Marie Read / Common redpoll (Carduelis flammea), male perched in alder (Alnus rugosa) during a snowstorm in winter, New York, USA, January.

Alex Hyde / Peacock butterfly (Inachis io) feeding on Goat Willow catkins (Salix caprea), an important food source for pollinators in early spring. Peak District National Park, UK. April.

Markus Varesvuo / Lesser Whitethroat (Sylvia curruca) with yellow pollen stain on head from willow catkins. Porvoo, Söderskär, Finland. May

Ammonite / Close-up of a Buff-tailed bumble bee (Bombus terrestris) feeding from Willow (Salix) catkins, before flying out of frame, France, March.

Ammonite / Mining bee (Andrena apicata) flying and feeding from Sallow (Salix caprea) catkins, with pollen sacs on hind legs, France, March.

Guy Edwardes / 2020VISION / Pussy willow catkins (Salix caprea), Somerset Levels, Somerset, England, UK, April

Colin Varndell / Hazel catkins (Corylus avellana) releasing pollen. Dorset, England, UK February

Alan Williams / Blue tit (Parus caeruleus) perched on twig with catkins, Essex, UK, February

Ross Hoddinott / Close-up of Hazel inflorescence (Corylus avellana) in spring, with both male catkins and female flower visible, near Bradworthy, Devon, UK. March 09.

Ross Hoddinott / Pussy willow / Sallow (Salix caprea) catkins, Cornwall, UK. April .

Andy Sands / Male Greenfinch (Carduelis chloris) amongst Pussy willow catkins, Hertfordshire, England, UK

Ross Hoddinott / Coal tit {Periparus ater} Cornwall. UK

Ross Hoddinott / Blue tit {Parus caeruleus} with hazel catkins, Cornwall. UK

Ross Hoddinott / Sallow / Pussy willow {Salix caprea} catkin, Cornwall. UK

David Kjaer / Bullfinch male {Pyrrhula pyrrhula} with Pussy willow catkins (Salix) Yorkshire, UK

Ross Hoddinott / Common alder (Alnus glutinosa) catkins silhouetted. Cornwall. UK

Dave Watts / Two Great tits with willow catkins in snow, France {Parus major}

David Kjaer / Brimstone butterfly on Pussy willow catkins, UK

David Tipling / Willow warbler singing, early spring, New Forest, Hampshire, England,

Hanne & Jens Eriksen / Eurasian blue tit (Cyanistes caeruleus) perched in tree amongst catkins. Denmark, February.

Nigel Cattlin / Goat willow (Salix caprea) male catkins. Kennett and Avon Canal, Berkshire, England, UK. April.

Heather Angel / Hazel (Corylus avellana) catkins dispersing pollen with female flower above on branch. Controlled condition.s Surrey, England, UK.

Heather Angel / Cedar (Cedrus sp), male catkins releasing pollen. Surrey England, UK.

Gary K. Smith / Common redpoll (Carduelis flammea) perched on corkscrew hazel with catkins, England, UK. March.

Gary K. Smith / Siskin (Carduelis spinus) perched on corkscrew hazel with catkins, England, UK. March.

Gary K. Smith / Common redpoll (Carduelis flammea) male, perched on corkscrew hazel with catkins, England, UK. March.

Gary K. Smith / Goldfinch (Carduelis carduelis) perched on corkscrew hazel with catkins, England, UK. March.

Nick Upton / Eurasian aspen (Populus tremula) catkins, major food souce for Siberian flying squirrel (Pteromys volans). Lisaku Park Forest, Estonia. April.

Nick Upton / Interior of a nest box used by Siberian flying squirrel (Pteromys volans) with stored Aspen (Populus tremula) catkins. Estonia. April.

Mike Read / Silver birch (Betula pendula) tree with catkins and young leaves. Wareham Forest, Dorset, England, UK. April.

Mike Read / Green alder (Alnus viridis suaveolens) catkins and leaves. Restonica Valley, Regional Natural Park of Corsica, France. July.

Mike Read / Goat willow (Salix caprea) catkins and new leaves. Blashford Lakes Nature Reserve, Hampshire and Isle of Wight Wildlife Trust Reserve, Ellingham, Ringwood, Hampshire, England, UK. April.

Mike Read / Silver birch (Betula pendula) catkins in morning light. Wareham Forest, Dorset, England, UK. April.

