A Sedge Warbler (Acrocephalus schoenobaenus) can be spotted singing agressively from a reed or willow perch in the old reed bed of the ponds south of Stangenhagen near Trebbin in southern Brandenburg.
The Sedge Warblers have only been in their breeding grounds in the extensive sedge beds, reed beds, reedmace and wet meadow of the Nuthe-Nieplitz-wetlands for a few days but are now abundandt in this wetland. They react spontaneously and vehemently to the imitation of fellow Sedge Warblers in order to defend their territory. One Sedge Warbler makes the wings vibrate at the slightest phrases typical of the species song, making itself bigger with a fluffed up belly and chest plumage and singing its song out loud. In between, the bird makes small vertical and horizontal jumps and jumps especially between the vertical stems of the reed to find a better – more exposed – branch or stem. In this conflict situation, the song is rarely interrupted by its otherwise typical song flights.
All in all, the Sedge Warbler delivers a loud and powerful mixture of pleasantly sounding sounds, which, in an immediate comparison, are free of the hard, strict, clattering passages that are otherwise used by the Eurasian Reed-Warbler (Acrocephalus scirpaceus). The Sedge Warblers’ song is faster, less rhythmic than the Eurasian Reed-Warbler’s song. Mimicry occurs occasionally. The song of the Sedge Warbler is also more abrupt, lively, less flowing and fluid than that of the Marsh Warbler (Acrocephalus palustris).
An individual-based examination shows in studies that the space requirement of the Sedge Warbler during the breeding season ranges from less than 1,000 to 5,000 m². The area sizes are (e.g. in Belgium) 350-2.229 m². However, the feeding area changes during the breeding season. The territory sizes are also dependent on vegetation and range from an average of 955 m² in blackberry scrub, an average of 1,034 m² in willow bushes with blackberries to an average of 1,152 m² in tall reed beds. The territories are on average 1,500-2,000 m², with the area spanning a range of 350-2,229 m². Agricultural areas in the Central European lowlands from the Netherlands to western Poland are used in addition to the Sedge Warblers and the Marsh Warbler and the Eurasian Reed-Warbler in the form of drainage ditches. The breeding habitat claims lead to a far-reaching separation, especially of the Marsh Warbler and the Eurasian Reed-Warbler. Only in the transition areas of reed beds and tall perennial vegetation, e.g. B. in melioration ditches weeded by eutrophication and dehydration, especially with Common Nettle (Urtica dioicd), there is a mixed vegetation that has an ecologically intermediate character and in which both species of Reed-Warblers can occur directly next to each other. Swampy, flooded areas between fields are also occasionally populated.
One reason for the remarkably aggressive behavior of the Sedge Warbler in spring could be to prevent the female from having illegitimate contacts with another male in the wider territory. Although 90% of the passerines live in socially monogamous pair bonds, molecular studies have shown that 86% of the passerines examined have genetic polygamy. This means that individuals are involved in copulations outside the couple bond (so-called extramarital copulations). Most of the explanations for the occurrence of extramarital copulations refer to the fact that females benefit indirectly from the unification with another male outside of marriage. The Sedge Warbler is a socially monogamous species in which 8% of the offspring (according to a study) came from extramarital copulations. The quality and complexity of the singing performed is recognized as one of the sexually relevant male characteristics used by females to choose a partner. The partners are selected based on a variety of male qualities. A common theory, the theory of “good genes”, predicts that females may be seduced by extramarital copulations with males of higher (genetic) quality than their social partner. This should lead to fitness benefits of the offspring. Interestingly, a study found that extra-marital associations took place frequently with males, which obviously had a reduced song repertoire and smaller territory than their respective social partners. This obvious preference for male Sedge Warblers with reduced song repertoire as partners contradicts the predictions of previous studies. One of the scientists’ assumptions is that the sudden cessation of singing activities after mating can result in acoustic signals being no longer available for later mating and in females possibly switching to other clues. Such behavior can lead to different patterns of female choice during social and extramarital mating. It is obvious that there are several reasons behind the patterns of electing by female Sedge Warblers.
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