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