The annual swarm of tiny baby spiders

Evewr since we built the cabin next to the patio I've noticed this amazing springtime sight of baby spiders all swarming together. I have no idea what they are nor what they become. But for a few days they can be found around the cabin, sometimes inside, sometimes out.

At this point they all gather together in a tight bunch and then gradually wander off a short distance until something spooks them and then they gather back in a close bunch, or swarm. They can't be more than about 3 to 4 mm in size. I notice them for a few days and then never see them again until the following spring. It's that time of year.

Here they are gathered just outside  one of the windows and under the edge of the felt roof.