Unfiltered sunlight is a natural source of vitamin D3. The only way to get this is through being outside - windows filter it out. So even if a bird is kept in a bright, sunny room, if the windows are closed then they can't synthesise their own vitamin D3 and will become deficient.
However, the sun is not the only source of vitamin D3; it is also effectively supplemented. If a bird (or human for that matter) is not getting enough unfiltered sunlight of an adequate strength, then they need a dietary supplement. Most humans in the UK and similarly northern latitudes are low on vitamin D3 during the winter - those of us with dark skin can get significantly deprived and suffer health consequences. I would always urge someone who lives somewhere with dark winters and/or whose bird does not get natural, unfiltered sunlight to feed a fortified pellet (and, to be honest, to consider their own vitamin D3 intake).
TOPs claims to contain a natural source of vitamin D3 but I'm not convinced that it contains enough to satisfy the requirements of a bird through a dark winter with little natural sunlight.
Hello! I have....some confusion. I feed primarily pellets, with chop and seeds. I usually mix pellets for availability reasons (the great toilet paper shortage of 2020 has me a little paranoid). I was doing some research, because my vet asked me if I provided my birds with cuttle bone and iodine...
forums.avianavenue.com
I also feed Roudybush and Psittacus and am happy with how my birds are on them. I like feeding a couple of different types of pellet with different textures and tastes to ensure that my birds don't get too used to one thing and won't eat other pellets. This also helps ensure that all nutritional bases are covered.
