Alterations in tissue aerobic capacity may play a role in premigratory fattening in shorebirds.

Migratory shorebirds show regulated seasonal increases in body mass (BM) even in captivity, consisting primarily, but not exclusively, of fat. We examined whether captive red knot (Calidris canutus) exhibited seasonal alterations in mitochondrial volume (liver, pectoral muscle) and/or succinate dehydrogenase (SDH) activity (liver, pectoral muscle, heart, small intestine) during three distinct life-cycle stages: stable BM, spring peak in BM, and as BM rapidly declined after the spring peak. Mitochondrial volume in liver and pectoral muscle and SDH activity in liver and heart did not alter with life-cycle stage. However, red knot undergoing premigratory fattening exhibited significantly lower pectoral muscle SDH activity in concert with significantly elevated activity in the small intestine compared with the other two time-points, suggesting that tissue metabolic rate alters with life-cycle stage. The increased intestinal SDH activity may indicate an elevation in energy assimilation at a time when intestine hypertrophy occurs, thus maximizing BM increase prior to putative migration. The concomitant decrease in pectoral muscle activity may act to reduce overall metabolic rate, or at least help counter the elevation in intestinal mass-specific metabolic rate. Both tissues hypertrophy prior to migration in wild red knot, but hypertrophy of the intestine precedes that of pectoral muscle. Indeed, it appears that the intestinal mass undergoes atrophy by the time pectoral muscle hypertrophy occurs in wild red knot. Thus, physiological adjustments in tissue metabolism may be an important factor in the life-history strategies of migrating shorebirds.