1. The water repellency of feathers is determined mainly, but not exclusively, by a structural parameter which can be expressed in terms of diameter and spacing of the barbs and barbules.
2. Effective waterproofing properties result from the optimal balance of structural parameter and resistance to water penetration.
3. Comparison of the structural parameters for water birds with the geological time interval of their earliest fossil record shows a phylogenetic tendency toward optimizing waterprooftng properties with the course of time.
4. Several behavioural patterns are discussed which may have evolved under the selective pressure of the quality of water repellency and resistance to water penetration.