Lucas Bustamante / Monarch butterflies (Danaus plexippus) flying during a warm morning. When the overwintering period draws to an end large numbers become active. Michoacan, Mexico, February.

Ingo Arndt / Flock of Brambling (Fringilla montifringilla) perched at roosting tree, overwintering, Steiermark, Austria

Laurent Geslin / Tufted Duck (Aythya fuligula) flock overwintering on Lake Leman / Geneva with city in background, Switzerland

Ingo Arndt / Monarch butterflies (Danaus plexippus) gathered to drink, overwintering colony, Michoacan, Mexico

Ingo Arndt / Cloud of Monarch butterflies (Danaus plexippus) flying, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) flying amongst trees, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) drinking from pool of water, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) flying in warmth of midday sun, overwintering colony in Michoacan, Mexico

Nigel Cattlin / 16-spot ladybird (Tytthaspis sedecimpunctata) group overwintering on old wood. Berkshire, England, UK. May.

Nigel Cattlin / 16-spot ladybird (Tytthaspis sedecimpunctata) overwintering on old wood, close up. Berkshire, England, UK. May.

Nigel Cattlin / 16-spot ladybird (Tytthaspis sedecimpunctata) overwintering on old wood, close up. Berkshire, England, UK. May.

Nigel Cattlin / 16-spot ladybird (Tytthaspis sedecimpunctata) overwintering on old wood, close up. Berkshire, England, UK. May.

Nigel Cattlin / Sweet bay (Laurus nobilis) leaves with overwintering cold or wind damage. Berkshire, England, UK. April.

Nigel Cattlin / European red mite (Panonychus ulmi) eggs overwintering on Apple (Malus domestica) wood.

Nigel Cattlin / European red mite (Panonychus ulmi) eggs overwintering on Apple (Malus domestica) wood.

Nigel Cattlin / European red mite (Panonychus ulmi) eggs overwintering on Apple (Malus domestica) wood.

Cyril Ruoso / Harlequin ladybird (Harmonia axyridis) group overwintering under bark in crack in tree, non-native invasive species. Bourgogne-Franche-Comte, France. March.

Marie Read / Bird photographers line up at sunrise to photograph mass takeoffs of overwintering Snow geese (Chen caerulescens), Bosque Del Apache National Wildlife Refuge, New Mexico, USA. December 2017.

Marie Read / Snow Geese (Chen caerulescens) large flock in pool at Bosque Del Apache National Wildlife Refuge with setting full moon at sunrise in December, New Mexico, USA. December.

Ingo Arndt / Brambling (Fringilla montifringilla) flock arriving at sleeping place while overwintering, Hessen, Germany, February.

Sven Zacek / Penduline tit (Remiz pendulinus) overwintering in Estonia and searching for food in reeds in Tartumaa, Estonia. January.

Dong Lei / Grandala (Grandala coelicolor) flock overwintering in Basongcuo National Park, Tibet, China, November.

Alex Hyde / Orange 16-Spot Ladybird (Halyzia 16-guttata / sedecimguttata) overwintering in recess of carved letter on a gravestone. Peak District National Park, Derbyshire, UK, March 2012.

Jeff Wilson / The crew of United States Coastguard Cutter 'Healy' prepare decks in the Bering sea, Alaska, USA. The ship served as a platform for the Frozen Planet crew whilst filming Spectacled eider ducks overwin...

Nick Upton / Single male of an overwintering species of Greenbottle (Eudasyphora cyanella) sunbasking in early spring sunshine on Forsythia flower. Wiltshire garden, UK, March.

Nick Upton / Male Greenbottle fly (Eudasyphora cyanella) sunbasking on a leaf. This is an overwintering species. Wiltshire, UK, March

Nick Upton / Two male Greenbottle flies (Eudasyphora cyanella) competing with one another as they search for mates. This is an overwintering species. Wiltshire, UK, March

Ingo Arndt / Flock of Bramblings (Fringilla montifringilla) flying to roosting site, overwintering, Steiermark, Austria

Ingo Arndt / Large flock of Bramblings (Fringilla montifringilla) in flight, overwintering, Steiermark, Austria

Angelo Gandolfi / Glossy Ibis (Plegadis falcinellus) flock flying over a Cork Oak tree (Quercus ruber), Coto del Rey area, Coto Donana National Park, Andalusia, Spain. Probably overwintering during migration.

Alan Williams / Whooper Swans (Cygnus cygnus) overwintering at Martin Mere WWT reserve, Lancashire, UK, February 2008

Gary K. Smith / Hosta plants overwintering in pots, Norfolk, UK, December 2008

Ingo Arndt / Aerial view of a colony of Monarch butterflies (Danaus plexippus) overwintering in trees, Michoacan, Mexico

Ingo Arndt / Cloud of Monarch butterflies (Danaus plexippus) flying, overwintering colony, Michoacan, Mexico

Ingo Arndt / Mass of Monarch butterflies (Danaus plexippus) flying, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) covering tree trunk, early morning, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) covering tree trunk, early morning, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) covering tree trunk, early morning, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) flying, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) drinking from pool of water, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) drinking from pool of water, overwintering colony, Michoacan, Mexico. Not available for print sales

Ingo Arndt / Monarch butterflies (Danaus plexippus) drinking from pool of water, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) gathered round water to drink, overwintering colony, Michoacan, Mexico

Ingo Arndt / Monarch butterflies (Danaus plexippus) flying and resting in trees, midday, overwintering colony, Michoacan, Mexico

Ingo Arndt / Tree trunk covered with members of a colony of Monarch butterflies (Danaus plexippus), some of them have their wings open and are basking in the sun, overwintering colony, Michoacan, Mexico

Ingo Arndt / Tree trunk covered in Monarch butterflies (Danaus plexippus) some of them have opened their wings and are basking in the sun, overwintering colony, Michoacan, Mexico

Michael D. Kern / Flock of Snow geese {Chen caerulescens} flying, overwintering in central america

Michael D. Kern / Flock of Snow geese {Chen caerulescens} some flying, some on ground, overwintering in central america