David Perpinan / Group of Common house martins (Delichon urbicum) preening on cable, Cuenca, Spain, August.

David Perpinan / Common house martin (Delichon urbicum) perched on a cable and then flies away, Cuenca, Spain, August.

David Perpinan / Group of Common house martins (Delichon urbicum) preening on cable, Cuenca, Spain, August. Migration.

Pal Hermansen / House martin (Delchion urbicum) taking off from under roof, viewed through window. Golsfjell, Viken, Norway. July.

Markus Varesvuo / House Martin (Delichon urbica) Spain, April.

Robin Chittenden / House martin (Delichon urbicum) group on ground, collecting sand for nests. Cyprus. April.

Robin Chittenden / House martin (Delichon urbicum) on ground, collecting sand for nest. Cyprus. April.

Nick Upton / House martin (Delichon urbicum) flying from its mud nest under the eaves of cottage, Lacock, Wiltshire, UK, May.

David Tipling / Common house martin (Delichon urbicum) Northumberland, England, UK. July

Roger Powell / House martin (Delichon urbicum) collecting straw for nest building. Alentejo, Portugal, April.

Roger Powell / House martins (Delichon urbicum) three collecting mud from a pool for nest building. Alentejo, Portugal, April.

Roger Powell / House martin (Delichon urbicum) collecting mud from a pool for nest building. Alentejo, Portugal, April.

Nick Upton / Judith Wakelam hand-feeding an orphaned House martin chick (Delichon urbicum) with insect food in her home, Worlington, Suffolk, UK, July. Model released.

Nick Upton / House martin (Delichon urbicum) flying overhead, Lacock, Wiltshire, UK, June.

Nick Upton / Orphaned House martin chick (Delichon urbicum) being hand fed with insect food by Judith Wakelam in her home, Worlington, Suffolk, UK, July. Model released.

David Kjaer / House martin (Delichon urbica) at nest, Spain, June.

Phil Savoie / House martin (Delichon urbicum) in flight, Monmouthshire, Wales UK, June.

Phil Savoie / House martin (Delichon urbicum) in flight, Monmouthshire, Wales UK, June.

Phil Savoie / House martin (Delichon urbicum) in flight, Monmouthshire, Wales UK, June.

Andy Sands / House Martin (Delichon urbicum) collecting mud from puddle, Hampshire, England, UK, May

Dave Bevan / House martin (Delichon urbicum) flying to nest to feed chicks, Cambridgeshire, England, UK. July.

Dave Bevan / House martin (Delichon urbicum) feeding chicks at nest, Cambridgeshire, England, UK. July.

Markus Varesvuo / House martin (Delichon urbica) in flight across sea, Finland, July

Jean E. Roche / Raven (Corvus corax) and House martin (Delichon urbicum) flying, Oppedette Canyon, Haute Provence, France, April.

Stephen Dalton / House martins (Delichon urbicum) at communal nesting site under house balcony, Extremadura, Spain, April

Robin Chittenden / House martin (Delichon urbica) at nests, in flight, Lesbos Lesvos Greece GR April 2015

Cyril Ruoso / House martin (Delichon urbicum) gathering mud to build nest, Picardy, France.

Cyril Ruoso / House martin (Delichon urbicum) perched on wire, France. rance

Pal Hermansen / House martin (Delichon urbica) two in flight, with habitat, Golsfjellet Mountain, Buskerud County, Norway, July.

Pal Hermansen / House martin (Delichon urbica) flying upwards, with clouds, Buskerud, Norway, July.

Alan Williams / House Martin (Delichon urbica) in flight, Wirral, Merseyside, UK, August.

Alan Williams / House Martin (Delichon urbica) in flight, Wirral, Merseyside, UK, July

Jussi Murtosaari / House Martin (Delichon urbicum) gathering mud to build nest, northern Finland, June.

Jussi Murtosaari / House Martins (Delichon urbicum) gathering mud to build nest, northern Finland, June.

David Pattyn / Sand martin (Riparia riparia) sitting on wires during migration. Greece. May

Juan Manuel Borrero / Illustration of Hirundinidae family: Common Sand Martin (Riparia riparia), Northen House-Martin (Delichon urbicum), Common Swift (Apus apus) and Barn Swallow (Hirundo rustica). Pencil and watercolor p...

Tim Martin / Male Common / House sparrow (Passer domesticus) feeding juvenile on roof. Isles of Scilly, UK. August

Tim Martin / Male Common / House sparrow (Passer domesticus) perched on sunflower. Isles of Scilly, UK. August

Tim Martin / Juvenile House / Common sparrow (Passer domesticus) begging from adult male, perched on sunflower. Isles of Scilly, UK. August

Tim Martin / Female Comon / House sparrow (Passer domesticus) feeding on sunflower, two juveniles waiting to be fed in background. Isles of Scilly, UK. August

Tim Martin / Male House / Common sparrow (Passer domesticus) on grass. Isles of Scilly, Cornwall, UK. August

Tim Martin / House / Common sparrow (Passer domesticus) female with bread in its beak. Isles of Scilly, Cornwall, UK. August

Rolf Nussbaumer / Common House Martin (Delichon urbicum) feeding chicks nest, Oberaegeri, Switzerland. June 1993

Mike Wilkes / Male Common (House) sparrow feeds young at nest made in old House martin's nest. England, UK

Mathias Schaef/ BIA / Common House Martin (Delichon urbicum) at pond, Pleven, Bulgaria

Anton Luhr/ BIA / Common House Martin (Delichon urbicum), Sweden

Ralph Martin / Common House Martin (Delichon urbicum) carrying insect prey, Baden-Wurttemberg, Germany

Konrad Wothe / Common House Martin (Delichon urbicum) collecting clay for nest building, Europe