DUCKS鈥� backs are celebrated for their water-repellent properties, and to keep them that way the birds use their bills to apply waxes from the 鈥減reen gland鈥�. Being glossy and waterproof comes at a cost, however. The waxes, usually mono-esters, are smelly and could be scented from afar by predators.
Birds are especially vulnerable when incubating eggs, so to cut the risk certain waders and ducks switch their preen oils to less volatile di-esters, which are harder to sniff out. Jeroen Reneerkens鈥檚 team at the University of Groningen in the Netherlands found that in species where one sex incubates, only that sex makes the switch; where both sexes incubate, both switch. The work, suggesting that di-esters are costlier to make, will appear in Ibis.
