One of the long skewers used with BBQs makes a great bird feeder. Just slip one end through a corner of the cage or flight wire, and attach a link to keep it from slipping through. Slip the other end out and push on a cork, a piece of plastic, or a bolt. The food is slipped onto the skewer and is in easy view to the birds. I often wonder if natural instincts are telling the bird just which vitamins its body requires at the moment and is really the deciding factor on which tidbit get tossed or eaten.
Birds eat food in different ways.
Some reach over the side and get picky. Others choose to sort through the whole dish, tossing items every which way, to see what is being offered. Then they climb down to the bottom to re-think the situation. My best thoughts are to offer a wide variety and keep the changes coming so that the bird can fill its own needs. Our birds are what they eat, which is what we serve them.