Veterinarians report that nutritional deficiencies are a major problem in pet birds, and it's frequently accompanied by obesity because they overeat in an attempt to get the nutrients that they're missing. So we really do need to pay enough attention to make sure our birds are getting the hard-to-obtain nutrients. A lot of birds don't get outside where they can make Vitamin D from sunshine, and if they're not eating pellets they're not likely to get it from their diet either. The form they need (D3) doesn't occur in plants so they're not going to get it from eating seeds and vegetables.
Getting enough protein from plants is tricky too, because the protein-to-calories ratio in seeds, grains and nuts isn't high enough and you need a significant percentage of beans/legumes in the mix to increase the amount of complete protein. Most of those beans need to be cooked too because of toxin problems in raw beans.
Most humans eat animal products so it's easier for us. Animal protein is a good source for some of the nutrients that are hard to find in plants, but there are potential problems with giving too much of it to a bird and we don't know what the safe limit is. So it's a complicated situation that can make the bird owner want to bang their head against a wall and scream.
