David Pattyn / Bewick's swans (Cygnus bewickii) four in flight, Den Bosch, The Netherlands, January

Paul Hobson / Bewick's swan (Cygnus columbianus bewickii) flying across a full moon, UK, February 2011

David Kjaer / Small flock of Bewick's / Tundra swans (Cygnus columbianus) on water, in snow covered landscape, Gloucestershire, England

Steve Knell / Two Bewick's swans (Cygnus columbianus bewickii) in flight, Slimbridge WWT, Gloucestershire, England, January

Steve Knell / Bewick's swan (Cygnus columbianus bewickii) alert on water, Slimbridge WWT, Gloucestershire, England, January

David Kjaer / Bewick swan {Cygnus columbianus columbianus} in flight. UK.

David Kjaer / Two Bewick swans {Cygnus columbianus bewickii} flying with full-moon in sky. UK.

Chris Packham / Bewicks swans in winter. (Cygnus columbianus bewickii) England Hampshire, UK

WILLIAM OSBORN / Bewicks swan standing on ice portrait (Cygnus columbianus bewickii) UK

Geoff Simpson / Bewick swan on water {Cygnus columbianus bewickii} Washington WWT, Co Durham, UK

Nick Upton / Bewick's swan (Cygnus columbiana bewickii) flapping its wings as it stands on a frozen marshland pool, Gloucestershire, UK, February.

Nick Upton / Bewick's swan (Cygnus columbiana bewickii) sliding on ice as it lands on a frozen marshland pool with other swans and Canada geese (Branta canadensis) in the background, Gloucestershire, UK, Febru...