2006. It's just ducky to be clean: the water repellency and water penetration resistance of swimming mallard Anas platyrhynchos ducklings. Á J. Avian Biol. 37: 561 Á571.Ducklings reared by a hen swim with dry plumage a day or two after hatching, while incubator-hatched ducklings of the same age may sink and even drown when placed in water. The common interpretation is that wild-reared chicks receive preen oil from the parent, and this oil makes them more waterproof. Using mallard ducklings Anas platyrhynchos we tested the effect of preen and other oils, as well as hydrophilic or surfactant contaminants, on the water resistance of down. We found that the true cause for the difference between incubator and hen-reared ducklings is the presence of hydrophilic hatching fluid residues in the down of incubator-hatched ducklings. Once well rinsed and dried, incubator-hatched ducklings can swim for over an hour with essentially dry down. Other tests showed that clean down is quite water resistant, and that water resistance was not improved by preen or other oils. Small amounts of preen or other oils had no effect on water repellency or wetting during surface swimming. However, oil decreased the hydraulic pressure needed to penetrate down. Down lacks the stabilizing interlocking structure of adult contour feathers, and small amounts of oil apparently cause barbules to stick together and reduce the effective number of down fibers. Detergent decreases water repellency and increases water retention, and has a more severe effect on oiled down or when applied in combination with oil. The penetration pressure of a clean down coat, 8669/154 Pa, could allow static immersion to ca. 8cm before water would saturate the down and increase thermal conductance. Thus, clean ducklings have a 2)/ Á3)/ safety margin for surface swimming. Saturation increases the thermal conductance of the down coat from 14.39/1.38 W/m 2 -8C to 1939/25 W/m 2 -8C. Thus, water pollution or down contamination causing wetting can significantly increase energy use and the frequency of hypothermia.