Murray Cooper / Plate-billed Mountain-Toucan (Andigena laminirostris) at nest cavity, Ecuador

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

Murray Cooper / Black-billed Mountain-Toucan (Andigena nigrirostris) calling from nest cavity, Ecuador

Murray Cooper / Black-billed Mountain-Toucan (Andigena nigrirostris) at nest cavity with fruit, Ecuador

Murray Cooper / White-tipped Quetzal (Pharomachrus fulgidus) emerging from nest cavity, Colombia

Murray Cooper / Golden-plumed Parakeet (Leptosittaca branickii) pair emerging from nest cavity, Ecuador

Murray Cooper / Golden-plumed Parakeet (Leptosittaca branickii) emerging from nest cavity, Ecuador

Murray Cooper / Golden-plumed Parakeet (Leptosittaca branickii) pair at nest cavity, Ecuador

Murray Cooper / Yellow-tufted Woodpecker (Melanerpes cruentatus) female bringing food to nest, Ecuador

Murray Cooper / Yellow-tufted Woodpecker (Melanerpes cruentatus) female emerging from nest cavity, Ecuador

Murray Cooper / Red-billed Parrot (Pionus sordidus) emerging from nest cavity, Ecuador

Murray Cooper / Crimson-rumped Toucanet (Aulacorhynchus haematopygus) bringing food to mate in nest, Ecuador

Murray Cooper / Crimson-rumped Toucanet (Aulacorhynchus haematopygus) emerging from nest cavity, Ecuador

Murray Cooper / Streak-headed Woodcreeper (Lepidocolaptes souleyetii) bringing food to nest

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

Gerrit Vyn / Eastern Screech-Owl (Otus / Megascops asio mccallii) roosting in a natural tree cavity, Starr County, Texas, USA

Konrad Wothe / Grey-headed Woodpecker (Picus canus) at nest cavity, Bavaria, Germany

Konrad Wothe / Great Spotted Woodpecker (Dendrocopos major) male feeding young, Bavaria, Germany

Konrad Wothe / Great Spotted Woodpecker (Dendrocopos major) at nest cavity with chick, Germany

Donald M. Jones / Common Starling (Sturnus vulgaris) chick pair peeking out of nest cavity, western Montana

Donald M. Jones / Common Starling (Sturnus vulgaris) chick peeking out of nest cavity, western Montana

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

Tui De Roy / Magellanic Penguin (Spheniscus magellanicus) incubating eggs inside freshly dug burrow, Isla Martilla, Tierra del Fuego, Argentina

Tui De Roy / Humboldt Penguin (Spheniscus humboldti) parent and chick on nest in rock burrow on desert island, Tilgo Island, La Serena, Chile

Tui De Roy / Humboldt Penguin (Spheniscus humboldti) in nesting cavity in rock burrow, Tilgo Island, La Serena, Chile

Tui De Roy / Humboldt Penguin (Spheniscus humboldti) nesting in rock burrow on desert island, Tilgo Island, La Serena, Chile

Tui De Roy / Humboldt Penguin (Spheniscus humboldti) peeking out of nest in rock burrow, Tilgo Island, La Serena, Chile

Tui De Roy / Humboldt Penguin (Spheniscus humboldti) nesting in rock burrow on desert island, Tilgo Island, La Serena, Chile

Tui De Roy / Austral Parakeet (Enicognathus ferrugineus) investigating potential nest cavity in Southern Beech (Nothofagus sp) forest, Ushuaia, Tierra del Fuego, Argentina

Suzi Eszterhas / Hyacinth Macaw (Anodorhynchus hyacinthinus) in tree cavity, Pantanal, Brazil

Suzi Eszterhas / Hyacinth Macaw (Anodorhynchus hyacinthinus) in tree cavity, Pantanal, Brazil

Wild Wonders of Europe / Lundgren / Amphipod close to the gastrovascular cavity of a Dahlia anemone (Urticina felina) Saltstraumen, Bodö, Norway, October 2008

Wild Wonders of Europe / Lundgren / Amphipod close to the gastrovascular cavity of a Dahlia anemone (Urticina felina) Saltstraumen, Bodö, Norway, October 2008

Donald M. Jones / White-breasted Nuthatch (Sitta carolinensis) peeking out of nesting cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) female coming out of nesting cavity with fecal sac, western Montana

Donald M. Jones / Lewis's Woodpecker (Melanerpes lewis) with insect prey at nest in Ponderosa Pine (Pinus ponderosa), western Montana

Roger Powell / Three young Barn owls (Tyto alba) in nest cavity. Northumberland, UK

Donald M. Jones / Williamson's Sapsucker (Sphyrapicus thyroideus) female at nest cavity, western Wyoming

Donald M. Jones / White-breasted Nuthatch (Sitta carolinensis) at nest cavity with insect prey, western Montana

Donald M. Jones / Western Bluebird (Sialia mexicana) female exiting nest cavity with fecal sac, western Montana

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

Donald M. Jones / Tree Swallow (Tachycineta bicolor) young in nest cavity, western Montana

Donald M. Jones / Tree Swallow (Tachycineta bicolor) male peeking out of nest cavity, western Montana

Donald M. Jones / Tree Swallow (Tachycineta bicolor) female at nest cavity, western Montana

Donald M. Jones / Tree Swallow (Tachycineta bicolor) male at nest cavity, western Montana

Donald M. Jones / Tree Swallow (Tachycineta bicolor) male at nest cavity with insect prey, western Montana

Donald M. Jones / Pygmy Nuthatch (Sitta pygmaea) at nest cavity with prey, western Montana

