Potential food resources and nestling food in the Great Reed Warbler (AcrocephMus arundinaceus arundinaceus) and Eastern Great Reed Warbler ( A crocephalus arundinaceus orientalis)

The main ecological difference between the Great Reed Warbler (Acrocephalus arundinaceus arundinaceus) and Eastern Great Reed Warbler (Acrocephalus acrocephalus orientalis) lies in the density of breeding population, which in the Eastern subspecies studied in Japan is on average 10 times higher than that of European populations of the Western subspecies. In this study it is shown that potential food resources of the Great Reed Warbler in Japan are more than ten times more abundant than those in Poland, with respect both to the number of potential prey items (invertebrates) and to their total biomass. This strongly suggests that such high densities of Great Reed Warbler in Japan are at least partly due to more plentiful food supply. Other factors which could explain the difference between breeding densities in Japan and Poland are the predation by Marsh Harrier (Circus aeruginosus) in Poland and lack of competition with Reed WarNer (Acrocephalus scirpaceus) in Japan. The abundance of food on the sample plot in Japan resulted primarily from outbreaks of dipterans of the suborder Nematocera, the most common of which were Chironomidae. Although Chironomidae were not the preferred prey, they made up a substantial proportion of the diet of nestlings. In both studied areas, apart from dipterans, the greatest proportion of nestlings’ diet constituted arachnids. Differences in nestlings’ diet between study sites resulted mainly from a greater proportion of Nematocera and lower proportion of Coleoptera and Odonata in Japan

Andrzej Dyrcz and Heiner Flinks, J. Ornithol. 141,351-360 (2000)