They can get blackhead from those meal worms if it is in your area, not sayen you birds have that but that is one way they get blackheadI meant worms in their coop, we have big black beetles in our backyard where my birds are, they are many, when i remove a plate from the ground i can see worms under it, i know these worms are beetles didn't develop yet, i don't have any other birds than peafowls, i will ask him on Friday which wormer he use.
Thank you so much again for your help, i appreciate every single word you wrote for me