allotment's resident robin

bobby our allotment robin he's very tame and eat's out of my hand ,he had 4 chick's that have left the nest and now has another nest of chicks how many i don't know this time ,but when i feed him he fly's back to the nest and has started returning with a beak full of bird dropping's and plonks them down in front of me before flying of with another beak full and returning with another beak full of droppings for me why ?

Done