These large black flies with dangly legs which hover in the wind are called St Mark's Flies. Put your bug spray away as they may be annoying but they only fly for 1 week and don't bite. The rest of their time is spent as larva and when flying they are only trying to attract a female.
As you can imagine slow flying juicey flies provide a tasty treat for many birds on the reserve such as sky larks and meadow pippits. They are a particularly important food source at a time in of the year where there are hungry chicks to feed.
But why are they called St Mark's fly? They normally emerge around the same day every year - 25th of April or the day where the Venice's patron saint St Mark is celebrated. Emerging in April they become more visible in May.