Photorefractoriness in Japanese quail: possible involvement of the thyroid gland.

Quail have a form of refractoriness to long daylengths that is relative and not absolute. The properties of "relative refractoriness" are compared with the more typical "absolute" form seen in starlings and the question is posed as to whether the two types represent the extremes of a single underlying process or are quite separate phenomena. A model is offered that assumes that a single process is involved, and some tests of the hypothesis, notably the involvement of the thyroid gland, are described.