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

Gerrit Vyn / Two Northern Saw-whet Owls (Aegolius acadicus) fledgling chicks, that have recently left the safety of their nest cavity. Alberta, Canada. June.

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

Tom Vezo / Acorn Woodpecker (Melanerpes formicivorus) female bringing food to nest cavity, Madera Canyon, Arizona

Konrad Wothe / Boreal Owl (Aegolius funereus) in nest cavity, Sweden

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

Michael & Patricia Fogden / Emerald Toucanet (Aulacorhynchus prasinus) eating Passion Fower (Passiflora sp) fruit at nest cavity, cloud forest, Costa Rica

Konrad Wothe / Boreal Owl (Aegolius funereus) in nest cavity, Sweden

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

Mitsuaki Iwago / Galah (Eolophus roseicapilla) pair in tree nest cavity with chick, Australia

Michael Quinton / American Robin (Turdus migratorius) parent feeds grasshopper to chicks in nest cavity at top of a snag, summer, Idaho

Michael Quinton / Great Gray Owl (Strix nebulosa) brooding female with chick in nest cavity at top of snag in the summer, Idaho

Michael Quinton / Great Gray Owl (Strix nebulosa) parent with owlets in nest cavity at top of snag, Idaho

Anup Shah / African harrier hawk (Polyboroides typus) investigating a cavity to see if there is a nest to steal eggs from, Masai Mara National Reserve, Kenya. March

Ingo Arndt / Garden Dormouse (Eliomys quercinus) hibernating in old blue tit nest inside hollow tree, Germany

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.

Scott Leslie / Snapping Turtle (Chelydra serpentina) nest cavity with eggshells, Canada

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

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

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

Tom Vezo / Western Screech Owl (Megascops kennicottii) juvenile at nest, Green Valley, Arizona

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

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

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

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

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

Pete Oxford / Chestnut-fronted Macaw (Ara severa) pair at nest cavity, Yasuni National Park, Amazon, Ecuador

Konrad Wothe / Great Spotted Woodpecker (Dendrocopos major) chick in nesthole, Upper Bavaria, Germany

Donald M. Jones / Red-shafted Flicker (Colaptes cafer) male cleaning fecal sac out of nest cavity, Troy, Montana

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

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

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

Donald M. Jones / Mountain Bluebird (Sialia currucoides) female in nest cavity, Troy, Montana

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

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

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

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

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

Chien Lee / Plain-pouched Hornbill (Aceros subruficollis) male approaching nest cavity being mobbed by Greater Racket-tailed Drongo (Dicrurus paradiseus), Uthai Thani, Thailand

Chien Lee / Plain-pouched Hornbill (Aceros subruficollis) male at nest cavity, Uthai Thani, Thailand

Martin Willis / Galah (Eolophus roseicapilla) female emerging from nest cavity, Simpson Desert, Northern Territory, Australia

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

Steve Gettle / Northern Flicker (Colaptes auratus) at nest cavity, Rifle River Recreation Area, Michigan

Steve Gettle / Red-bellied Woodpecker (Melanerpes carolinus) male at nest cavity, Huron Meadows Metropark, Michigan

Steve Gettle / Downy Woodpecker (Picoides pubescens) female at nest cavity, Kensington Metropark, Milford, Michigan

Steve Gettle / Red-bellied Woodpecker (Melanerpes carolinus) male in nest cavity, Kensington Metropark, Milford, Michigan

Steve Gettle / Red-bellied Woodpecker (Melanerpes carolinus) male in nest cavity, Kensington Metropark, Milford, Michigan

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

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

Pete Oxford / Crimson-rumped Toucanet (Aulacorhynchus haematopygus) peeking out of nest hole, Intag Valley, northwest Ecuador

B&S Draker / Pair of Golden-fronted Woodpecker (Melanerpes aurifrons), one taking flight from their nest cavity. Sinton, Corpus Christi, Coastal Bend, Texas, USA, May.

