That special wax lasts after courtship. (Zoology).

A survey of 19 species of sandpipers is changing scientists' ideas about why the birds switch from one glandular wax for preening their feathers to a different one.

Ornithologists had long presumed that each bird species makes only one kind of wax. Its purpose, researchers have speculated, is to slow down feather wear and make the feathers flexible and water resistant.

Wax switching came to scientific attention in 1999, when Theunis Piersma of the University of Groningen in the Netherlands and his colleagues described an abrupt seasonal shift in wax composition among sandpipers called red knots (Calidris canutus). As breeding season approached, the birds stopped producing a wax made of molecules known as monoesters and secreted a more viscous diester wax. At first, the researchers proposed that the birds were putting on a special make-up for their dates.

That may have been too simple, Jeroen Reneekens at the Royal Netherlands Institute for Sea Research and his colleagues now say in an upcoming issue of the Proceedings of the Royal Society of London B. They analyzed wax from sandpipers at different points in their life cycles. All species made some kind of wax switch, but to the researchers' surprise, the birds kept preening with the diester wax while they incubated eggs. Among ruff and curlew sandpipers, however, only the females switched waxes. These were the only two species in the survey with female-only incubation. The diesters may play some role in courtship, the researchers conclude, but its effect on incubation seems more important.

--S.M.