Balance Sheet of Breeding Bird Population 1994
First of all, the breeding bird habitats were certified area-specifically according to the break-down of Flade (1994). By means of an addition of all habitats occurring in a grid square, it was possible to prepare a species list of the indicator species to be expected for these areas (ideal value). By means of the assignment of actually mapped breeding birds, as obtained from the breeding bird atlases, to the grid squares, the actual breeding bird occurrence was determined (actual value). From these ideal and actual values, a balance sheet of the occurrence of breeding bird indicator species in Berlin could then be prepared. Finally, for better assignment, the balance-sheet results were underpinned with a potential avifaunistic evaluation of the habitats (with reference to the index values according to Witt 1991), and a survey of the occurrence of Red Data Book species was compiled.
Certification of Breeding Bird Habitats
The basis of habitat mapping was the study of breeding bird habitat types of Germany north of the crest of the Central Mountains, as described comprehensively by Flade, from which the habitats occurring in Berlin were selected and supplemented by inclusion of the type Airports (cf. Flade 1994). The differentiated habitat types of young conifer forests were subsumed under the Type Conifer Thickets and Pole Woods. Altogether, 44 habitat types were distinguished (cf. Map Legend).
In the certification, only areas larger than 5 ha were considered; in cases of valuable mini-habitats (Bodies of Water, Gravel Quarries, Open Forest Bogs, Field Woods, Soft-Wood Riparian Forests, Sedge Marshes and Reed-Beds; the latter shown larger-than-scale), the minimum area size was 1 ha. These minimum sizes were considered, since the occurrence of demanding bird species is generally tied to a certain minimum area size. Also, the occurrence probability of characteristic bird species increases with area size (cf. Tab. 1, Fig. 3 and Bezzel 1982).
For reasons of representability, canals and all other linearly distinctive habitats, such as railroad embankments, ditches or wooded fringes, were not considered.
Decisive for the assignment of an area to one of the 44 classes was its structure (type of vegetation, type and density of development) and use. This is also true for mosaic areas (several habitat types in a small area) and special areas, for which case-by-case decisions were made. Thus, a mosaic area consisted of allotment gardens, dry ruderal areas and tree nurseries was assigned to the class allotment gardens, since its structure was generally very allotment garden-like. Recultivated dumps as examples of special areas were assigned either to the class dry ruderal areas or to the class parks, depending on their structure and condition. In sum, almost all classes were supplemented by case-by-case assignments of similar structures, which was always preceded by a thorough examination with aerial photography.
Subsequently, all open space losses after 1980 were marked, in order to obtain a comparable reference time of mapped habitat types and breeding bird mappings. Small-area islands and fringes of changed use thus created were likewise excluded, since in these cases, negative influences at the edges of the open spaces can be assumed.
Ascertainment of the Species Lists by Grid Square (Ideal Value)
To be able to use the grid mapping of the breeding bird atlases, indicator species lists were prepared for each grid square.
For this purpose, the habitat map was first of all overlaid with the grid from the atlases. Since the map grid squares differed in the two breeding bird atlases, small areas along the former boundary between East and West Berlin could not be covered, and no balance-sheet could be prepared for them.
Subsequently a habitat type list was prepared for each grid square through summation, whereby only the splinter areas at the grid boundaries remained unconsidered. Small-area habitats, too, were incorporated in the list, and primarily associated, if necessary, to a grid square.
Flade assigns each breeding-bird habitat to a potential indicator species group (cf. Tab. 2).
Using the work of Flade as a standard, and with consideration of the breeding bird species found in Berlin according to the Red Data Book of breeding birds (Witt 1991), indicator species groups were formed for the 44 Berlin habitat types. In cooperation with Berlin ornithologists, the supplementary habitat type Airports and the comprehensive type Conifer Thickets and Pole Woods were also provided with indicator species groups, and Dry-Shrub Ruderal Areas and Ruderal Areas were modified (cf. Tab. 3).
A total of 118 breeding-bird species were covered (cf. Tab. 4).
Some of these species occur only in one of the two city halves or have become extinct or lost since 1945; in the latter case, the possibility of restocking in Berlin still exists.
With the aid of the grid square-covered habitat type lists and the indicator species groups, it was ultimately possible to prepare a list of the indicator species to be expected for each grid square, the number of which was entered as the ideal-value in the balance.
Survey of the Actual Population
The grid square-related ideal-values thus ascertained were counterposed to the data of actually encountered species (actual values) from the breeding bird atlases. The occurrence of potential indicator species was checked by means of the ascertainment categories used in the atlases: B (possible range), C (probable broods) and D (certain broods), as well as for the black kite, by means of surface cross-hatching (range marking). Subsequently, the number of indicator species mapped in each grid square was determined and compared with the number of indicator species to be expected.
Grid squares with predominantly changed use since 1980 and grid squares with no major share of Berlin territory (grid squares on the border with Brandenburg) were not considered.
The balance sheet results are shown on the present map with the aid of the ideal and actual values and the proportional share of mapped indicator species (in 20 % stages). The bird species missing or encountered, respectively, may also be obtained from the resulting file.
Data for the distribution and population development of the individual species are listed in the breeding-bird atlases. More current and more detailed single-area investigations (test areas) can be requested from the literature file of the Berlin State Commissioner for Conservation (cf. ÖKOGRUBE).
Inquiry into the Potential Avifaunistic Value
The potential avifaunistic value should serve as background information for the estimate of the value of the indicator species groups missing or occurring (balance-sheet result).
The Red Data Book of breeding birds in Berlin (Witt 1991) contains index values in accordance with Bezzel (1980) for each of the bird species used (cf. Tab. 4). It describes the distribution of the species throughout the whole city (index A), their presence in major areas (index B), their range figures (index C) and their population development (index D); these were used in the present map to derive a simple sum (index A + index B + index C + index D = simple sum) as a measure for habitat evaluation (cf. Tab. 5).
Only for two species, and in the special case of species lost or extinct since 1945, did new values have to be established in cooperation with Berlin ornithologists (cf. Tab. 6).
Finally, with the aid of these sum-index values of breeding bird species, average index values were calculated for the indicator species groups of each Berlin habitat type, which were used, in cooperation with the Berlin ornithologists, for the formation by five evaluation levels for the Berlin breeding bird habitat types.
Class I thus contains bird habitats which are potentially especially rare in Berlin and/or offer declining or rare species suitable brood habitats; Class V, on the other hand, encompasses habitats which are widespread in Berlin, and are not home to any, or to hardly any, rare or declining species (cf. Tab. 7).
Such conspicuous features of this value-level assignment as the relatively poor values for deciduous forests, ruderal areas, allotment gardens and garden-apartment areas, or the relatively good values for pine forests, dumps, old buildings and city center-areas, can be explained by the evaluation approach: The indicator species potentially occupying the respective habitat type show corresponding index-values according to Witt, 1991. The evaluation thus gives the avifaunistic potential of each habitat type, regardless of the actual condition and size of the individual habitat; it is not therefore referenced toward single areas.
Evaluation of the Breeding Bird Population According to Occurrences of Red Data Book Species
The number of Red Data Book species occurring per mapped grid square was determined with the aid of the breeding bird atlases and shown as a vignette. The species of the Classes 1 (threatened by extinction), 2 (highly endangered), 3 (endangered) and P (potentially endangered) of the Red Data Book of breeding birds in Berlin (Witt 1991) were used. In addition, populations of the species red kite, black kite, honey buzzard, hobby, sparrow hawk and raven which, for species protection reasons, are documented in the breeding bird atlases only by means of surface cross-hatching or not at all, were incorporated into the evaluation.