Complex sentence: Second-year birds lag behind full adults in developing breeding plumage; they have a lower body mass and a slower weight increase than full adults, and perhaps the demands made on their energy reserves during the migration flight are the main reason of the delayed moult.
Split into simpler sentences: Second-year birds lag behind full adults in developing breeding plumage. They have a lower body mass and a slower weight increase than full adults, and perhaps the demands made on their energy reserves during the migration flight are the main reason of the delayed moult.