Data from: Evolutionary design of a flexible, seasonally migratory, avian phenotype: why trade gizzard mass against pectoral muscle mass?

Migratory birds undergo impressive body remodelling over the course of an
annual cycle. Prior to long-distance flights, red knots (Calidris canutus
islandica) reduce gizzard mass while increasing body mass and pectoral
muscle mass. Although body mass and pectoral muscle mass are functionally
linked via their joint effects on flight performance, gizzard and pectoral
muscle mass are thought to be independently regulated. Current hypotheses
for observed negative within-individual covariation between gizzard and
pectoral muscle mass in free-living knots are based on a common factor
(e.g., migration) simultaneously affecting both traits, and/or protein
limitation forcing allocation decisions. We used diet manipulations to
generate within-individual variation in gizzard mass and test for
independence between gizzard and pectoral muscle mass within-individuals
outside the period of migration and under conditions of high protein
availability. Contrary to our prediction, we observed a negative
within-individual covariation between gizzard and pectoral muscle mass. We
discuss this result as a potential outcome of an evolved mechanism
underlying body remodelling associated with migration. Although our
proposed mechanism requires empirical testing, this study echoes earlier
calls for greater integration of studies of function and mechanism, and in
particular, the need for more explicit consideration of the evolution of
mechanisms underlying phenotypic design.