Yes, female birds are programmed to lay a certain number of eggs throughout their lives. During mating season, with or without a male, a female swan will still lay eggs due to the increased hormone levels. Same and opposite gendered swans will also mate during the mating/nesting season. Male paired swans will obviously not produce eggs, but will still try to mate and will sit on a nest for a couple of weeks until the hormones level-off. So, if your non-bred female swans are laying eggs, it is perfectly normal. The Regal Swan