Phenotypic flexibility during migration: optimization of organ size contingent on the risks and rewards of fueling and flight?

Avian long-distance migration involves the storage and expenditure of very large fuel loads. Birds may double in weight before take-off on flights of many 1000 km, and they may lose half their body mass over the subsequent fe w days that such trips take. Recent studies indicate that in addition to th e storage and depletion of fat, the muscles and belly organs also undergo c onsiderable changes in size in the course of such migrations. Such intraind ividual and repeatedly reversed changes in stores and organ sizes represent a class of phenotypic plasticity called 'phenotypic flexibility'. Using pr eliminary comparative data for different populations of Bar-tailed Godwits Limosa lapponica, and several other shorebird species adding variation to t he migration strategies sampled (Golden Plover Pluvialis apricaria, Ruff Ph ilomachus pugnax, Red Knot Calidris canutus and Bristle-thighed Curlew Nume nius tahitiensis), the thesis is developed that the size of the organs carr ied during take-off represent evolutionary compromises between their functi ons during the storage, flight and post-arrival phases of migration. In all cases fat-free tissue along with fat is deposited during fuel storage, but the proportions vary a great deal between similarly sized species. Just be fore departure on long-distance flights, exercise organs (pectoral muscle a nd heart) tend to show hypertrophy and nutritional organs (stomach, intesti ne and liver) tend to show atrophy. Reductions in nutritional organs appear most pronounced in (sub-) species that are about to overfly barren oceans with few or no opportunities for emergency landings. Migrant birds seem to show a great deal of adaptive flexibility, and the study of this flexibilit y may shed light on (presently unknown) physiological mechanisms as well as on correlated ecological constraints on bird migration.