Donald M. Jones / Western Bluebird (Sialia mexicana) male in nest cavity entrance, Montana

Donald M. Jones / Hairy Woodpecker (Picoides villosus) male with prey at nest cavity entrance, western Montana

Michael Quinton / Red Squirrel (Tamiasciurus hudsonicus) four young at entrance to their nest in a tree cavity, Alaska

John Chan / Female Downy woodpecker nestling (Picoides pubescens) peering out and calling from the entrance of its nest cavity, begging to be fed, Southern California, USA, May.

John Chan / Male Downy woodpecker nestling (Picoides pubescens) peering out and calling from the entrance of its nest cavity, begging to be fed, Southern California, USA, May.

Marie Read / Equipment set up for remote control photography at nest hole of Great Crested Flycatcher (Myiarchus crinitus), New York, USA. Camera, fitted with wide angle lens, mounted on monopod attached to ladder...

Marie Read / Great crested flycatcher (Myiarchus crinitus), female alighting at nest cavity entrance carrying pine needles as nest material, wide angle captured with remote camera to show habitat, New York, USA. M...

Nick Upton / German / European wasp (Vespula germanica) worker carrying some small sticks while excavating a nest cavity to deposit them outside as two other workers defend the nest entrance, chalk grassland meado...

Donald M. Jones / Western Bluebird (Sialia mexicana) male at nest cavity entrance, Montana

Marie Read / Red-breasted Sapsucker (Sphyrapicus ruber), outside entrance to nest cavity in treetrunk, holding beakful of insects for young. Lee Vining Canyon, Mono Lake Basin, California, USA, June.

Martin Willis / Black-throated Finch (Poephila cincta) at entrance to nest cavity, Flat Creek Station, Georgetown, Queensland, Australia

Murray Cooper / Powerful Woodpecker (Campephilus pollens) female at entrance to nest cavity, Ecuador

Murray Cooper / Long-tailed Woodcreeper (Deconychura longicauda) at entrance to nest cavity, Ecuador

Shin Yoshino / Indian Scops Owl (Otus bakkamoena) at entrance to nest cavity, Japan

Laurie Campbell / Barn Owl (Tyto alba) adult at entrance to nest cavity in dead Elm tree at dawn, Berwickshire, Scotland, UK, July

Laurie Campbell / Barn Owl (Tyto alba) adult at entrance to nest cavity in dead Elm tree, Berwickshire, Scotland, UK, July

Laurie Campbell / Barn Owl (Tyto alba) adult at entrance to nest cavity in dead Elm tree, Berwickshire, Scotland, UK, July

Laurie Campbell / Barn Owl (Tyto alba) adult at entrance to nest cavity in dead Elm tree, Berwickshire, Scotland, UK, July

Tui De Roy / Magellanic Woodpecker (Campephilus magellanicus) male at nest entrance, in Southern Beech (Nothofagus sp), Tierra del Fuego National Park, Argentina

Donald M. Jones / Western Bluebird (Sialia mexicana) female at nest cavity entrance with insect prey, western Montana

Donald M. Jones / Western Bluebird (Sialia mexicana) female at nest cavity entrance with insect in beak, western Montana

Pete Oxford / Rose-ringed Parakeet (Psittacula krameri) at nest entrance, Ranthambhore National Park, Rajasthan, India

Roger Powell / Barn Owl (Tyto alba) juveniles nestling in together at entrance to nest cavity, Northumberland, UK

Doug Wechsler / Cuban amazon parrot (Amazona leucocephala) at nest cavity entrance, Zapata Swamp, Cuba

Flip de Nooyer / Robust Woodpecker (Campephilus robustus) at entrance to nest cavity, Pantanal, Brazil

Thomas Mangelsen / Mountain Bluebird (Sialia currucoides) at entrance of nest cavity, Yellowstone National Park, Wyoming

Claus Meyer / Campo Flicker (Colaptes campestris) at entrance of nest cavity, Pantanal ecosystem, Brazil

Michael Quinton / Red Squirrel (Tamiasciurus hudsonicus) four young at entrance to their nest in a tree cavity, Alaska

Michael Quinton / Bufflehead (Bucephala albeola) chick at nest entrance in tree, boreal pond habitat, Alaska

Michael Quinton / Bufflehead (Bucephala albeola) mother flying to nest entrance, boreal pond habitat, Alaska

Michael Quinton / Bufflehead (Bucephala albeola) chicks in nest in tree trunk with mother at nest entrance, boreal pond habitat, Alaska

Tui De Roy / King Vulture (Sarcoramphus papa) at the entrance of its nest cavity in snag, Tambopata River, Peruvian Amazon, Peru

Lucas Bustamante / Tawny-bellied screech owl (Megascops watsonii) perched at entrance to nest hole in tree cavity, Yasuni National Park, Orellana, Ecuador.

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male perched above a potential nest cavity in Dipterocarp tree as female investigates the inside whilst perched at the entrance, Kalimantan Province, Borneo, Indone...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) female perched at entrance to nest cavity in Dipterocarp tree, Kalimantan, Borneo, Indonesia. Critically endangered.

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it whilst perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically enda...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it whilst perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically enda...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it whilst perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically enda...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it whilst perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically enda...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) female examining nest cavity in a large Dipterocarp tree,as male perches above entrance, Kalimantan, Borneo, Indonesia. Critically endangered.

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it while perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically endan...

Tim Laman / Helmeted hornbill (Rhinoplax vigil) male delivering fruit to female in nest cavity by regurgitating it whilst perched at entrance, Budo-Sungai Padi National Park, Narathiwat, Thailand. Critically enda...

Ingo Arndt / Thermal image of a Honey bee (Apis mellifera) colony in an abandoned Black woodpecker nest in a tree cavity, viewed from the nest entrance at night. Cooler parts appear darker, warmer parts appear bri...

Ingo Arndt / Honey bee (Apis mellifera) worker bees cleaning the wall around the entrance of the nest cavity, view of the cavity / Germany

Ingo Arndt / Honey bees (Apis mellifera) starting a colony in an abandoned Black woodpecker nest cavity, Germany. May.

Ingo Arndt / Honey bees (Apis mellifera) starting a beehive in a tree nest cavity, Germany.

Ingo Arndt / Honey bee (Apis mellifera) nest within an old black woodpecker nest cavity with honeycombs visible through front entrance, Germany.

Ingo Arndt / Honey bee (Apis mellifera) colony building a beehive inside an old black woodpecker nest cavity, forming a 'ball' at the top, Germany.

Ingo Arndt / Honey bee (Apis mellifera) workers cleaning the wall around the entrance of their tree hole beehive as seen from within the cavity, Germany.

Ingo Arndt / Honey bee (Apis mellifera) colony beginning to establish a hive inside an old black woodpecker nest cavity as viewed from outside, Germany

Ingo Arndt / Honey bee (Apis mellifera) colony closing nest entrance to their hive inside an old black woodpecker nest cavity as viewed from outside, Germany.

Ingo Arndt / Honey bee (Apis mellifera) worker bees isolate wall inside their nest cavity, close to the entrance, with propolis or bee glue, Germany.

Ingo Arndt / Honey bee (Apis mellifera) swarm starting a colony inside an abandoned Black woodpecker nest cavity, showing nest entrance from outside, Germany. May.

Ingo Arndt / Viewed from inside a tree hole nest cavity, a Honey bee (Apis mellifera) swarm arriving at the entrance to establish a hive, viewed from inside, Germany.

Tim Laman / Golden-shouldered parrot (Psephotellus chrysopterygius) male at entrance to nest cavity in a termite mound. Cape York Peninsula, Queensland, Australia. May.