Steven David Miller / Red-collared lorikeet (Trichoglossus rubritorquis) pair examining hole in Eucalyptus as a potential nesting hollow. Katherine, Northern Territory, Australia.

Dave Bevan / Pair of Sand martins (Riparia riparia) feeding chicks, Carmarthenshire, Wales, UK, July.

Dave Bevan / Slow motion clip of a pair of Common swifts (Apus apus) approaching and entering a nest hole in limestone cliff, Norfolk, England, UK, May.

David Perpinan / Common rock sparrow (Petronia petronia) feeding chicks in nest, filmed using an infra-red camera, Cuenca, Castile-La Mancha, Spain, June.

David Perpinan / Common rock sparrow (Petronia petronia) chick exploring outside nest, Cuenca, Castile-La Mancha, Spain, June.

David Perpinan / Adult Common rock sparrow (Petronia petronia) feeding chicks, Cuenca, Castile-La Mancha, Spain, June.

David Perpinan / Female Rock sparrow (Petronia petronia) feeding chick inside nest, filmed with an infrared camera, Spain, June.

David Perpinan / Rock sparrow (Petronia petronia) feeding chick outside nest, Spain, June.

Bence Mate / European roller (Coracias garrulus) peering out of nest with feather, Lake Csaj, Pusztaszer, Kiskunsagi National Park, Hungary, February.

Dave Bevan / Male Great spotted woodpecker (Dendrocopos major) flying to a nest hole in an electricty pole and feeding chick, Carmarthenshire, Wales, UK, June.

Dave Bevan / Slow motion clip of two pairs of Jackdaws (Corvus monedula) squabbling over nest site, Worcestershire, England, UK, May.

Dave Bevan / Slow motion clip of a Tawny owl (Strix aluco) leaving nest site in hollow tree, Carmarthenshire, Wales, UK, April.