The cygnets could have suffocated in the egg if the nest got too wet, especially, if you had a torrential downpour. If you had an extreme temperature range, too cold, then hot and cold again, or vice versa, the mother swan may not have been able to regulate the cygnet’s body temperature and this extreme range could have caused the birds to freeze or overheat. Finally, if a predator (human, wild or domestic) was interfering with the nest, the parent’s may have abandoned the nest to protect themselves. In nature, it is always better to save a viable breeding pair of adults than young which have not been hatched and cannot produce to increase the flock. The Regal Swan
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