My Ph.D. studyalready started before I got my M.Sc. degree. It was on a peculiar dimorphism among male ruffs, birds that bear conspicuous ruffs of feathers around their faces. In spring, these males stay together for considerable parts of the day at special sites (leks), most often close to the water (see picture below). Some of the males (resident males) defend very small territories (residences). Others act as guests (satellite males) of the resident males. Females also come to the communal places, but only for short visits. They select a male, mate, and depart again. They nest alone, lay eggs, incubate and tend their chicks, also without any help of the other sex. Resident males sometimes fight and have dark plumages. The satellite males have light plumages (often white ruffs), behave submissively and never fight. Females are considerably smaller than males, are inconspicuous and possess no ruff. A male plays only one of the two roles during his whole life.
Later, on the basis of breeding experiments, David Lank demonstrated that there are genetical differences between resident- and satellite males.
Still later, simply by measuring birds that were trapped for ringing, Joop Jukema discovered a third type of male (the faeder), looking like a female, somewhat bigger, but smaller than a male. Faeders are treated like females by other males. They visit leks and residences together with females and copulate in a hurry as soon as a real female invites a resident male for mating.
In the past ruffs were rather common in the low parts of the Netherlands. At present, ruffs only pass (less and less) during migration. In very exceptional cases, a few may stay for breeding.