How about letting the ducks or
chicken into the garden and observe what happens?
Depending on their normal food they might like to eat some protein.
Once they are filled up, change their appetite or the grasshoppers are all eaten up, you take the ducks/chickens out of the garden.
If you would like to keep the tasty plants (and the grasshoppers left any), leaving the ducks and chickens unattended in the garden is probably not wise.
Or you could try to put individual ducks/chickens into the garden and keep those that prefer the grasshoppers over the plants…