Bird droppings or mashed kiwi?

On our garbage cans, especially the paper recycling bins, there are always these strange little piles about 3-4 cm in diameter that look a bit like squashed kiwi and are a bit sticky (yes, I accidentally touched them once, so I'm starting to wonder what they are). My first thought was that it was bird droppings. But I'm usually only used to white or some other color. Does anyone have any ideas?