Little Flies

What's with those lttle flies with striped wings? I never knew their name. They're the kind you sometimes find in the trash can - and wonder how they got in there 'cause you keep the lid real tight.

I just went outside - first time this morning - and found hundreds if not thousands of them on the south wall and deck railing of our house. That's the sunny side. I didn't see any in the shadows so I assume for some reason they want to be in the sun. I've NEVER seen this many at one time.

Does anyone know what kind of fly they are? Is it mating season? Dang!