectoral muscle and body temperature in response to a 1-week period of inclement weather. The pectoral muscle is the largest reserves of proteins, and nestlings have to devote a large amount of energy in the maintenance of body tem- perature. The results show that nestling pectoral muscle size and body temperature were significantly reduced during the episode of inclement weather. Assuming that these physiological changes are adaptive, our study sug- gests that nestling swifts spare energy by a pronounced reduction (up to 18°C) in body temperature and use pro- teins from the pectoral muscle as a source of extra energy to survive prolonged periods of fasting. Keywords Pectoral muscle Á Starvation Á Torpor Á Weather condition Introduction Offspring of species for which food availability varies in time and space in an unpredictable manner are under strong selection to evolve behavioural and physiological strategies to cope with periods of acute food shortage (e.g. Prinzinger and Siedle 1988; Drumond and Chavelas 1989; Brzek and Konarzewski 2001; Love et al. 2003). At least two non- mutually exclusive physiological strategies can increase the survival of dependent young during periods of food shortage. Firstly, offspring can accumulate large energy reserves in periods of food abundance and use those reserves in periods of food shortage. Birds are known to rely on two major sources of energy, namely lipids from fat tissues and proteins from pectoral muscles and other internal organs (Cherel et al. 1988). At adulthood, fat is the primary source of energy for breeding individuals that have to spontane- ously fast during incubation and rearing of the brood (Groscolas and Robin 2001), whereas pectoral muscle can rapidly change in size in response to protein use and disuse during egg production (Cottam et al. 2002), moulting (Fox and Kahlert 2005) and migration (Piersma et al. 1999; Lindstrom et al. 2000). Although offspring are known to accumulate fat reserves, little is known on the use and disuse of proteins and on the dynamics of pectoral muscle size in response to food shortage at the nestling stage. Secondly, organisms can spare resources during periods of undernutrition by strategically reducing energy expen- diture in costly activities which have low immediate ben- efits for survival. Offspring vigilance and begging behaviour are crucial to ensure being fed by parents (Roulin 2001), and thus there should be strong selection on dependent nestlings to stay active during periods of food shortage. Conversely, because under cool ambient Communicated by F. Bairlein. P. Bize (&) Division of Environmental and Evolutionary Biology, Glasgow University, Graham Kerr Building, Glasgow G12 8QQ, UK e-mail: P.Bize@bio.gla.ac.uk A. Klopfenstein Á C. Jeanneret Á A. Roulin Department of Ecology and Evolution, Biophore, University of Lausanne, 1015 Lausanne, Switzerland 123 J Ornithol (2007) 148:387–393 DOI 10.1007/s10336-007-0141-5