Anthomyia procellaris is quite an attractive little fly. It is light greyish with black markings. It does however have two look-a-likes: Anthomyia pluvialis and Anthomyia imbrida. With care however these species can be told apart. Anthomyia pluvialis has two seperate spots just at the base of the wing. In the other two species these two spots are melted into one. The shape of this one spot is different in the two species: In Anthomyia procellaris the spot ends in a straight line, while in Anthomyia imbrida the spot ends in a bended or notched line. To be absolutely certain you will have to catch the fly and count the number of hairs on the hindtibia (the fibula of insects). Anthomyia procellaris en Anthomyia pluvialis have less than 9 hairs there, where Anthomyia imbrida has at least 9 hairs there. By the way Anthomyia imbrida only inhabits England and the south of Wales. So in Scotland or Ireland don't worry about this species.
Anthomyia procellaris appears, sometimes in great numbers by the end of April or in May. Numbers then steadily decline till mid-September, after which sightings are very rare. The adult flies love sun bathing and have been seen feeding on bird's droppings. Other aspects regarding its life is still not known. It is uncertain where the maggots live. Water has been suggested and so have bird's nests, but no one knows for sure. The fly has been seen in various habitats, so this doesn't help at all. Should you ever happen to see an egg-depositing female, please inform me, for many fly specialist are interested in exactly that piece of information.