JANSEN PB & NIE HW DE (1986) Thirty years of passerine breeding bird monitoring in a mixed wood. LIMOSA 59 (3): 127-134.

In 1951-81, the numbers of passerine breeding birds (including Turtle Dove Streptopelia turtur) have been determined in 50 ha of mixed woodland, mainly pine-, spruce-, beech- and red oakwood (fig. 1, tab. 1). The number of territories was determined by the mapping method or by the maximum number registered in a series of 9 to 63 visits (17 minllO ha) per season. The censuses have been carried out by the first author, who already had much experience when the study started. The internal consistency of the entire data set is discussed. For 10 of the 18 species, the registration efficiency did not differ for years with 44-63 visits per season (1953-68) and years with 9-24 visits (1969-81, app.). Turtle Dove, Blackbird Turdus rnerula, Song Thrush T. philornelos, Blackcap Sylvia atricapilla, Willow Warbler Phylloscopus trochilus, and Chaffinch Fringilla caelebs showed higher efficiencies during the second period, presumably because from 1969 onwards the author made his censuses mainly under favourable weather conditions. There was no positive correlation between the numbers of territories found and the numbers of visits during the second period. The average registration efficiency of all (81- 100%) of the territories (Jn,aJ was up to 17 times larger than expected according to theoretically derived levels by Kwak & Meijer (1985). Species with low densities in the area had lower registration efficiencies than found elsewhere in the Netherlands. Turtle Dove, Robin Erithacus rubecula, Mistle Thrush Turdus viscivorus Garden Warbler Sylvia borin, Chiffchaff Phylloscopus collybita, Willow Warbler, Willow Tit Parus montanus, Coal Tit P. ater, and Yellowhammer Emberiza citrinella decreased from a total of 89 to 48 territories per 50 ha on average (fig. 3, 4). Tree Pipit Anthus trivialis, Dunnock Prunella modularis, Song Thrush, Whitethroat Sylvia communis, and Blackcap had in 1951 together 17 territories/ 50 ha; around 1965 they increased by a factor 1.8, but in 1980 the number decreased to 12 per 50 ha. Whitethroat and Yellowhammer disappeared from the study area. Only Wren and Blackbird definitely increased in numbers. The main cause of the decreases can be attributed to increased human recreational activities which have caused disturbance and also damage of the herbaceous vegetation. Changes in the plantation scheme and succession of the vegetation in the wood also played an important role in changes in the number of breeding birds. Wren Troglodytes troglodytes and Robin decreased in 1956, 1963 and 1979 because of severe winters. In 1956, Song Thrush and Blackcap also declined, probably because the frost reached deeper south in Europe. The decline of the Whitethroat started earlier than the decline of other species breeding in the lower part of the vegetation. Obviously, factors in the wintenng area mainly affected the breeding population; the same is suggested for the Dutch population of Redstart, Garden Warbler, and Willow Warbler. Intoxicated seeds might have negatively influenced the Chaffinch in the western part of the Netherlands. ##### Winter Wren Troglodytes troglodytes European Robin Erithacus rubecula Common Redstart Phoenicurus phoenicurus Common Whitethroat Sylvia communis Garden Warbler Sylvia borin Blackcap Sylvia atricapilla Common Chiffchaff Phylloscopus collybita Willow Warbler Phylloscopus trochilus Yellowhammer Emberiza citrinella European Turtle Dove Streptopelia turtur Tree Pipit Anthus trivialis Dunnock Prunella modularis Common Blackbird Turdus merula Song Thrush Turdus philomelos Mistle Thrush Turdus viscivorus Willow Tit Poecile montanus Coal Tit Periparus ater Common Chaffinch Fringilla coelebs

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