Andy Sands / Goat wIllow (Salix caprea) Young male catkins often known as pussy willow, Hertfordshire, England, UK, April

Andy Sands / Bullfinch (Pyrrhula pyrrhula) male among Pussy Willow catkins, Hertfordshire, England, UK, February

Andy Sands / Blue tit (Cyanistes caeruleus) perched among Pussy Willow catkins, Hertfordshire, England, UK, February

Philippe Clement / Common hazel (Corylus avellana) close up of male catkins dispersing pollen in early spring, Belgium, February

Chris Mattison / Mountain willow (Salix arbuscula) male catkins. Cultivated plant.

Ashley Cooper / Pollen from Willow catkins (Salix sp) on black background.

Nick Upton / Common osier (Salix viminalis) female catkins releasing seeds, Wiltshire, UK, April.

Nick Upton / Common osier (Salix viminalis) female catkins releasing seeds, Wiltshire, UK, April.

Nick Upton / Hazel (Corylus avellana) male catkins and small female inflorescence with snow in the background, Wiltshire, UK, March.

Stephen Dalton / Hazel catkins (Corylus avellana) Sussex, England, UK, February.

Robert Thompson / Hazel catkins (Corylus avellana) and female flower, Northern Ireleand.

Staffan Widstrand / Wild Wonders of China / Birch tree (Betula sp) with catkins, humid montane mixed forest, Laba He National Nature Reserve, Sichuan, China

Jussi Murtosaari / Eurasian siskin (Carduelis spinus), male on branch with catkins, Finland, April.

Phil Savoie / Goat Willow (Salix caprea), male catkins, Monmouthshire, Wales, UK, April.

Phil Savoie / Garden bumblebee (Bombus hortorum) visiting Goat willow (Salix caprea) male catkins, Monmouthshire, Wales, UK. March.

Phil Savoie / Garden bumblebee (Bombus hortorum) visiting Goat willow (Salix caprea) male catkins, Monmouthshire, Wales, UK. March.

Stephen Dalton / Hazel (Corylus avellana) catkins shedding pollen, Primroses (Primula vulgaris) in woodland setting, Rookery Wood, Sussex, England, UK, February.

Nick Upton / Juvenile Eurasian beaver (Castor fiber) at night, standing up to reach for Willow (Salix) catkins just out of reach, before swimming off, Devon Wildlife Trust's Devon Beaver Project, Devon, UK, April.

Colin Varndell / Common alder (Alnus glutinosa) male catkins in early spring, Dorset, UK, March.

Juan Carlos Munoz / Willow (Salix sp) catkins and leaves. Shumen, Bulgaria, April.

Juan Carlos Munoz / Willow (Salix sp) catkins and leaves. Shumen, Bulgaria, April.

Gary K. Smith / Chaffinch (Fringilla coelebs) female perched on hazel twig with catkins, Norfolk, UK February

Mike Read / Green alder (Alnus alnobetula subsp. suaveolens) catkins Restonica Valley, Corsica, France, May 2016

Mike Read / Green alder (Alnus alnobetula subsp. suaveolens) catkins Restonica Valley, Corsica, France, May 2016.

Mike Read / Alder (Alnus glutinosa) catkins, Testwood Lakes Reserve, Hampshire and Isle of Wight Wildlife Trust Reserve Southampton, Hampshire, England, UK, February 2016.

Mike Read / Alder (Alnus glutinosa) catkins, Testwood Lakes Reserve, Hampshire and Isle of Wight Wildlife Trust Reserve, Southampton, Hampshire, England, UK. February 2016.

Colin Varndell / Male catkins of Bog myrtle/Sweet gale (Myrica gale), Dorset, UK, April.

Will Watson / Pussy willow catkins of Green-leaved Willow (Salix x rubra), Bodenham Lake Nature Reserve, Herefordshire, England - SMALL REPRO ONLY

Colin Varndell / Sweetgale (Myrica gale) catkins, Dorset, England, UK, April.

