An avian equivalent of make-up?

We report that a long-distance migrating shorebird, the red knot, makes a c omplete switch from commonly occurring monoester preen waxes to a much rare r class of higher-molecular-weight diester waxes at the time of take-off to the high arctic breeding grounds. The cold arctic climate would have requi red a lowering of wax-viscosity, and thus, a shift in the reverse direction . We propose that a sexually selected need for a brilliant plumage has lead to this center-intuitive temporary shift from monoesters to diester waxes. The difficulty of application of the diester preen waxes under cold condit ions would ensure the reliability of the quality-signalling function of thi s most probably sexually selected trait.