I'm sorry
this is very disappointing news. You did all you could to make your yard space safe for them and they even used the ramps you built. This is in no way your fault and there is no way to know for sure what happened. There is a reason that mama ducks have so many ducklings in nature, it's challenging for many to survive. 
this is very disappointing news. You did all you could to make your yard space safe for them and they even used the ramps you built. This is in no way your fault and there is no way to know for sure what happened. There is a reason that mama ducks have so many ducklings in nature, it's challenging for many to survive. 

