Yesterday I found a bumble bee nest in an old brick wall (about two metres above the ground). I haven't yet identified the species as the bees tend to fly in and out at high speed. (The flowers below the wall attract more than one species.)
What I could see very clearly however was that as each bee flew in, most of them were carrying in their back legs a small piece of leaf (possibly petal?) a bit less than a square centimetre. The bees which emerged weren't carrying anything, as far as I could see.
I saw this four times over the course of about twenty minutes' observation.
What on Earth would they be doing these leaf fragments?