Sven Zacek / Brown bear (Ursus arctos) in coniferous forest. Ida-Virumaa, Eastern Estonia. September.

Sven Zacek / Meandering Loobu River with surrounding meadows and woodlands, aerial view. Lahemaa National Park in Northern Estonia in Laane-Virumaa, Estonia. May 2017.

Sven Zacek / Two Black Grouse (Tetrao tetrix) fighting at a lek. Virumaa, Estonia, Europe, April.

Sven Zacek / Roe Deer (Capreolus capreolus) herd travelling through snow and fog. Virumaa, Estonia, Europe, March.

Sven Zacek / Roe Deer (Capreolus capreolus) from different angles, only one in focus. Virumaa, Estonia, Europe, March. Winner of the Mammals category in GDT competition 2013. 2nd Prize in the Mammals Category of M...

Sven Zacek / Roe Deer (Capreolus capreolus) males with developing antlers in velvet, one in focus, one not. Virumaa, Estonia, Europe, March. Winner of the Animal stories portfolio in the Melvita Nature Images Awar...

Sven Zacek / Small herd of Roe Deer (Capreolus capreolus) looking back at the photographer against forested background. Virumaa, Estonia, Europe, February.

Sven Zacek / Large herd of Roe Deer (Caperolus capreolus) running across snow. Virumaa, Estonia, Europe, March.

Sven Zacek / Roe Deer (Capreolus capreolus) pronking away from the photographer. Virumaa, Estonia, Europe, February.

Sven Zacek / Ural Owl (Strix uralensis) in flight over snow. Virumaa, Estonia, Europe, February.

Sven Zacek / Herd of Roe Deer (Capreolus capreolus) on the horizon with another out-of-focus in foreground. Virumaa, Estonia, Europe, February.

Sven Zacek / Herd of Roe Deer (Capreolus capreolus) feeding through snow as the sun sets. Virumaa, Estonia, Europe, February.

Sven Zacek / Portrait of two Roe Deer (Capreolus capreolus) in soft evening light. Virumaa, Estonia, Europe, February.

Nick Upton / Eurasian beaver (Castor fiber) dam made from cut branches, surrounded by wet woodland. Near Lisaku, Ida-Viru County, Estonia. April.

Nick Upton / Scots Pine tree (Pinus sylvestris) forest on northern shore of Lake Peipsi, near Alajoe, Ida-Viru County, Estonia. April 2018.

Nick Upton / Wind-blown lake ice piled up near shore, Lake Peipsi, Estonia. April 2018.

Sven Zacek / Silhouette of Red squirrel (Sciurus vulgaris) gathering nesting material for winter nest. Ida-Virumaa, Eastern Estonia. November.

Sven Zacek / Brown bear (Ursus arctos) in coniferous forest. Ida-Virumaa, Eastern Estonia. September.

Sven Zacek / Narva River, the border between Estonia and Russia with medieval castles on each side, in evening light. Ida-Virumaa, Estonia. May 2016.

Sven Zacek / Red squirrel (Sciurus vulgaris) crossing log over river surrounded by ferns. Lahemaa National Park, Laane-Virumaa, Estonia. July.

Sven Zacek / Self portrait of photographer holding photograph of the same woodland scene taken the previous day. Lahemaa National Park, Laane-Virumaa, Northern Estonia. May 2016.

Sven Zacek / Pattern of Conifer (Pinaceae) needles on snow, revealed following spring melt. Agusalu Nature Reserve, Ida-Virumaa, Eastern Estonia. April.

Sven Zacek / Pine (Pinus sp) forest canopy viewed from below. Agusalu Nature Reserve, Ida-Virumaa, Eastern Estonia. April.

Sven Zacek / Marsh marigold (Caltha palustris) flowering along Altja river, Lahemaa National Park, Laane-Virumaa, Northern Estonia. May.

Nick Upton / Siberian flying squirrel (Pteromys volans) caught in emergence trap over nest hole entrance in Aspen (Populus tremula) tree. Muraka Forest Reserve, near Lisaku, Estonia. April.

Nick Upton / Siberian flying squirrel (Pteromys volans) caught in trap, prior to fitting of radio collar. Muraka Forest Reserve, near Lisaku, Estonia. April.

