To feed or not to feed

Hello, 

I have been feeding wild birds in my balcony for almost two years now. Mostly sunflower seeds because it is what they choose to eat. I have been happy to be able to help them survive the cold and stormy winters. However, lately, I noticed that the birds are depending on me so much that they don't find the other food sources in nature anymore. Also, I noticed their droppings are pale green to grey in colors which mean they eat mainly the sunflower seeds or their kidney had gone fatty. I tried to reduce the amount of seeds put in the feeders to encourage them to eat other seeds or flowers in spring, but some of them just stayed and waited. Their dropping became just white (urates) showing that they haven't eaten much.

I love seeing them happy and noisy when they see me outside, but I have a mixed feeling now as I don't want them to get sick or even die when I am not around (for holiday or for good). 

If the birds came to my feeder for their meals everyday ever since they fledged, I don't think they can or want to eat other food sources. 

What do you guys think? Any solution to encourage them to find other food sources other than my feeders? Thanks!

PS. Most of the birds are Lesser Goldfinch and House finch.