Many domestic ducks are descended from mallards, so maybe mallards occasionally have yellow babies. I don't have time to do good research, but a very quick glance over various websites suggests that this is the case. I was carried away by the romance of a wild mallard raising domestic ducklings. 8-)

As for the attrition, I've seen the same on repeated visits to state parks where ducks are raising their young, just fewer and fewer as time goes by. I guess that's why they lay so many eggs. But the whole "fuzzy babies should LIVE" feeling is very strong.

P.