Dropping

Dropping
 

p.284
-when a bird dropping landed on Seamus's head as one of Hermione's canaries whizzed over them.

Does DROPPING really mean waste? My teacher told me this meaning would always be plural, so I'm not sure.

Yes, 'a bird dropping' means bird dung. 鸟粪

The story is describing a single occurrence.
Had the bird defecated more than once, the plural "when bird droppings landed" would have been used.

Thank you.