The message
from "David Hill" contains these
words:

Lets just say that the blackbirds turn over the mulch looking for food.
In so doing they also stop any weed seed taking rot in the mulch.

Quite. As they endlessly turn over the mulch here, they are doing the
same job as turning a compost heap..letting in air and rain and
encouraging its decomposition which in turn encourages more worm
activity, to our mutual benefit.

Janet.