Hi Randall
There could be any number of reasons, but if they are healthy one day and weak the next, it may be some type of toxin in the water due to run-off, algae bloom or something they are ingesting. Another prime suspect would be an infection such as fungal (aspergilliosis), bacterial or viral. Additionnally, if the cygnets are not getting proper nutrients, i.e., vitamins and minerals, they could develop neurological issues which would manifest in weakness. Again, there could be any number of causes. We would suggest in the future to use poultry layer pellets or poultry layer mash mixed with the cracked corn to eliminate any mineral or vitamin deficiency. We hope that this information is of benefit. The Regal Swan