Once a pair's young are well grown and ready to make their own way in the world the male bird will often begin the process of moulting his main flight feathers. The process will involve replacing certain feathers each year and the whole process can take four years until all of its flight feathers have been renewed. By contast the female will replace her flight feathers whilst she is confined to the nest site caring for her offspring. Sometimes, especially if she is going to double-brood, she will not begin her moult process until later in the year.
Inspection of nest sites can often reveal evidence of these flight feathers being replaced and can be a good indicator of the bird's activities. This nest site that was checked upon yesterday had produced four young that had fledged in July and now the parents are roosting together following the juvenile dispersal.