Yo-yo Dieting

During these snowy and cold days of late January I anxiously await the spring bird migrations and the emergence of the first wildflowers. Soon!!

Many birds display seasonal changes in fat deposits and body physiology as they prepare to migrate. However, the eared grebe (Podiceps nigricollis) exhibits the most dramatic fluctuations known in birds, undergoing three to six cycles of atrophy (wasting away) and hypertrophy (exaggerated growth) a year. The most dramatic of these cycles takes place as the grebes stage for their winter migration. They accumulate huge stores of fat (up to 46% of lean body mass) and their digestive organs grow, while pectoral muscles shrink below the mass necessary for flight, rendering the grebe flightless. Then just before the eared grebes depart for their wintering areas a reversal occurs – fat deposits decrease, digestive organ mass is reduced by up to 50% and the pectoral muscles increase in size. This pattern, to a lesser degree, is repeated at the wintering and breeding (summer) sites, and in some grebes, at the spring staging area and during the fall molt. Talk about a yo-yo diet!

Why does this happen? According to Joseph R. Jehl, Jr. in an article in the June 1997 “Journal of Avian Biology” the reason for the pectoral muscle atrophy is unknown. The increase in fat deposits to the point of obesity in the fall is thought to insure that the grebes postpone their migration until months of maximum darkness thus avoiding predation on their flight. Grebes are one of the latest birds to migrate in the fall and they only fly at night. The reduction of digestive organ mass would make flight easier and increase flight speed. Yet one would think the same goal could be reached without the extreme atrophy/hypertrophy cycles. I have not found any further explanations for this phenomenon but would be interested to learn of further research on the subject.

These pictures were taken at Eagle Lake (Lassen County CA) in late November. I do not know if they were staging but they were busy eating.

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