Das Ressourcenangebot verschiedener Grünland-Gesellschaften und dessen Nutzung durch Brutvögel. Eine biozönologische Fallstudie zur Habitatnutzung des Braunkehlchens (Saxicola rubetra) in Südwestdeutschland
The relationships between avifauna, vegetation and invertebrate fauna were investigated in several grassland areas of Southwest Germany. The following methods have been applied: for the avifauna, mapping of singing males or alarm-calls of warning pairs or families as well as recording of hunting activities of particular breeding-pairs; for the invertebrate fauna, a standardized catching net-method and for the vegetation phytosociological methods with special consideration of structure analyses and phenological investigations. The focus of this investigation was the whinchat (Saxicola rubetra) in the area "Mittelmeß" (58 ha area, 690 m altitude, 47°57' latitude, 8°35' east longitude) in the "Baar"-region. The vegetation of this low marshy country comprises different types of Arrhenatheretum and Cirsietum rivularis meadows which are mown one to three times a year for fodder, as well as extensive, loosely growing Molinetalia and Caricion fuscae communities and Thymo-Festucetum, all three being cut once a year for late mown litter (straw). Moreover, different fallow-lying Magnocaricion, Filipendulion and heterogeneously structured Molinietalia communities occur, as well as ruderal vegetation. Trees and shrub species are almost completely absent. The overall food supply (number of invertebrates) was highest in a Cirsietum rivularis and in a shrubby Angelica sylvestris-Molinietalia-fallow area and was about 2.5 times higher than in the sample areas poorest in invertebrate individuals (Thymo-Festucetum, Carex acutiformis-Magnocaricion, Melilotus officinalis-ruderal area). The invertebrate spectrum of similar structured sample areas showed high values of dominance identity (Renkonen). Invertebrate numbers and flower diversity of the sample areas increased parallel with the vegetation. Vegetation structure types were mapped to determinate habitat preferences. The types could be distinguished by the physiognomic characteristics of the vegetation - especially grass-to-herb ratio, degree of coverage 15 cm above ground, flower diversity, and density of "Überständer". Comparing the floristic and the physiognomic methods (of mapping vegetation units), the typical forms of plant associations are represented only by (one or few) structure types while those plant associations changing through natural succession or agricultural amelioration show several structure types. By comparing the utilization of structure types by the whinchat with the "supply" of structure types, the habitat preferences could be quantified (calculation of a "utilization coefficient"). The most preferred habitats of the whinchat were fallow-lying Molinietalia and Magnocaricion communities as well as the less agriculturally intensive Cirsietum rivularis and Arrhenatheretum stands with high forbs serving as hunting perches. The habitat preference could be differentiated for several stages of the breeding season and be explained by the development of vegetation (e.g. growing of Überständer) and the development of food supply. With artificially installed perches, it turned out that areas which had not been utilized by the whinchat before installation were utilized afterwards, but only as long as the food supply in these places was at least as high as in most of the other places. It could be shown, that both structure type and food supply can be limiting factors in whinchat habitats. The mapping of vegetation structure types provided a good basis for determination of habitat preferences of the whinchat, because both the hunting strategy as well as the dependence of invertebrate fauna of the vegetation could be well explained by structural parameters of the vegetation. Furthermore, relating structure types to plant associations is possible, and provides a new approach for vegetation science. It would be interesting to apply the same techniques in several more areas, to compare structure types of plant associations and (relative) habitat preferences for different bird species.