I have not been on this site for years, married at chickenstock 2006 in TX at Allen Wranch's..., I was miss amara - official henbassador... LOL. anywho I had this issue with some duck babies I had and found little help here or anywhere. Hope I can give a head's up. If your ducklings are young and show distress with foam in the mouth and nose they are choking. First off if you run out of feed and water always rewater first before feed. That doesn't always work so if you still end up with chokers feel the neck. If unsure how it should feel grab a happy duck and feel his neck. Should feel smooth. The difference between him and the choking duck should be obvious. On the choker push the lump up. It is a jacked up Heimlich Maneuver. You will see the lump of feed come into the mouth and use a tooth pick or something to scrape out of the mouth. He will be all happy... and alive.
I hope any others doing a google search find it helpful too, because when it happened to me and what we dubbed my "suicidal flock" I was in a complete panic and lost the first guy while hitting the net for help. We saved 3 after that with the push throat method same day. My goodness, I was holding them upside down, jiggling, poking... Truly a trial and error scene and not pretty. Also, we used a dropper to help siphon a choker (from my cuticle oil bottle no less after a hot water rinse) . I totally forgot that one. I bet a baby "snot sucker" from the hospital would work well since they are huge bulbs with long skinny "suckers". Remember to always squish the bulb out before inserting! On your kids too 
