We had an e mail at the end of March from a man who had photographed some poo on his bat box asking whether it could be a bat’s or whether it was a bird.
The clue is in the size of the droppings – too big for a bat and the other indicator is the whiteness of the droppings. Birds produce this white uric acid, bats do not. 
Another way to be sure it to rub the droppings through your fingers. If it crumbles to dust, you have bat poo. If it sticks to your fingers it isn’t and you need to wash your hands.The poo on the left if from under a pipistrelle roost. ( We hand netted one of the occupants)
Identifying what sort of bat made the droppings is fraught with difficulty – the only way to be certain is to have the poo’s DNA analysed but even then you have to be careful that it is not contaminated. If we catch a rare bat which is hard to identify, we scour the bag it is kept in for poo. Most bats are generous in the donations they make.
We know from experience that birds will often take advantage of a bat box, especially early in the year. When we check boxes early in the ear there is often a bird nest there, in which case we close the box and tiptoe quietly away
