Silkosiya (Bulgarian: Силкосия) is a nature reserve in Strandzha Nature Park, located in the homonymous mountain, southeastern Bulgaria. Its territory close to the villages Kosti and Balgari. Silkosiya is the oldest reserve in the country, declared on 23 July 1931 in order to protect the evergreen bushes unique for Europe. It encompasses part of the Veleka catchment area with a territory of 396 ha. The terrain is various, in the lower parts there are predominantly swamp areas with typical Central European flora. The reserve encompasses territory between 100 and 250 m altitude and is thus among the lowest-lying nature reserves in the country.
The variety of plant species is very high. There are specimen from different geographical latitudes which makes it interesting for the scientists. Plant inversion can be observed in the reserve. Usually, the beech trees are located in the higher and cooler areas, while the oak trees occupy the lower parts. In Silkosia in is the other way round: the oak forests are located higher in the mountain.
In the wet valley rare species such as the strandzhan periwinkle, laurel, caucausian blueberry, common yew can be observed. It is also the only place in Europe, where the wild medlar tree can be found. The most common flowers are the caucausian cowslip and the spring cyclamen which blossom in February and March.
Typical reptilians in the reserve are the spur-thighed tortoise, Hermann's tortoise, Anguis fragilis and scheltopusik. There are also many species of snakes - coluber caspius, Aesculapian snake, Elaphe sauromates, Malpolon monspessulanus among the others.
There are various bird species. The golden oriole, nightingale, Sardinian warbler, whitethroat, common blackbird, Eurasian jay, common chaffinch nest in the reserve, as well as different species of Falconiformes, woodpeckers and many others.
- Георгиев, Георги. Националните и природните паркове и резерватите в България. Гея-Либрис, София, 2004, с. 130 - 131 ISBN 954-300-028-X