Something that has really surprised me since I got my OH to build an aviary to free my caged mule and add canaries to keep him company is how much they enjoy 'wet' food such as lettuce, cress, brocolli and apples.
They fly onto it as soon as I put it into the cage and the 8 of them can finish a halved apple in days. I'm afraid that I was guilty of only giving my caged bird something like this occasionally. They also enjoy a bowl of black sunflower seeds.
I hope that you all 'treat' your birds