I give my birds hard boiled eggs all the time with no ill effects. Though, I usually only give them eggs if they're breeding or to prime them for breeding - extra protein always helps in breeding season
I wouldn't give too much, but my current birds are on:
- budgie seed (main part of their diet)
- greens (no fruits because it attracts fruit flies in my climate) - hard veggies like capsicum and weeds / herb trimmings
- boiled eggs for extra protein in breeding season
Though if you wanted to do it year round just make sure it's not in excess. Mine go nuts over it and the chickens in the bottom of the aviary clean up the rest. No waste!