Peat bogs and marshland cover about 26 % of the nation, while the soil of about 70 % of Belarusian territory is podzolic acidic with fairly large amounts of iron oxides. The forest region, though considerable, is not contiguous. Coniferous forests predominate, with pine the principal tree; spruce, oak, birch, and ash trees also are found. The Belovezhskaya Pushcha Reserve in the southwest is part of the oldest existing European forest and the sanctuary of the virtually extinct European bison, or wisent. Belarus has more than 69 mammal species, including deer, fox, wild pig, wolves, and the common squirrel. There are 280 bird species, including doves, kestrels, wrens, bullfinches, and woodpeckers.