Nick Upton / Siberian flying squirrel (Pteromys volans) in the hand, prior to fitting of radio collar. Muraka Forest Reserve, near Lisaku, Estonia. April.

Nick Upton / Siberian flying squirrel (Pteromys volans) entering nest hole in Downy birch (Betula pubescens) tree trunk. Mature mixed forest, near Lisaku, Ida Virumaa, Estonia. April.

Nick Upton / Researcher labelling collection tube of Siberian flying squirrel (Pteromys volans) droppings. Droppings found below old deciduous tree with suitable nest hole. Muraka Forest Reserve, near Lisaku, Ida...

Nick Upton / Siberian flying squirrel (Pteromys volans) droppings in moss. Found underneath old deciduous tree with an occupied nest hole. Lisaku Park Forest, Estonia. April.

Nick Upton / Siberian flying squirrel (Pteromys volans) droppings. Found below an old deciduous tree with nest hole. Muraka Forest Reserve, near Lisaku, Estonia. April.

Nick Upton / Laptop screen showing trailcam footage of Siberian flying squirrel (Pteromys volans) returning to nest hole in Aspen (Populus tremula) tree. Near Lisaku, Estonia. April 2018.

Nick Upton / Felled conifer trees stacked beside forestry track through mixed forest. Near Muraka Forest Reserve, a rare surviving home for Siberian flying squirrel (Pteromys volans). Near Lisaku, Estonia. April 2...

Nick Upton / Felled conifers stacked at edge of forest.Near Muraka Forest Reserve, a rare surviving home for the Siberian flying squirrel (Pteromys volans). Estonia. April 2018.

Nick Upton / Felled Aspen (Populus tremula) trees with tit nests in hollows. Treeholes suitable for Siberan flying squirrel (Pteromys volans). Near Lisaku, Estonia. April 2018.

Nick Upton / Felled Aspen (Populus tremula) trees with tit nests in hollows. Treeholes suitable for Siberan flying squirrel (Pteromys volans). Near Lisaku, Estonia. April 2018.

Nick Upton / Downy birch (Betula pubescens) trees in mixed woodland, the tree on the left houses a Siberian flying squirrel (Pteromys volans) nest hole. Near Lisaku, Estonia. April.

Nick Upton / Aspen (Populus tremula) with Aspen trunk rot / False tinder conk mushroom (Phellinus tremulae). Trees with rotten cores provide ideal nest holes for the Siberian flying squirrel (Pteromys volans). Mur...

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

Nick Upton / Researcher radiotracking Siberian flying squirrel (Pteromys volans) in mature mixed forest. Muraka Forest Reseve, near Lisaku, Estonia. April 2018.

Nick Upton / Research team preparing to radio collar Siberian flying squirrel (Pteromys volans) caught in a trap. Muraka Forest Reserve, near Lisaku, Estonia. April 2018. Model released.

Nick Upton / Researcher setting trailcam to monitor Siberian flying squirrel (Pteromys volans) nest hole. Mature mixed forest, near Lisaku, Estonia. April 2018. Model released.

Nick Upton / Researcher reviewing trailcam footage on laptop of Siberian flying squirrel (Pteromys volans) emerging from nest hole in Aspen (Populus tremula) tree at night. Mature mixed forest, near Lisaku, Estoni...

Nick Upton / Researcher radio tracking Siberian flying squirrel (Pteromys volans) in mature mixed forest. Muraka Forest Reseve, near Lisaku, Estonia. April 2018. Model released.

Nick Upton / Researcher inspecting a Siberian flying squirrel (Pteromys volans) caught in trap on old Aspen (Populus tremula) tree with occupied nest hole. Muraka Forest Reserve, near Lisaku, Estonia. April 2018....

Nick Upton / Researcher examining Siberian flying squirrel (Pteromys volans) droppings. Mature mixed forest, near Lisaku, Estonia. April 2018. Model released.

Nick Upton / Researcher examining and sexing Siberian flying squirrel (Pteromys volans) prior to fitting radio collar. Muraka Forest Reserve, near Lisaku, Estonia. April 2018. Model released.

Nick Upton / Researcher wearing thick gloves collecting Siberian flying squirrel (Pteromys volans) from trap prior to fitting radio collar. Muraka Forest Reserve, Lisaku, Estonia. April 2018. Model released.