Back in Oregon, we raised a batch of ducks for slug protection. In the Backyard Chicken forum, I had read about the technique of painting round-ish rocks with red paint, and leaving them in the garden near the strawberries to train/trick your chickens into avoiding the strawberries. The picture above is an example of how they looked; nothing fancy, just red. My husband used some red non-voc exterior paint he happened to have laying around.
We did it, and it worked with our ducks! The key was to put the rocks in long before the strawberries fruited. The ducks - specifically the female ducks, we noticed - would try to eat the rocks a few times, and then finally give up. Then they didn't touch anything red after that, even raspberries. It was great... no need for netting.
Our ducks were young and thus had never been exposed to a strawberry. So I don't know if this would work for birds who have already eaten strawberries. But that would be a good test - how long is the memory of a duck or chicken?
Back to our observation among our ducks, it was clearly the females who were testing different foods. The males never seemed to try anything new, they just ate what the females did. Which makes me wonder if one had only male ducks if they might be less likely to eat one's desired garden plants?
One day I noticed that one of the female ducks was nibbling the new growth off an Autumn Olive. I decided to taste it, and the leaves were really good and interesting. They all ended up eating most of the new leaves off one plant, and then left it alone the rest of the season. It recovered just fine. (I'm not advocating eating what you see animals eat; I'm just willing to take my own risks. It's good to keep in mind that bear can eat poison oak berries, though.)
One more possible key to our success...our ducks had a huge area to roam, and lots to do. Ponds and creeks to frolick in, lots of slugs to eat, and garden beds to explore for five ducks. Whether this method would work if the ducks had less space, more pressure to compete for forage, I don't know. But it is a simple enough thing to try!
Be joyful, though you have considered all the facts. ~Wendell Berry
I wonder if the ducks will learn over time be able to tell the difference. If they do, find some nice round rocks, and get a sling shot. If they are also bright red, this will help you to relocate your ammo.