Time course and reversibility of changes in the gizzards of red knots alternately eating hard and soft food

The ability to change organ size reversibly can be advantageous to birds th at perform long migrations. During winter, red knots (Calidris canutus) fee d on shellfish and carry a muscular gizzard that weighs 10% of their body m ass. Gizzard size decreases when these birds eat soft foods, e,g, while bre eding in the tundra, We studied the reversibility and time course of such c hanges using ultrasonography, Two groups of shellfish-adapted knots (N=9 an d N=10) were fed alternately a hard and a soft food type. Diet switches eli cited rapid reversible changes, Switches from hard to soft food induced dec reases to 60% of initial gizzard mass within 8.5 days, while switches to ha rd food induced increases in gizzard mass to 147% within 6.2 days. A third group of knots (N=11), adapted to soft food for more than I year, initially had very small gizzards (25% of the mass of shellfish-adapted gizzards), b ut showed a similar capacity to increase gizzard size when fed shellfish. T his is the first non-invasive study showing rapid digestive organ adjustmen ts in non-domesticated birds.