BULT H (1999) Pitfalls during Marsh Tit Parus palustris surveys in West-Brabant. LIMOSA 72 (3): 85-88.

The first (Fig. I, Teixeira 1979) and many subsequent (SOVON 1987) records of Marsh Tit in West-Brabant are solely based on registrations of song or calls by myself and similar "outsiders", i.e. birders coming from areas where this species is much more common. Later I noticed that the call of the Marsh Tit, until then assumed to be diagnostic (ef Cramp & Perrins 1993), can be "mimicked" by Red squirrel Sciurus vulgaris and most other Parus-species. Moreover, occasional aberrant songs of Blue Tit P. eaeruleus are indistinguishable from that of a Marsh Tit. Since 1988, keen local birders failed to discover Marsh Tits during a thorough survey of West-Brabant, but again a few territories were reported by "outsiders". To investigate the puzzling status of the Marsh Tit, a tape with calls and two song variants was repeatedly played during one to three visits to the sites in West-Brabant. The technique proved to be highly efficient and a single visit elicited reactions in 67% of the territories in Terworm (Limburg), a stronghold of the species. In contrast, not a single Marsh Tit was found at 24 sites where it was reported from in West-Brabant, only territories of mimicking species were apparent. Moreover, 80% of the territories of tit species with a similar registration efficiency was relocated after three visits to the Liesbos (Tab. I). It thus seems that the presence of Marsh Tits in West-Brabant is questionable and that future claims should be perfectly documented in view of the likelihood of pitfalls.

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