Donald M. Jones / Pygmy Nuthatch (Sitta pygmaea) at nest cavity, western Montana

Donald M. Jones / Pygmy Nuthatch (Sitta pygmaea) at nest cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) young in nest cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) male feeding young at nest cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) male at nest cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) female feeding young at nest cavity, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) male coming out of nest cavity with fecal sac, western Montana

Donald M. Jones / Northern Flicker (Colaptes auratus) male at nest cavity, western Montana

Donald M. Jones / Mountain Bluebird (Sialia currucoides) male returning to nest cavity with prey, western Montana

Donald M. Jones / Mountain Bluebird (Sialia currucoides) male at nest cavity, western Montana

Donald M. Jones / Mountain Bluebird (Sialia currucoides) male calling at edge of nest cavity, western Montana

Donald M. Jones / Lewis's Woodpecker (Melanerpes lewis) at nest cavity in Ponderosa Pine (Pinus ponderosa), western Montana

Donald M. Jones / Lewis's Woodpecker (Melanerpes lewis) at nest cavity in Ponderosa Pine (Pinus ponderosa), western Montana

Donald M. Jones / Lewis's Woodpecker (Melanerpes lewis) at nest cavity in Ponderosa Pine (Pinus ponderosa), western Montana

Tim Laman / Great indian hornbill (Buceros bicornis) female inside nest cavity in hollow tree with tail feathers raised. Khao Yai NP, Thailand

Tim Laman / Great indian hornbill (Buceros bicornis) female feeding chick inside nest cavity in hollow tree, male has brought the food to the nest. Khao Yai NP, Thailand

Tim Laman / Various seeds of rainforest trees on the ground below the nest cavity, regurgitated by Rufous-necked hornbills (Aceros nipalensis) note also uneaten cicadas, Huai Kha Khaeng Wildlife Sanctuary, Thaila...

Tim Laman / Great indian hornbill (Buceros bicornis) female feeding chick inside nest cavity in hollow tree, male has brought the food to the nest. Khao Yai NP, Thailand

Tim Laman / Wreathed hornbill (Rhyticeros / Aceros undulatus) female inside nest cavity, adding mud plaster to the side of the nest hole, Khao Yai National Park, Thailand

Tim Laman / Bushy-crested Hornbill (Anorrhinus galeritus) juvenile, shortly after emerging from nest cavity. Gunung Palung National Park, Borneo, Indonesia

Tim Laman / Great indian hornbill (Buceros bicornis) female and chick inside nest cavity in hollow tree, Khao Yai NP, Thailand

Tim Laman / Great indian hornbill (Buceros bicornis) female and chick inside nest cavity in hollow tree, Khao Yai NP, Thailand

Tim Laman / Oriental pied hornbill (Anthracoceros albirostris) female in nest cavity. Khao Yai National Park, Thailand

Tim Laman / Great indian hornbill (Buceros bicornis) female feeding chick inside nest cavity in hollow tree, male has brought the food to the nest. Khao Yai NP, Thailand

Tim Laman / Great indian hornbill (Buceros bicornis) female and chick inside nest cavity in hollow tree, Khao Yai NP, Thailand

Kevin Schafer / Resplendent Quetzal (Pharomachrus mocinno) male at nest cavity, Talamanca Cloud Forest, Costa Rica

Vincent Grafhorst / Crested Barbet (Trachyphonus vaillantii) sticking its head outside nest hole, Botsalano Game Reserve, South Africa

Jan Vermeer / Ural Owl (Strix uralensis) on the lookout from nest cavity, Hokkaido, Japan

Jan Vermeer / Ural Owl (Strix uralensis) on the lookout from nest cavity, Hokkaido, Japan

Jan Vermeer / Ural Owl (Strix uralensis) on the lookout from nest cavity, Hokkaido, Japan

Michael & Patricia Fogden / Peach-faced Lovebird (Agapornis roseicollis) pair mating in nest hole in rock cliff, Waterberg Plateau Park, Namibia

Michael & Patricia Fogden / Golden-olive Woodpecker (Colaptes rubiginosus) male at nest hole, Monteverde Cloud Forest, Costa Rica

Michael & Patricia Fogden / Golden-olive Woodpecker (Colaptes rubiginosus) female at nest hole, Monteverde Cloud Forest, Costa Rica

Tim Fitzharris / Pileated Woodpecker (Dryocopus pileatus) in nesthole, North America

Scott Leslie / Tree Swallow (Tachycineta bicolor) landing near nest cavity with partner calling, Tobeatic Wilderness Area, Nova Scotia, Canada

Michael Quinton / Bufflehead (Bucephala albeola) female leaving nest cavity with eggs, Slana, Alaska

Michael Quinton / Northern Red-backed Vole (Clethrionomys rutilus) in abandoned northern flicker tree cavity with star tracks in background, Slana, Alaska

Michael Quinton / Mountain Bluebird (Sialia currucoides) flying out of abandoned northern flicker nest cavity, Wade Lake, Montana

Michael Quinton / American Kestrel (Falco sparverius) three week old chicks in abandoned northern flicker nest cavity, Slana, Alaska

Michael Quinton / American Kestrel (Falco sparverius) parent feeding chicks in abandoned northern flicker nest cavity, Slana, Alaska

Michael Quinton / American Kestrel (Falco sparverius) parent and chicks in abandoned northern flicker nest cavity, Slana, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) woodpecker adult taking flight from tree with nest cavity, Slana, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) woodpecker parent feeding chicks in nest, Wade Lake, Montana

Michael Quinton / Northern Flicker (Colaptes auratus) woodpecker in nest cavity, Slana, Alaska