Description
More than 230 species of birds have been recorded in and around Østensjøvannet. In summer the lake hosts a large colony of Karabağ martı, as well as populations of Bahri, Sakarmeke, Sutavuðu and Tepeli patka. You can also find Bataklık kirazkuşu, Saz Kamışçını and the occational Orman çıvgını here.
It is a popular area for recreation, and there is a path all around the lake with plenty of good viewpoints. The path brings you quite close to the water's edge. There is also a bird hide on the west side of the lake, which provies good views of the central parts.
During spring and autumn the lake is used as a stopover by numerous species, such as Kaşıkgaga, Kulaklı batağan and waders.
In winter, large flocks of Yeşilbaş stay at the lake, with the occational Kilkuyruk and Fiyu thrown in. Various geese and Sakarmeke are also known to winter here. As a bonus, encountering Uzunkuyruklu baştankara is not uncommon during the colder months.
Details
Access
There is a parking site south of the lake. The area can also be reached by bus (Abildsø) and the subway (Skullerudåsen, Bøler). The paths around the lake are mostly prepared for biking, but walking around the lake (with plenty of stops) won't take more than about 2 hours (4,5 km).

