If you look carefully, you'll see that the nest is made up of a lower layer of mostly dry grasses, which is completed with a downy layer of feathers. The birds create a cup shape as they are building the nest by pushing towards the sides with their wings outstretched. When they do this you can hear their little legs trying to get a good grip on the floor.
If by any chance you have violet-green swallows nesting near you, I suggest you try offering light, downy feathers to them. If you blow one up into the air, they'll probably swoop down to catch it and take back to their nest. If you are interested in plans for building your own nestbox, we'll be sure to post.
We hope you enjoy viewing these birds over the coming weeks as much as we do! I'll post more information soon, including photos.