More wing exercises

Our two young swifts in box 3 are busy exercising their wings, or at least one of them is, while the other is looking out of the entrance hole. Notice how the rear swiftlet raises and fans it's tail out as well. This is the box with the entrance on the underside so the view is probably more difficult to see from their angle.

They are lucky, there is plenty of room in their nestbox (all our boxes are roughly the size of a shoebox with at least a 7-8" width to give plenty of wing space) but how do other swifts that are nesting in very small spaces or under roof tiles manage to excercise themselves like this?

According to the date they hatched they are due to fledge on around the 27th August...but they look nearly ready to me!