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Der Bestand der Ringeltaube (Columba palumbus) in der Innenstadt von Münster/Westfalen 1989 und 2008
(2009)
Eine Erfassung der Ringeltaubennester im Herbst 2008 ergab 1134 Nester auf einer Fläche von 3,4 km² in der Innenstadt von Münster. Das entspricht einer durchschnittlichen Dichte von etwa 334 Nestern / km². Zuvor waren auf gleicher Fläche von Scharlau & Scharlau (1990) 1214 Nester gefunden worden. Diese geringfügige Abnahme um ca. 7 % des Ringeltaubenbestandes liegt im Fehlerbereich der Erhebung. Die hohe Dichte wird auf die Lage der Solitärstadt Münster in landwirtschaftlich geprägtem Umfeld zurückgeführt. Die Auswahl der Nistbäume entspricht weitgehend dem Angebot. Linden sind der häufigste Nistbaum.
The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.