Steve Gettle / Eastern Screech Owl (Megascops asio) in nest cavity, Howell, Michigan

Steve Gettle / Eastern Screech Owl (Megascops asio) at nest cavity, Howell, Michigan

Pete Oxford / Fernandina's Flicker (Colaptes fernandinae) woodpecker female emerging from nest cavity, Cuba

Pete Oxford / Fernandina's Flicker (Colaptes fernandinae) woodpecker female at nest cavity, Cuba

Tui De Roy / Nubian Woodpecker (Campethera nubica) at nest cavity, Lewa Wildlife Conservation Area, Kenya

Tui De Roy / Meyer's Parrot (Poicephalus meyeri) emerging from nest cavity, Solio Game Reserve, Kenya

Tui De Roy / Meyer's Parrot (Poicephalus meyeri) at nest cavity, Solio Game Reserve, Kenya

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

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

Konrad Wothe / Common Starling (Sturnus vulgaris) parent at nest cavity with begging chicks, Bulgaria

Konrad Wothe / European Bee-eater (Merops apiaster) at nest cavity, Bulgaria

Konrad Wothe / European Roller (Coracias garrulus) at nest cavity, Bulgaria

Konrad Wothe / Eclectus Parrot (Eclectus roratus) female emerging from nest cavity, Cape York Peninsula, North Queensland, Queensland, Australia

Murray Cooper / Choco Toucan (Ramphastos brevis) emerging from nest cavity, Ecuador

Murray Cooper / Choco Toucan (Ramphastos brevis) at nest cavity, Ecuador

Marie Read / Lewis's Woodpecker (Melanerpes lewis) outside its nest hole in burned Jeffrey Pine (Pinus jeffreyi), with beakful of insects to feed its young, Mono Lake Basin, California, USA. (Digitally retouched...

Marie Read / Black-backed Woodpecker (Picoides arcticus) female with food for young (beetle larva) in bill, clinging near its nest hole in burned Jeffrey Pine (Pinus jeffreyi) trunk, Mono Lake Basin, California,...

Steve Gettle / Black-capped Chickadee (Poecile atricapillus) at nest cavity, Kensington Metropark, Michigan

Steve Gettle / House Wren (Troglodytes aedon), returning to next with insect prey, Huron Meadows Metropark, Michigan

Steve Gettle / House Wren (Troglodytes aedon) returning to nest with prey, Huron Meadows Metropark, Michigan

Steve Gettle / Eastern Bluebird (Sialia sialis) male at nest cavity, Brighton, Michigan

Steve Gettle / Eastern Bluebird (Sialia sialis) male at nest cavity, Brighton, Michigan

Luciano Candisani / Blue-fronted Parrot (Amazona aestiva) emerging from nest cavity, southern Pantanal, Brazil

Luciano Candisani / Blue-fronted Parrot (Amazona aestiva) emerging from nest cavity, southern Pantanal, Brazil

Luciano Candisani / Blue-fronted Parrot (Amazona aestiva) emerging from nest cavity, southern Pantanal, Brazil

Gerrit Vyn / Two Northern Saw-whet Owls (Aegolius acadicus) fledgling chicks, that have recently left the safety of their nest cavity. Alberta, Canada. June.

Gerrit Vyn / American Three-toed Woodpecker (Picoides dorsalis) female, with head emerging from nest cavity in tree trunk. Alberta, Canada. May.

Tim Laman / Great indian hornbill (Buceros bicornis) female inside her nest cavity in hollow tree, with chick. Female passes food brought by the male to the chick, Borneo. IUCN Red List: Near Threatened.

Tim Laman / Great indian hornbill (Buceros bicornis) female inside her nest cavity in hollow tree, with chick. Female passes food brought by the male to the chick, Borneo. IUCN Red List: Near Threatened.

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

James Christensen / Golden-headed Quetzal (Pharomachrus auriceps) male bringing katydid to nest, Mindo, Ecuador

James Christensen / Golden-headed Quetzal (Pharomachrus auriceps) male bringing katydid to nest, Mindo, Ecuador

Murray Cooper / Tarantula (Theraphosidae) emerging from nest cavity, Amazon, Ecuador

Murray Cooper / Golden-headed Quetzal (Pharomachrus auriceps) male at nest cavity, Ecuador

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

Murray Cooper / Orange-fronted Barbet (Capito squamatus) male bringing food to nest cavity, Ecuador

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