The vast majority of Palaearctic birds have now finished breeding and are either heading south, dispersing locally or going into moult. Many of the birds now heading south are in a terrible state, ready to drop the old feathers and replace them with new ones. Some start the moult, then pause it in order to migrate and complete it in the winter quarters. All this makes sense. Migrate south as the days start to get shorter and don’t leave it until it is too late. Shorter days mean less feeding time and you certainly don’t want to have to raise a brood in those conditions. One mouth is enough to feed with the shortening days.