Anything avian relates to birds. Avian flu is the bird flu, which birds can pass to humans.
Someone who flies a plane is an aviator, and that may help you remember that avian things have to do with birds. Feathers are part of avian anatomy. Laying eggs is the avian method of reproduction. Bird-watchers are avian experts who know how to tell birds apart. Ornithologists are also avian experts: they’re scientists who study birds.