Nick Upton / Common swift (Apus apus) nest box fitted by Roger Becket and Peter Grayshon into a louvred window in the bell tower of Holy Trinity church, Bradford-on-Avon, Wiltshire, UK, June.

Nick Upton / Common swift (Apus apus) call playback system positioned below a nest box just fitted by Roger Becket and Peter Grayshon into a louvred window in the bell tower of Holy Trinity church, Bradford-on-Avo...

Nick Upton / Simon Evans ringing a Common swift chick (Apus apus) removed briefly from a nest box in All Saints Church belfry with over 40 swift nestboxes, Worlington, Suffolk, UK, July. Model released. Winner of...

Nick Upton / Simon Evans climbing a 40ft ladder to reach a church belfry with over 40 nestboxes for Common swifts (Apus apus) to ring the chicks, Worlington, Suffolk, UK, July. Model released.

Nick Upton / Swifts (Apus apus) flying and calling around All Saints Church bell tower at dusk, where a large colony breeds in nestboxes behind the window louvres, Worlington, Suffolk, July.

Nick Upton / Peter Grayshon replaces inspection covers to a Common swift (Apus apus) nest box he's just helped to attach to a louvred window in the bell tower of Holy Trinity church, Bradford-on-Avon, Wiltshire, U...

Wild Wonders of Europe / Widstrand / Rab town, Rab island, Velebit Mountains Nature Park, Croatia, April 2014.

Philippe Clement / Old anchor and belfry at the entrance gate to the medieval Ville Close at Concarneau, Finistere, Brittany, France, September 2015

Gary K. Smith / Adriatic coastline and waterfront of Venice with prominent bell tower in St Marks Square, Italy, April.

David Tipling / White Storks (Ciconia ciconia) nesting on Alfaro Cathedral, La Rioja Spain, May 2010.

Gavin Hellier / Bell Tower of Orthodox Church overlooking the Caldera in Fira, Santorini (Thira), Cyclades Islands, Aegean Sea, Greece, 2010

Rob Cousins / Lighthouse and bell tower at the entrance to the river Avon at Portbury. March 2009.