Seasonal Variation in Daily Mass Amplitude and Minimum Body Mass: A Test of a Recent Model

Abstract

Two recent models of body regulation suggest that wintering birds will show one of two energy storage strategies. Both models suggest that birds will increase the amplitude of the daily cycle of body as midwinter approaches and decrease the amplitude toward spring. Seasonal changes in night length and night temperature cause seasonal variation in thermoregulatory demands, hence seasonal variation in amplitude of the daily cycle. The models differ in that only the first but not the second model proposes an increase in dawn body toward midwinter, and a subsequent decrease in toward spring; in the second model, the daily minimum remains seasonally constant. Because fat is a major component of daily and seasonal variation in body mass, these contrasting strategies for body regulation have been termed true winter fattening and constant dawn mass strategies, respectively. In the former case, the seasonal increase in the dawn body/fat is considered to represent an emergency energy supply to be used during sudden poor feeding conditions, i.e. snow and ice storms. We present data on four species of temperate-dwelling passerines that support the first but not the second model. In the Dark-eyed Junco Junco hyemalis wintering in southern Indiana, amplitude of the daily cycle of body mass, evening and dawn body mass, and evening and dawn visible fat class all increased toward midwinter and decreased toward spring. A highly significant linear relationship between visible fat and chemically extracted body lipid in this species verified that the visible fat class system reliably measured body fat content. In addition, the seasonal variation in amplitude was explained by seasonal variation in nighttime roosting temperature and night length. The Northern Cardinal Cardinalis cardinalis and Fox Sparrow Passerella iliaca wintering in southern Indiana and the Song Sparrow Melospiza melodia wintering in southwest British Columbia all displayed a seasonal increase and decrease in dawn body and visible fat class. Results of this study and previous studies indicate that 13 of 14 species studied in Europe, Asia, and North America display this strategy for regulating winter body mass. We conclude that among birds that winter in the temperate zone, there has been widespread natural selection for a seasonal increase in the daily minima of body and fat reserve.