Fasting in tree sparrows spizella arborea and dark eyed juncos junco hyemalis ecological implications
Stuebe, M.M.; Ketterson, E.D.
Auk 99(2): 299-308
ISSN/ISBN: 0004-8038 Accession: 005447906
Despite similarities in winter distribution, habitat selection and food choice, dark-eyed juncos (J. hyemalis) and tree sparrows (S. arborea) differ in the extent to which they store fat during winter, with juncos accumulating greater stores. Anticipating that tree sparrows might have some means of conserving energy durng fasting and thus suffer no disadvantage when weather prevents feeding, the species for weight loss, body temperature and locomotor activity during fasting were compared. Relative fasting endurance was noticed. Because both species exhibit geographic variation in sex ratio during winter, we also made sexual comparisons, anticipating that males would be able to fast longer than females. The species responded similarly to fasting by lowering body temperature, especially at night and becoming hyperactive, progressively more so as fasting time increased. Tree sparrows did not exhibit these responses to a greater degree (although they became hyperactive sooner) and were not able to fast as long as juncos. No sexual differences in fasting ability were observed. Because the species-specific difference in tendency toward fat accumulation cannot be attributed to differences in energy expenditure while fasting, at least in the laboratory, other explanations are considered.