Nick Upton / People watching Swifts (Apus apus) flying around Holy Trinity church during a swift walk organised by the local swift group, Bradford-on-Avon, Wiltshire, UK, June. Digital composite. Model released.

Nick Upton / Common swift (Apus apus) flying to its nest in a Swift nest box in the belfry of All Saints Church, Worlington, Suffolk, UK, July.

Nick Upton / Screaming party of Common swifts (Apus apus) flying around a church bell tower at dusk, where a large colony breeds in nestboxes behind the window louvres, All Saints Church, Worlington, Suffolk, UK,...

Nick Upton / Screaming party of Common swifts (Apus apus) flying around a church bell tower at dusk, where a large colony breeds in nestboxes behind the window louvres, All Saints Church, Worlington, Suffolk, UK,...

Nick Upton / Nest box for Common swifts (Apus apus) carried by Roger Becket into Holy Trinity church to be fitted in the bell tower, Bradford-on-Avon, Wiltshire, UK, June. Model released.

Nick Upton / Common swift (Apus apus) call playback system positioned and turned on successfully by Roger Becket and Peter Grayshon below a nest box they've just fitted into a louvred window in the bell tower of H...

Nick Upton / Nest box for Common swifts (Apus apus) carried by Roger Becket and Peter Grayshon into Holy Trinity church to be fitted in the bell tower, Bradford-on-Avon, Wiltshire, UK, June.

Nick Upton / Common swift (Apus apus) nest box carried by Roger Becket into Holy Trinity church to be fitted in the bell tower, Bradford-on-Avon, Wiltshire, UK, June.

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) 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) 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) 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) 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.

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

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

Onne van der Wal / Church against city background. Valparaiso, Chile, 2008.

Angelo Gandolfi / Barn Owl (Tyto alba) flying over a bell tower of a church in Pitigliano, at night, Tuscany, Italy.

Stephen Dalton / Barn owl (Tyto alba) flying to nest in bell tower, Sussex, UK

Stephen Dalton / Barn owl ( Tyto alba) flying with prey towards nest in bell tower, Sussex, UK

Stephen Dalton / Barn owl (Tyto alba) in flight, approaching bell tower, England, UK

Juan Carlos Munoz / Degioz village with bell tower, Gran Paradiso NP, Valsavarenche, Aosta valley, Italian alps

Juan Manuel Borrero / Bridge and church bell tower in Isil, the Pyrenees mountains, Catalonia, Lerida, Spain

Juan Manuel Borrero / Bell tower and archway in the village of Farrera, the Pyrenees mountain, Catalonia, Lerida, Spain

Juan Manuel Borrero / The bell tower at Castle of Molina de Aragón, Guadalajara, Spain

Mike Potts / Car, street and bell tower in Trinidad, colonial city of central Cuba

Gavin Hellier / Looking down towards the Duomo and Bell Tower from the Town Hall, Siena, Tuscany, Italy

John Cancalosi / White stork nesting next to bell tower {Ciconia cinconia} Spain

John Cancalosi / White stork nesting beside bell tower {Ciconia ciconia} Spain

Angelo Giampiccolo / Late Romanesque bell tower of the cathedral in Gaeta, Latina, Lazio, Italy. The tower has a roof made of colored ceramic tiles, situated in the medieval area of Gaeta, known as Sant' Erasmo.

Angelo Giampiccolo / Late Romanesque bell tower of the cathedral in Gaeta, Latina, Lazio, Italy. The tower has a roof made of colored ceramic tiles, situated in the medieval area of Gaeta, known as Sant' Erasmo.

Angelo Giampiccolo / Clock and bell tower of San Rufino Cathedral, Assisi, Italy.

Roberto Rinaldi / Amalfi cathedral, the bell tower originates from 1180, Amalfi, Italy

Michele Westmorland / Bell tower at the Brimstone Hill Fortress on the island of St. Kitts, Dutch Caribbean, Netherlands Antilles, West Indies. August 2006.

Roberto Rinaldi / Across the canal to St Mark's Campanile (Bell Tower), Venice, Italy.

Roberto Rinaldi / Campanile di San Marco (St Mark's Bell Tower) that can be seen from all over the canals in the Venice Lagoon, Venice, Italy.

Roberto Rinaldi / View of the bell tower (campanile) and monastery San Giorgio Maggiore. Masts of yachts moored in the island of San Giorgio's marina can be seen, Venice, Italy.

Roberto Rinaldi / Campanile (Bell Tower), St Mark's Square, Venice, Italy.

Robin Chittenden / Female juvenile Peregrine falcon (Falco peregrinus) caught as had come to ground, released higher up on the Bell Tower Norwich Cathedral UK, June.

Stephen Dalton / Barn owl (Tyto alba) in flight approaching bell-tower, carrying rodent prey to young in nest. Sussex, UK.