It happens in domestic ducklings there may just be one that isn't processing the nutrients properly and needs extra help whether it's feed or vitamins . Hard to say if this is what is happening with your little one because Mergansers aren't domestic. But I sure enjoyed reading up on them. They really are pretty. Might want to consider getting some type of insects for these little ones. as their main diet and just have the game bird as back up. I wonder if you can buy feed for wild ducks Strombergs comes to mind.
