1. The barbules of the abdominal feathers of sandgrouse exhibit structural characteristics which are favourable for the uptake of large quantities of water. This feature greatly assists the male of the species in his efforts to bring water to the young.
2. The structural details of feather keratin, peculiar to many fibrous proteins, provide a straightforward explanation of the mechanical transformation of sandgrouse barbules, when wetted, in physico-chemical terms appropriate to macromolecular systems.
3. X-ray analysis of dry and wetted sandgrouse barbules qualitatively support the proposed mechanism of a reversible melting-recrystallization cycle for the barbule keratin.