from what I got from here there are a few birds that need seeds, but most, are better with pellets, if your bird doesn't want seeds, it's akin to a three year old not wanting candy, a good thing.
Assuming they're used to eating a variety of foods and are willing to try new things, I believe in listening to what your bird says about their diet. Obviously, get an understanding of their natural ecosystem, the niche they occupy, and the foods they eat in the wild. It's also important to have a basic understanding of their dietary requirements in captivity, nutritional diseases they're prone to, etc. However, I also think it's important to watch what they eat, as they tend to have a basic understanding of what their bodies need. However, make sure they aren't just pigging out on certain things because they're addicted to them, otherwise you'll run into trouble.
For example, my budgies have made it clear that they need to have millet in their diet. They don't eat it just because they think it tastes good, but because they seem to have some sort of biological or instinctual NEED for it. Obviously, they eat veggies, pellets, and other foods too, but they definitely need to eat millet too. And when it comes to my kākāriki, despite being fed a terrible diet prior to adopting them, they make it clear that they need lots of leafy greens in their diet (specifically, superfoods like spinach, chard, dandelion greens, etc.), as well as high fat and high protein foods like nuts, sunflower seeds, and birdie bread. Noah eats pretty much everything. But when it comes to eating veggies, he's very specific about what parts he eats, which undoubtedly reflect his species' unique diet in desert-like ecosystems. He also actively hunts bugs (confused flour beetles, aphids, spiders), which is definitely interesting.
