Apparently they flap their wings for the entire trip. Also: "Their internal organs undergo a “strategic restructuring” before departure. The gizzards, kidneys,

Apparently they flap their wings for the entire trip. Also: "Their internal organs undergo a “strategic restructuring” before departure. The gizzards, kidneys, livers and guts shrink to lighten the load for the trans-Pacific journey. Pectoral muscles grow before takeoff to support the constant flapping the trip requires."