Hungarian birch mouse

It was long thought to be a subspecies of the southern birch mouse (S. subtilis), but a 2016 study found sufficient genetic and anatomical divergence for it to be considered its own species. The same study found the Romanian population of the species to have enough genetic divergence from the Hungarian population to be described as its own subspecies, the Transylvanian birch mouse (S. t. transylvanica).[3][4]

The Hungarian birch mouse was formerly found throughout the Pannonian Basin, including most of Hungary, Austria, Slovakia, and Serbia. It is now thought to be extirpated from Austria and is likely also extirpated from Slovakia and Serbia. Based on records, it was likely abundant in Hungary prior to 1950, but it is now restricted to the Borsodi Mezőség Protected Landscape Area.[5]

The Transylvanian birch mouse was likely found throughout the Transylvanian Plateau in the past, but its range has likely shrunk due to agriculture. It is known only from several locations in Cluj County. Its full range has not yet been fully established.[6]

This species depends on undisturbed grassland habitat or grassland grazed by livestock, which has become rare due to mechanized agriculture. The Hungarian subspecies is most abundant in remnant patches of tall vegetation such as thistle in pastures that are otherwise grazed by livestock. It is also known from saline fescue meadows, wormwood-fescue steppes, and abandoned agricultural fields at lower densities. The Transylvanian subspecies is known from slopes that are highly encroached upon by shrubs, also in areas otherwise used for livestock grazing.[6]

S. trizona is one of the rarest and least known small mammal species in Europe.[5] The Hungarian subspecies has been extirpated from most of its range due to mechanized agriculture allowing for larger areas to be mowed at once, which reduces the diversity of plant species and makes the habitat unsuitable for the birch mice. Although some areas have been set aside for the protection of the species, this protected area is relatively small and the surrounding area is still mechanically cut or overgrazed. The Transylvanian birch mouse is also threatened by intensive agriculture as well as overgrazing by sheep.[6]

Both populations that were later classified into this species were feared to be extinct throughout much of the 20th century until being rediscovered the 21st century; the Hungarian subspecies was not seen for over 70 years until being rediscovered in 2006, while the Transylvanian subspecies was only rediscovered in 2014.[5][7] These threats and rarity have led to the species being classified as Endangered on the IUCN Red List.[6]