POOT M, ENGELEN F & VAN DER WINDEN J (1999) A mixed breeding pair of Blyth's Reed Warbler Acrocephalus dumetorum and Marsh Warbler A. palustris near Utrecht in spring 1998. LIMOSA 72 (4): 151-157.

In 1998 a mixed pair of a male Blyth's Reed Warbler and a female Marsh Warbler bred in the Netherlands, in a ruderal strip along an industrial area on the outskirts of Utrecht. The habitat consisted of dry marshy vegetation and some willows. The Blyth's Reed Warbler sang from 14 till 19 June. On 18 June, interactions were seen with a Marsh Warbler, ending on the ground in dense vegetation where peeping sounds were heard, likely made during copulation. On the same day, nest building was started. During the nest phase both birds became very secretive and only contact calls were heard near the nest. On 8 July four young of c. 2-3 days old were present. We estimate that the first egg was laid on 22-23 June. Only two young were in the nest on 13 July and were ringed. They were fed by both parents. On 18 July the nest was empty. On 23 July an attempt was made to recapture the juveniles. Only the adult male was caught and ringed. Our observations resembled the picture of mixed breeding described by Koskimies (1980) for a population in SE-Finland, where hybridisation of Blyth's Reed Warbler and Marsh Warbler is relatively common. As this case shows, the possibility of breeding of Blyth's Reed Warbler should be kept in mind when birds show up in Western Europe and Marsh Warblers are around. If singing stops, this does not need to imply that the male has disappeared, but could be a indication of the onset of breeding..

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