pale comb/beak

I'm a newbie with ducks. I recently (about a month & half ago) purchased 4 at a farm store. 2 pekin & 2 Cayuga. Not to sure on sex.This illness has came on all of a sudden with 1 of the pekin. One morning(a few days ago) I let them out to the pen in the yard then later on that evening he/she...