I know this has more than likely been discussed before, but I thought I would just mention it anyway. I recently started planting a few vegetables so I can give my birds some fresh greens. So far I've discovered they love the following, all of which are easy to grow:

Pak choy
Cilantro (the plant from which you get coriander seeds)
Flowering pak choy
Chinese kale
Chinese water spinach

The Chinese water spinach really seems to be a favorite though as the birds really go mad for it. It's very easy to grow in soil or in water. If growing in a pond, make sure your ducks can't get to it because they finish the lot. I recently heard from a lady in the UK that she used to feed it to her family and to her birds all the time, but she can't seem to get the seeds in the UK. Well, that's all it took, and now I've ended up selling seeds on eBay. lol