Swifts are very noisily investigating the three unoccupied nestboxes on my house. I believe that these are immature birds (fledging locally last year or the year before) that are now looking for nesting sites to use next year. They are screaming low around the house, and have several times landed on top of the boxes and on a nearby drainpipe; I guess they are testing out the ease of landing at and taking off from the boxes. I would suggest that this is a good time for anybody with unoccupied swift boxes to play the CD of swifts screaming, to attract these immature birds. The pair of swifts nesting in my fourth box are having to dodge the screaming youngsters to get in and out to feed their chicks.