Migrant warblers have different moult strategies. This is an adult Reed Warbler which does a complete moult including its primary feathers in Africa during the winter so it is in tip top condition to fly to Fleet Pond and breed it. Now it has very worn feathers and has to fly back to Africa before it can moult them again. However Blackcaps do a complete moult in the summer after breeding so they fly back to Africa in good condition but they then have have to keep those feathers through the winter, flying back to Fleet Pond, finding a mate and breeding before they moult again. Willow warblers have the best of both worlds they do both a summer and winter complete moult this is great as they are in great condition just before both of their flight to Africa and back, but it makes them difficult to age.