Doug Wechsler / Yellow-plumed parakeets (Leptosittaca branickii) at nest cavity in wax palm stump, Tapichalaca Biological Reserve, Ecuador.

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

Anup Shah / African Harrier-Hawk (Polyboroides typus) adult using its legs to probe into cavity nests for young of birds such as swifts and weavers. Masai Mara National Reserve, Kenya. February.

Rolf Nussbaumer / Eastern Fox Squirrel {Sciurus niger} in tree cavity, Hill Country, Texas, USA

Steve Gettle / Tree Swallow (Tachycineta bicolor) at nest cavity, Huron Meadows Metropark, Michigan

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

Martin Willis / Budgerigar (Melopsittacus undulatus) father with begging chick at nest cavity, Queensland, Australia

Murray Cooper / Spot-breasted Woodpecker (Colaptes punctigula) at nest cavity, South America

Michael Quinton / Northern Flicker (Colaptes auratus) carrying fecal sac from nest cavity in forest, Alaska

Donald M. Jones / Red-breasted Nuthatch (Sitta canadensis) chick in nest cavity, western Montana

Michael Quinton / Northern Flicker (Colaptes auratus) at nest cavity with chicks, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) parent at nest cavity with chick, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) chicks in nest cavity ready to fledge, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) chicks in nest cavity ready to fledge, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) chick in nest cavity ready to fledge, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) entering nest cavity with chicks, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) chicks in nest cavity ready to fledge, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) entering nest cavity with begging chicks, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) parent brooding chicks in nest cavity, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) at nest cavity with begging chicks, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) leaving nest cavity in forest, Alaska

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

Michael Quinton / Northern Flicker (Colaptes auratus) leaving nest cavity in forest, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) at nest cavity, Alaska

Michael Quinton / Northern Flicker (Colaptes auratus) at nest cavity, Alaska

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

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

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

Alan Murphy / Northern Flicker (Colaptes auratus) female at nest cavity with chicks, British Columbia, Canada

Alan Murphy / Northern Flicker (Colaptes auratus) male at nest cavity with chicks, British Columbia, Canada

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

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

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

Graeme Guy / Northern Flicker (Colaptes auratus) male at nest cavity, British Columbia, Canada

Tui De Roy / Plate-billed Mountain-Toucan (Andigena laminirostris) at nest cavity, Bellavista Cloud Forest Reserve, Tandayapa Valley, Andes, Ecuador

Tui De Roy / Plate-billed Mountain-Toucan (Andigena laminirostris) at nest cavity, Bellavista Cloud Forest Reserve, Tandayapa Valley, Andes, Ecuador

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

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

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

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 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

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

Winfried Wisniewski / White-fronted Bee-eater (Merops bullockoides) trio at nest entrances, Chobe National Park, Botswana