Preening
Preen, pencil drawing of a Great Blue Heron by Terry Miller	Preening is a commonly observd behaviour involving the careful cleaning, rearrangement, and oiling of the feathers with the bill. Preening is essential in preserving those delicate structures so critical both for flight and, because of their insulating properties, for regulating body temperature. Most birds have a "preen gland" on the rump at the base of the upper trail feathers. The bill is used to work oil squeezed from this gland into the feathers, and head scratching may be an attempt to distribute preen oil over the head, where the bill obviously cannot do the job. The oil apparently has several functions: to help keep the feathers flexible and waterproof and to inhibit the growth of fungi and bacteria. In Ross' Gulls and some other gulls and terns, the preen oil contains a pink colorant.
The intensity of color seems to depend on the diet and whether or not the bird is in breeding condition. But in these species the head gets little color, appparently because of the difficulty of spreading the oil to the head.
SEE: Temperature Regulation and Behavior; Head Scratching. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.