If the swans cannot fly (pinioned), they may not get enough lift off the water to fly at a higher level to get from one area to another. If there are other swans at the end of the lake that they are flying toward, this could be a means to gather with the other swans quickly for greeting or to chase the other swans from the area (territorial). So, the context is important to know if there are other swans at the end they are flying or if the swans are just not willing or able to get lift off the water when flying. The Regal Swan