Sleeping and roosting in the Grey Starling may be two quite different behaviours from the point of view of ethology. The sleeping appetite is of entirely physiological one. The roosting, on the other hand, is a series of behaviours, commenced while the starlings are on the feeding ground.
Feeding in the open field, the starlings defend themselves against the predators by joining into a flock. Towards the evening, the anxiety caused by lowering light intensity may stimulate the appetite for mutual dependence which releases their flocking behaviour in form of joint feeding and pre-roosting flight assembly.
Then, still lowered light intensity may cause the next appetite to arise, the appetite for taking shelter, which releases the roosting flight towards the roost (or shelter).
Arriving at the roost, they make a pre-roosting assembly and by gradual loss of light intensity (to almost O Lux), their final appetite for entering in shelter may be evoked.
After this, an entirely physiological impulse to sleep may be activated. These postulated roosting procedure is diagrammatically shown.
