[order] Ciconiiformes | [family] Ardeidae | [latin] Botaurus stellaris | [UK] Great Bittern | [FR] Butor étoilé | [DE] Rohrdommel | [ES] Mirasol Común | [IT] Tarabuso eurasiatico | [NL] Roerdomp

Roerdomp determination

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Stocky, rather bulky, thick-necked heron, generally golden-brown with black crown and mottling above and dusky stripes below. This plumage is in their favorite habitat, extensive reedbeds, extremely cryptic. In flight, primaries and all except innermost secondaries pale reddish-brown barred and spotted with black. More often heard than seen, it's deep booming song is often likened to the sound produced by blowing across the top of an empty milk bottle.

A secretive species which has been very little studied to date. It is found mainly in beds of common reed Phragmites australis, either pure stands or mixed with other fen species, though birds have bred occasionally in other dense aquatic vegetation such as saw sedge Cladium mariscus and lesser reed mace Typha angustifolia. Adults have been shown by analysis of records of booming to be largely site faithful in any one year. Analysis of breeding sites shows that Bitterns utilise large, wet reedbeds with areas of open water. Research in England originally indicated that booming males required a reedbed of at least 20 hectares in extent, but it now appears that smaller sites may be utilised provided that other feeding areas are available nearby. In continental Europe, where there are highly productive reedbeds, Bitterns can certainly exist in sites of less than 20 hectares. Some much smaller reedbeds support breeding birds, but these birds are dependent on a network of reed-fringed dykes or more open wetland habitats and other reedbeds over a much wider area for foraging.
In reedbeds established in shallow water (less than 30cm deep), foraging areas (at least in winter) are changed according to the water level. In these reedbeds, marked and rapid variations during the breeding season could reduce the availability of aquatic food, so Bitterns tend to avoid wetlands with water levels which fluctuate widely. Reedbeds established in deeper water (over 30cm) appear to be less prone to drying out.

This solitary bird inhabits the permanent marshes of Eurasia, between 30°N and 60°N, and from the Iberian Peninsula to China. It is also known from northern and southern Africa. European birds are partly sedentary, partly migratory, and some of them reach equatorial regions. The populations of the European Union are strongly fragmented, and declining everywhere (except in Denmark). They are totalling not more than 970-1400 breeding pairs, while the total European population is estimated at 19000-43000 pairs. The main reason for this decline is the reclamation of wetlands, especially the disappearance of large Phragmites beds, the mechanical harvesting of reeds, the pollution and disturbance of the breeding areas by human recreation activities. However, breeding populations also fluctuate very strongly according to the severity of climatic conditions in winter
The Bittern breeds throughout much of Europ, north Africa and central and eastern Asia, with an outlying population in southern Africa. In northern Europe it is largely migratory, in central and western Europe it is partially resident, and in southern European countries, such as Greece, breeding is sporadic but wintering birds are widespread.
The European population, excluding the former USSR, was estimated to be between 2500 and 2700 pairs in 1976. However, the hard winter of 1978/79 resulted in a 30-50% decline in European populations (Day 1981). In southern Europe the Bittern tends to be a winter visitor and passage migrant. In Greece, for example, birds are regularly seen in winter and at times of severe cold weather exhausted individuals turn up in remarkably high numbers.

Bitterns are fairly eclectic and flexible in their choice of food, which is predominantly fish, amphibians and insects, but also small birds and mammals. In the Netherlands, small mammals are regarded as important in severe winters. Bitterns require adequate feeding opportunities for primarily aquatic animals - fish, amphibians, invertebrates and some birds. In mainland Europe, amphibians may be a more important part of the diet; in Britain this may also be the case where they are numerous, because they are available at an important time of the year (pre-breeding), but most reedbeds have poor invertebrate populations. At some sites in northern Europe eels Anguilla anguilla appear to be the principal food and here the availability of an unimpeded run for eels is important. Eels of 3-4 years of age, about 100 grams in weight and 35cm long, appear to be the optimum size for Bittern. Larger eels frequent dykes whilst the smaller ones are more likely to penetrate into wet reedbed. By contrast, in at least one Bittern site in Italy, wintering (and probably breeding) Bitterns feed largely on very small (2-5cm) fish, predominantly two species of mosquito fish Gambusia holbrooki and Aphanius fasciatus.

This species has a large range, with an estimated global Extent of Occurrence of 10,000,000 km². It has a large global population estimated to be 110,000-340,000 individuals (Wetlands International 2002). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern. [conservation status from birdlife.org]

Males advertise their territories by booming and there is some evidence that they are polygamous. The nest consists of a platform of reed stems amongst standing reeds. Usually four to five eggs are laid in April-May. As far as is known, all incubation and chick rearing is carried out by the female. The young can leave the nest 12 days after hatching but may stay on the nest platform longer before dispersing into the reedbed. The female may build further platforms for the young while they are dependent on her for food. Fledging takes place between June and early August in northern Europe, May-June in Mediterranean Countries. Little is known about fledging success, chick survival or chick diet

The migratory patterns of populations in the EU depend on their location. In United Kingdom and Atlantic coastal areas Bitterns tend to be resident, since the climate is relatively mild in most winters, and normally abundant rainfall keeps the reedbeds moist in all but the driest summers. The resident breeding population is augmented by wintering birds from further north and east.
Continental birds inhabit northern and inland regions of the EU, where winters are cold and shallow water freezes solid for long periods. These Bitterns are mainly migratory, wandering south and west, move mostly to South and South East Asia to reach suitable wintering areas. They are highly vulnerable at this time, and populations may crash after particularly severe weather in their wintering areas (more so than the Atlantic coast birds).
Mediterranean birds are mainly resident, since freezing up of the water bodies in their wetland habitats is rare. However, extremely dry summers cause severe problems in some areas due to drying out of water bodies, and it is thought that Bitterns may have to subsist on a variety of non-aquatic prey, such as small mammals, at this time.