Reneerkens J.& Piersma T. (2004) Ongoing study. LIMOSA 77 (4): 149-156.

Birds daily maintain their plumage by applying waxes secreted from their uropygial gland. The preen wax coat probably keeps the feathers flexible and waterproof. Recent studies show that the chemical composition of preen wax of several shorebird species rapidly shifts from the usual monoester waxes to more complicated diester wax molecules in incubating individuals. In this article we present (preliminary) data of experiments performed to unravel the functional aspects of this shift. Measurements of light reflection with a photospectrometer showed that preen wax composition did not affect plumage coloration. Therefore, diesters probably have no function as 'avian make-up'. We could not find evidence for an improved protection against abrasion of the wing feathers by diester wax. As incubating sandpipers create a microclimate that is not only profitable for the hatching of eggs, but also for growth of bacteria, we are currently testing the idea that diester preen waxes offer better protection against these malicious bacteria. The data so far suggest that mono- and diester preen waxes inhibit the growth of feather- degrading bacteria equally well. During incubation sandpipers and their clutches are vulnerable to predation by especially mammalian predators. Because diesters are less volatile than monoester preen wax, they could be less easily detected by predators that use their sense of smell. A trained dog used in an experiment indeed had more difficulty in locating diesters than monoesters, suggesting that ground-breeding birds that secrete diesters during incubation are selectively favoured above birds that apply less olfactory cryptic monoester wax to their plumage.

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