I had a closer look at the blowflies on the window sill. Turns out that there were two species, both native. A common brown blowfly (Calliphora stygia) was joined by a lesser brown blowfly (C. augur). As you can see from the top picture, the two species are very similar but C. augur is distinguished by a steel blue patch on the abdomen.
Compare the photos of C. augur above with the images of C. stygia below.
Whereas C. stygia gets in early to lay its eggs, Calliphora augur can afford to be a little more relaxed. It doesn't waste time with eggs but produces young already at the maggot stage.
As annoying as they are, I can't help feeling a little bit impressed by flies. I understand if I'm alone in that.