Loic Poidevin / Brambling (Fringilla montifringilla) female perched amongst catkins, Loire Atlantique, France, January

Loic Poidevin / Brambling (Fringilla montifringilla) male perched amongst catkins, Loire Atlantique, France, January

Loic Poidevin / Blue tit (Cyanistes caeruleus) sitting on Hazel (Corylus) with catkins, Loire Atlantique, France, January

Loic Poidevin / Great tit (Parus major) sitting on Hazel (Corylus) with catkins, Loire Atlantique, France, January

Loic Poidevin / Great tit (Parus major) sitting on Hazel (Corylus) with catkins, Loire Atlantique, France, January

Rod Williams / Pussy willow catkins (Salix caprea) Sutcliffe Park Nature Reserve, Eltham, London. April.

Ernie Janes / Long tailed tit (Aegiyhalos caudatus) on hazel catkins, England, UK, February.

Ernie Janes / Long tailed tit (Aegithalos caudatus) on Hazel branch with catkins, England, UK, February.

Ernie Janes / Blue tit (Cyanistes caeruleus) perched on branch with catkins in winter, England, UK, February.

Nick Upton / Hazel (Corylus avellana) young male catkins, GWT Lower Woods reserve, Gloucestershire, UK, October.

Solvin Zankl / Male flowers (catkins) producing pollen on English oak tree (Quercus robur) Niedersechsische Elbtalaue Biosphere Reserve, Elbe Valley, Lower Saxony, Germany

Solvin Zankl / Male flowers (catkins) producing pollen on English oak tree (Quercus robur) Niedersechsische Elbtalaue Biosphere Reserve, Elbe Valley, Lower Saxony, Germany

Solvin Zankl / English oak tree (Quercus robur) male flowers (catkins) producing pollen. Niedersechsische Elbtalaue Biosphere Reserve, Elbe Valley, Lower Saxony, Germany

Solvin Zankl / English oak tree (Quercus robur) male flowers (catkins) producing pollen. Niedersechsische Elbtalaue Biosphere Reserve, Elbe Valley, Lower Saxony, Germany

Markus Varesvuo / Male Black grouse (Tetrao / Lyrurus tetrix) jumping to reach Willow catkins, Utajarvi, Finland, May.

MYN / Niall Benvie / Crack willow (Salix fragilis) catkins, Angus, Scotland, UK, May.

Mike Read / Common alder (Alnus glutinosa) and catkins on white background.

Fabrice Cahez / Pussy willow (Salix caprea) catkins, Vosges, France, March.

Alex Hyde / Sessile oak (Quercus petraea) leaves and catkins emerging in spring. Peak District National Park, Derbyshire, UK. April.

Alex Hyde / Peacock butterfly (Inachis io) feeding on Goat Willow catkins (Salix caprea), an important food source for pollinators in early spring. Peak District National Park, UK. April.

Alex Hyde / Peacock butterfly (Inachis io) feeding on Goat Willow catkins (Salix caprea), an important food source for pollinators in early spring. Peak District National Park, UK. April.

Alex Hyde / Peacock butterfly (Inachis io) feeding on Goat Willow catkins (Salix caprea), an important food source for pollinators in early spring. Peak District National Park, UK. April.

Martin Gabriel / Caucasian oak (Quercus macranthera) catkins and leaves, Northern Armenia, May.

Russell Cooper / Female Goat willow catkins (Salix caprea) at the point of opening, south-west London, UK, March.

Russell Cooper / Silver birch catkins (Betula pendula) male and female, South-west London. UK, April.

Russell Cooper / Male Goat willow catkins (Salix caprea) South-west London, UK, April.

Roger Powell / Blue tit (Cyanistes caeruleus) on a Hazel (Corylus) branch with catkins. Lower Saxony, Germany. February.

Roger Powell / Blue tit (Cyanistes caeruleus) on a Hazel (Corylus) branch with catkins. Lower Saxony, Germany. February.

Roger Powell / Hawfinch (Coccothraustes coccothraustes coccothraustes) female perched on a Hazel branch (Corylus) surrounded by catkins. Lower Saxony, Germany, Febraury.

Roger Powell / European greenfinch (Chloris chloris chloris) male on branch with Hazel catkins (Corylus) Lower Saxony, Germany, February.

Roger Powell / European greenfinch (Chloris chloris chloris) male on branch with Hazel catkins (Corylus) Lower Saxony, Germany, February.

Roger Powell / Eurasian bullfinch (Pyrrhula pyrrhula) with a seed in its beak, perched on a Hazel (Corylus) branch with catkins. Lower Saxony, Germany

Colin Varndell / Male and female Alder (Alnus glutinosa) catkins developing. Dorset, UK, February.