This is entirely from personal experience, but I’ve been able to get even very reluctant vegetable eaters like cockatiels to eat lots of green like beet greens, mustard, kale, lettuce, carrots, asparagus, and bok choy. It’s a simple suggestion: use clothes pins.
If I put vegetables in a bowl, the cockatiels especially take no notice of it, but something about a clothes pin makes it more like a game or perhaps provides counter-pressure that makes it easier to eat them.
I’ve had success to traditional clothes pins and some plastic ones, although each may be prone to getting destroyed by birds, depending on the species. Here are Amazon links to both kinds I’ve used.
I use the Oxo ones, which you can clip into the dishwasher to thoroughly clean them. Clothespins should work just as well, but it depends on the dishwasher whether you’ll be able to position them to get fully cleaned.…