VOOUS KH & DIJK T VAN (1972) Mysterious songsters in the Province of Drente. LIMOSA 45 (1): 36-41.

During the summer seasons of 1964, 1965, 1966 and 1967 at least two, at present still unidentified Phylloscopi were observed singing in a mixed deciduous broad-leaved forest of Mensinge Estate, Roden, province of Drente. These birds have been observed singing persistently throughout the spring and early summer and did not seem to have got mates. In the same habitat Phylloscopus trochilus and P. collybita were plentifully present as breeding birds. Of P. sibilatrix only a few singing males were around. The ratio of abundance of these species was in 1967 about 18 : 8 : 1. The unidentified Phylloscopi, which reappeared each year in practically the same site (distances varying up to about 50 metres), have been observed and heard by several Dutch field-ornithologists and were subsequently mentioned by Van Orden & Braaksma (1966) as Phylloscopus trochiloides. The song of one of these birds was recorded by Tj. van Dijk on 21 June 1966. The song phrases, sonagrams of which were made in the Biological Laboratory of the Free University, Amsterdam, did not agree with the recordings of songs of P. trochi[ oides viridanus available, nor with the song of any other species of Phylloscopus of which recordings had been collected. The recording has been demonstrated to a great number of people and has been sent to specialists in Germany (Prof. E. Schilz, Prof. A. Faber, Dr. M. Schubert), but so far nobody has been able to identify it, or to remember ever to have heard this song. H does not agree with the song of mixed songsters of P. trochilus and P. collybita either (Schubert 1969). The birds themselves looked like pale buffy greyish coloured P. trochilus and seemed to have a very narrow white wing bar and brownish feet. A bird with identical song has been observed in the nearby Alteveerse Bos on 10 June 1965 and again in almost the same spot (distance of 30 metres) on 13 May 1967. The recording of the song was made available for demonstration in the Bird Sound Corner of the XV International Ornithological Congress in The Hague in 1970, where it was listened at by numerous Congress members. Though it was generally agreed that the song showed some relation to that of P. trochilus nobody ventured to identify it specifically. The birds themselves were more lively in their movements than either the Willow Warbler, the Cliffchaff and the Wood Warbler, kept to higher elevations in the tree tops most of the day and performed their song most persistently throughout the day, apparently not hampered by strong wind and rain. Grauwe Fitis Phylloscopus trochiloides

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