I'm not sure how big your scoop is but I have 10 girls that get about a third to a half a cup of scratch and not every day. I used to give them more but since doing necropsies on birds fed too much scratch, I am much more judicious with it now. A scoop seems like it might be a lot of scratch to me even between 18 birds.
Necropsies are really the only way to establish what your birds die of and I find it fascinating to perform them myself once you get over the emotional trauma of losing the bird. If you feel that you are able to try this yourself with the next bird that dies, take plenty of photos both of the organs in situ and then removed and post them here. There are several of us that are getting more experienced at knowing what is normal and what is not and it is surprising how often we can come up with a probable diagnosis after discussing it.
Necropsies are really the only way to establish what your birds die of and I find it fascinating to perform them myself once you get over the emotional trauma of losing the bird. If you feel that you are able to try this yourself with the next bird that dies, take plenty of photos both of the organs in situ and then removed and post them here. There are several of us that are getting more experienced at knowing what is normal and what is not and it is surprising how often we can come up with a probable diagnosis after discussing it.