Pete Oxford / Red and Green Macaw (Ara chloroptera) adult emerging from nest cavity, central Pantanal, Mato Grosso do Sul, Brazil

Michael & Patricia Fogden / Resplendent Quetzal (Pharomachrus mocinno) male flying near nest hole in Monteverde Cloud Forest Reserve, Costa Rica

Michael & Patricia Fogden / Chestnut-mandibled Toucan (Ramphastos swainsonii) nest hole, rainforest, Costa Rica

Michael & Patricia Fogden / Resplendent Quetzal (Pharomachrus mocinno) male, near nest hole, Monteverde Cloud Forest Reserve, Costa Rica

Michael & Patricia Fogden / Brown-billed Scythebill (Campylorhamphus pusillus) hummingbird, in tree-cavity nest, Monteverde Cloud Forest Reserve, Costa Rica

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

Pete Oxford / Plate-billed Mountain Toucan (Andigena laminirostris) parent bringing food to young in nest cavity, Andes Mountains, Ecuador

Pete Oxford / Plate-billed Mountain Toucan (Andigena laminirostris) parent bringing food to young who are hiding inside nest cavity, Andes Mountains, Ecuador

Pete Oxford / Plate-billed Mountain Toucan (Andigena laminirostris) parent bringing food to young in nest cavity, Andes Mountains, Ecuador

Thomas Mangelsen / Bluebird (Sialia sp) parent feeding young in nest cavity, North America

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

Thomas Mangelsen / Northern Saw-whet Owl (Aegolius acadicus) in nest cavity in tree, Wyoming

Jan Vermeer / Pine Marten (Martes martes) young looking out of tree hole, Europe

Jan Vermeer / Pine Marten (Martes martes) female looking out of tree hole, Europe

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

Patricio Robles Gil / Yellow-headed Parrot (Amazona oratrix) in nest cavity, Tamaulipas, Mexico

Jim Brandenburg / Pileated Woodpecker (Dryocopus pileatus) at nest

Konrad Wothe / Saguaro (Carnegiea gigantea) with nesting cavities made by birds, Sonora Desert, Arizona

Michael Quinton / Northern Hawk Owl (Surnia ulula) parent delivering fresh prey to mate and chicks in nest, Alaska

Michael Quinton / Northern Hawk Owl (Surnia ulula) incubating eggs on nest built in top of snag, Alaska

Michio Hoshino / Boreal Owl (Aegolius funereus) in nest cavity, Alaska

Tui De Roy / Tarictic Hornbill (Penelopides panini) male attending nest cavity in Strangler Fig, Tangkoko-Dua Saudara Nature Reserve, Sulawesi, Indonesia

Tui De Roy / Tarictic Hornbill (Penelopides panini) male standing guard over nest cavity, Tangkoko-Dua Saudara Nature Reserve, Sulawesi, Indonesia

Tui De Roy / Sulawesi Red-knobbed Hornbill (Aceros cassidix) female before sealing herself inside nest cavity for four months, Tangkoko-Dua Saudara Nature Reserve, Indonesia

Tui De Roy / Sulawesi Red-knobbed Hornbill (Aceros cassidix) male delivering Figs to female sealed in hollow tree nest cavity, Tangkoko-Dua Saudara Nature Reserve, Sulawesi, Indonesia

Tui De Roy / Sulawesi Red-knobbed Hornbill (Aceros cassidix) male delivering Figs to female sealed in hollow tree nest cavity, Tangkoko-Dua Saudara Nature Reserve, Sulawesi, Indonesia

Michael Quinton / Boreal Owl (Aegolius funereus) in tree cavity in winter, Alaska

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

Michael Quinton / Great Gray Owl (Strix nebulosa) parent with chick in nest cavity in the spring, Idaho

Michael Quinton / Great Gray Owl (Strix nebulosa) couple in nest cavity at top of snag in the spring, Idaho

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

Tui De Roy / Weeping Fig (Ficus benjamina) with Tarictic Hornbill (Penelopides panini) nest cavity in host Natu tree trunk, Tangkoko-Dua Saudara Nature Reserve, Sulawesi, Indonesia

Shin Yoshino / Northern Flicker (Colaptes auratus) parent feeding two chicks at nest, North America

Donald M. Jones / Pileated Woodpecker (Dryocopus pileatus) mother with chicks in nest cavity, Troy, Montana

Donald M. Jones / Red-shafted Flicker (Colaptes cafer) female in nest cavity extending tongue, Troy, Montana

Donald M. Jones / Mountain Bluebird (Sialia currucoides) male bringing food to nest cavity, Troy, Montana

Pete Oxford / Desert Wheatear (Oenanthe deserti) at nest with prey, Gobi Desert, Mongolia

Michael & Patricia Fogden / Streaked Tuftedcheek (Pseudocolaptes boissonneautii) tending to chick in nest in cloud forest, Tandayapa Valley, western slope of Andes, Ecuador

Kevin Schafer / Hyacinth Macaw (Anodorhynchus hyacinthinus) in nesting hole, Pantanal, Brazil

Steve Gettle / Tree Swallow (Tachycineta bicolor) male at nest cavity, Michigan

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

Pete Oxford / Hyacinth Macaw (Anodorhynchus hyacinthinus) pair in nest cavity, Pantanal, Brazil

Pete Oxford / Hyacinth Macaw (Anodorhynchus hyacinthinus) in nest cavity, Pantanal, Brazil

Murray Cooper / Blue-headed Parrot (Pionus menstruus) peeking from nest cavity, Ecuador

Kevin Schafer / Resplendent Quetzal (Pharomachrus mocinno) male in nest cavity, Talamancas, Costa Rica

Michael Quinton / Northern Flicker (Colaptes auratus) woodpecker chicks inside nest cavity, Wade Lake, Montana

Michael Quinton / Northern Flicker (Colaptes auratus) woodpecker chicks inside nest cavity, Wade Lake, Montana

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

Luciano Candisani / Hyacinth Macaw (Anodorhynchus hyacinthinus) researcher, Neiva Guedes, director of Macaw Conservation Program, holding young in front of nest cavity, endangered species, Pantanal Ecosystem, Brazil

Luciano Candisani / Hyacinth Macaw (Anodorhynchus hyacinthinus) emerging from nest in a Panama Tree (Sterculia apetala) cavity, endangered species, Pantanal Ecosystem, Brazil

Luciano Candisani / Hyacinth Macaw (Anodorhynchus hyacinthinus) emerging from nest in a Panama Tree (Sterculia apetala) cavity, endangered species, Pantanal Ecosystem, Brazil

Claus Meyer / Hyacinth Macaw (Anodorhynchus hyacinthinus) pair perching on nest cavity in tree, Pantanal, Brazil