Bittern (Botaurus stellaris) is a stocky bird of the heron family. It has a long, strong bill, a broad, long neck, and powerful legs. From the tip of the bill to the end of the tail it measures between 70 and 80 cm, and its wingspan is between 125 and 135 cm. It spends most of its time in the shelter of reeds, with which it blends very well thanks to its distinctive cryptic coloration and is therefore rarely seen. In spring days and nights its call betrays it. The unmistakable "BUUH", which resembles the sound produced when blowing into an empty bottle, echoes up to 5 km away. In flight the bittern resembles a cross between a heron and an owl. It is a solitary bird, active mainly at dusk, when it searches for food. It preys on fish, amphibians, reptiles, bird nestlings and small mammals, and among invertebrates it feeds on larvae and adult insects, spiders, crayfish and molluscs.
Populations of bitterns living in warmer regions do not migrate, while populations in the continental parts of Europe migrate in autumn from breeding areas to warmer places. In Slovenia the bittern can be observed year-round, but its nesting has not been confirmed for a long time. In Europe it nests from March to June. It builds a cup-shaped nest from reeds and other vegetation in dense reed beds. The nest is usually right by the water, less often it is floating. Although males on large wetlands can mate with several females, they generally do not nest in colonies.
Although the bittern has a wide distribution and its population is currently relatively large, the situation can quickly reverse, as it has very strict requirements for nest-site selection. It prefers quiet lowland wetlands by lakes or rivers, where young reed beds overgrow extensive areas flooded with shallow water. The water level must not fluctuate greatly; around reed beds it needs larger areas of deep water that are not overgrown and are present also in the dry part of the year. It is threatened by the loss of reed beds, which change due to drying up and changes in the traditional regime of harvesting reeds and due to pollution of waters with excessive inputs of mineral fertilizers and contaminants. Disturbances during the breeding season also have fatal consequences. Outside the breeding season the bittern is not so particular and can be found in other habitats, for example by ponds, gravel pits and on wet meadows.
