Can Ducks and Chickens share feed?

Yes, Ducks and Chickens can share feed, but there are things that need to be thought of before doing so.

Avoid Mixed Flock feeds as they sometimes have more protein then the laying feeds. These feeds are designed to put weight on them for consumption and can cause defects in your flock such as leg and foot problems and Angel Wing (When the wings begin to curve away from the body and makes it hard for them to fly).

A regular layer feed is fine for both adult Chickens and Ducks. Ducks do require more nicotinic acid, so adding brewers yeast into the feed (up to 2.5%) will help with that and wont hurt the chickens if they eat it. Shell grit and extra nutrient filled treats is also a great idea.

Ducklings and Chicks are a bit different however. Ducklings are not as susceptible to coccidiosis as Chicks, so they don't need the medicated chick starter feeds. Ducklings are also only on chick starter for 2 weeks after hatching and then onto a grower feed till they're 18 weeks. Chicks need to be given medicated chick starter feed for 8-12 weeks and then can go onto adult laying feed. 

So if you wish to have a mixed flock and cut down costs on feed, its definitely possible. It's just remembering that some feeds aren't made for laying Ducks and Chickens, Ducklings are different to Chicks till 18 weeks of age and making sure extra nutrients is given where required to maintain a happy and healthy flock.