Budgies regurgitate (bring food up from the crop) for their mates or friends as a sign of affection and a means of bonding. However, not all humans appreciate having their finger covered in undigested seed! Still, if your budgie does this you should feel honored and loved. It's best to ignore this behavior when it's done to us. If your budgie is becoming "obsessed" with a mirror, toy, or other object it might be wise to remove the favorite object and replace it with something else. Allowing this sort of behavior to continue can lead to a very frustrated, hormonal budgie which can in turn, lead to biting and other behavioral problems.

Additional reading to ensure your bird is actually regurgitation vs vomiting (which can indicate sickness) Reguritation vs Vomiting