Bioenergetic Basis of Light-induced Fat Deposition in the White-crowned Sparrow. ∗

Summary

In Zonotrichia leucophrys gambelii there is a striking deposition of fat in subcutaneous and visceral depots following prenuptial molt and preceding beginning of vernal migration. During the last 2 weeks of April, in natural populations, mean lipid indices increase from about 5% to more than 20%. A similar premigratory fat deposition occurs in captive birds under natural conditions of photoperiod and temperature. Similarly in these captive birds this fat deposition is followed by development of migratory behavior (Zugunruhe). Measurements of metabolizable energy intake in these birds show that fat deposition is accompanied by a marked increase in energy intake to levels of 30 to 50% above the preceding minima. This temporary hyperphagia represents an altered physiologic state rather than simply the effect of more daylight for feeding since, during hyperphagia, the amount of daylight increases only 9%. The temporary hyperphagia is the principal factor responsible for positive energy balance required for premigratory fat deposition. The role of daily photoperiod in development of temporary hyperphagia was demonstrated experimentally by subjecting birds in mid-winter to 20 hours of light per day. These developed a hyperphagia and an accompanying fat deposition very similar to those observed in spring under natural conditions.