You may have also noticed that this procedure does not require gloves. If this were a dangerous procedure, like if we were using chemicals or touching blood, we would have to be wearing gloves. Good scientists are always careful. The pieces are bones, teeth, and fur from the small animals an owl has eaten.
Owls eat rodents like mice and voles.
It is not uncommon to find them on the ground wherever owls live and hunt. Owls, and other birds of prey, have no teeth. They swallow their prey whole, then absorb the softer parts by digesting them and passing them on to their intestines. However, they are not able to digest all parts of their meal. The bones, teeth, feathers and fur are compressed in the owl's digestive system.